aboutsummaryrefslogtreecommitdiff
path: root/Documentation/filesystems/devfs/README
blob: 54366ecc241fee30aaf1a77ec91b15c310b766a3 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
Devfs (Device File System) FAQ


Linux Devfs (Device File System) FAQ
Richard Gooch
20-AUG-2002


Document languages:







-----------------------------------------------------------------------------

NOTE: the master copy of this document is available online at:

http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html
and looks much better than the text version distributed with the
kernel sources. A mirror site is available at:

http://www.ras.ucalgary.ca/~rgooch/linux/docs/devfs.html

There is also an optional daemon that may be used with devfs. You can
find out more about it at:

http://www.atnf.csiro.au/~rgooch/linux/

A mailing list is available which you may subscribe to. Send
email
to majordomo@oss.sgi.com with the following line in the
body of the message:
subscribe devfs
To unsubscribe, send the message body:
unsubscribe devfs
instead. The list is archived at

http://oss.sgi.com/projects/devfs/archive/.

-----------------------------------------------------------------------------

Contents


What is it?

Why do it?

Who else does it?

How it works

Operational issues (essential reading)

Instructions for the impatient
Permissions persistence across reboots
Dealing with drivers without devfs support
All the way with Devfs
Other Issues
Kernel Naming Scheme
Devfsd Naming Scheme
Old Compatibility Names
SCSI Host Probing Issues



Device drivers currently ported

Allocation of Device Numbers

Questions and Answers

Making things work
Alternatives to devfs
What I don't like about devfs
How to report bugs
Strange kernel messages
Compilation problems with devfsd


Other resources

Translations of this document


-----------------------------------------------------------------------------


What is it?

Devfs is an alternative to "real" character and block special devices
on your root filesystem. Kernel device drivers can register devices by
name rather than major and minor numbers. These devices will appear in
devfs automatically, with whatever default ownership and
protection the driver specified. A daemon (devfsd) can be used to
override these defaults. Devfs has been in the kernel since 2.3.46.

NOTE that devfs is entirely optional. If you prefer the old
disc-based device nodes, then simply leave CONFIG_DEVFS_FS=n (the
default). In this case, nothing will change.  ALSO NOTE that if you do
enable devfs, the defaults are such that full compatibility is
maintained with the old devices names.

There are two aspects to devfs: one is the underlying device
namespace, which is a namespace just like any mounted filesystem. The
other aspect is the filesystem code which provides a view of the
device namespace. The reason I make a distinction is because devfs
can be mounted many times, with each mount showing the same device
namespace. Changes made are global to all mounted devfs filesystems.
Also, because the devfs namespace exists without any devfs mounts, you
can easily mount the root filesystem by referring to an entry in the
devfs namespace.


The cost of devfs is a small increase in kernel code size and memory
usage. About 7 pages of code (some of that in __init sections) and 72
bytes for each entry in the namespace. A modest system has only a
couple of hundred device entries, so this costs a few more
pages. Compare this with the suggestion to put /dev on a <a
href="#why-faq-ramdisc">ramdisc.

On a typical machine, the cost is under 0.2 percent. On a modest
system with 64 MBytes of RAM, the cost is under 0.1 percent.  The
accusations of "bloatware" levelled at devfs are not justified.

-----------------------------------------------------------------------------


Why do it?

There are several problems that devfs addresses. Some of these
problems are more serious than others (depending on your point of
view), and some can be solved without devfs. However, the totality of
these problems really calls out for devfs.

The choice is a patchwork of inefficient user space solutions, which
are complex and likely to be fragile, or to use a simple and efficient
devfs which is robust.

There have been many counter-proposals to devfs, all seeking to
provide some of the benefits without actually implementing devfs. So
far there has been an absence of code and no proposed alternative has
been able to provide all the features that devfs does. Further,
alternative proposals require far more complexity in user-space (and
still deliver less functionality than devfs). Some people have the
mantra of reducing "kernel bloat", but don't consider the effects on
user-space.

A good solution limits the total complexity of kernel-space and
user-space.


Major&minor allocation

The existing scheme requires the allocation of major and minor device
numbers for each and every device. This means that a central
co-ordinating authority is required to issue these device numbers
(unless you're developing a "private" device driver), in order to
preserve uniqueness. Devfs shifts the burden to a namespace. This may
not seem like a huge benefit, but actually it is. Since driver authors
will naturally choose a device name which reflects the functionality
of the device, there is far less potential for namespace conflict.
Solving this requires a kernel change.

/dev management

Because you currently access devices through device nodes, these must
be created by the system administrator. For standard devices you can
usually find a MAKEDEV programme which creates all these (hundreds!)
of nodes. This means that changes in the kernel must be reflected by
changes in the MAKEDEV programme, or else the system administrator
creates device nodes by hand.

The basic problem is that there are two separate databases of
major and minor numbers. One is in the kernel and one is in /dev (or
in a MAKEDEV programme, if you want to look at it that way). This is
duplication of information, which is not good practice.
Solving this requires a kernel change.

/dev growth

A typical /dev has over 1200 nodes! Most of these devices simply don't
exist because the hardware is not available. A huge /dev increases the
time to access devices (I'm just referring to the dentry lookup times
and the time taken to read inodes off disc: the next subsection shows
some more horrors).

An example of how big /dev can grow is if we consider SCSI devices:

host           6  bits  (say up to 64 hosts on a really big machine)
channel        4  bits  (say up to 16 SCSI buses per host)
id             4  bits
lun            3  bits
partition      6  bits
TOTAL          23 bits


This requires 8 Mega (1024*1024) inodes if we want to store all
possible device nodes. Even if we scrap everything but id,partition
and assume a single host adapter with a single SCSI bus and only one
logical unit per SCSI target (id), that's still 10 bits or 1024
inodes. Each VFS inode takes around 256 bytes (kernel 2.1.78), so
that's 256 kBytes of inode storage on disc (assuming real inodes take
a similar amount of space as VFS inodes). This is actually not so bad,
because disc is cheap these days. Embedded systems would care about
256 kBytes of /dev inodes, but you could argue that embedded systems
would have hand-tuned /dev directories. I've had to do just that on my
embedded systems, but I would rather just leave it to devfs.

Another issue is the time taken to lookup an inode when first
referenced. Not only does this take time in scanning through a list in
memory, but also the seek times to read the inodes off disc.
This could be solved in user-space using a clever programme which
scanned the kernel logs and deleted /dev entries which are not
available and created them when they were available. This programme
would need to be run every time a new module was loaded, which would
slow things down a lot.

There is an existing programme called scsidev which will automatically
create device nodes for SCSI devices. It can do this by scanning files
in /proc/scsi. Unfortunately, to extend this idea to other device
nodes would require significant modifications to existing drivers (so
they too would provide information in /proc). This is a non-trivial
change (I should know: devfs has had to do something similar). Once
you go to this much effort, you may as well use devfs itself (which
also provides this information).  Furthermore, such a system would
likely be implemented in an ad-hoc fashion, as different drivers will
provide their information in different ways.

Devfs is much cleaner, because it (naturally) has a uniform mechanism
to provide this information: the device nodes themselves!


Node to driver file_operations translation

There is an important difference between the way disc-based character
and block nodes and devfs entries make the connection between an entry
in /dev and the actual device driver.

With the current 8 bit major and minor numbers the connection between
disc-based c&b nodes and per-major drivers is done through a
fixed-length table of 128 entries. The various filesystem types set
the inode operations for c&b nodes to {chr,blk}dev_inode_operations,
so when a device is opened a few quick levels of indirection bring us
to the driver file_operations.

For miscellaneous character devices a second step is required: there
is a scan for the driver entry with the same minor number as the file
that was opened, and the appropriate minor open method is called. This
scanning is done *every time* you open a device node. Potentially, you
may be searching through dozens of misc. entries before you find your
open method. While not an enormous performance overhead, this does
seem pointless.

Linux *must* move beyond the 8 bit major and minor barrier,
somehow. If we simply increase each to 16 bits, then the indexing
scheme used for major driver lookup becomes untenable, because the
major tables (one each for character and block devices) would need to
be 64 k entries long (512 kBytes on x86, 1 MByte for 64 bit
systems). So we would have to use a scheme like that used for
miscellaneous character devices, which means the search time goes up
linearly with the average number of major device drivers on your
system. Not all "devices" are hardware, some are higher-level drivers
like KGI, so you can get more "devices" without adding hardware
You can improve this by creating an ordered (balanced:-)
binary tree, in which case your search time becomes log(N).
Alternatively, you can use hashing to speed up the search.
But why do that search at all if you don't have to? Once again, it
seems pointless.

Note that devfs doesn't use the major&minor system. For devfs
entries, the connection is done when you lookup the /dev entry. When
devfs_register() is called, an internal table is appended which has
the entry name and the file_operations. If the dentry cache doesn't
have the /dev entry already, this internal table is scanned to get the
file_operations, and an inode is created. If the dentry cache already
has the entry, there is *no lookup time* (other than the dentry scan
itself, but we can't avoid that anyway, and besides Linux dentries
cream other OS's which don't have them:-). Furthermore, the number of
node entries in a devfs is only the number of available device
entries, not the number of *conceivable* entries. Even if you remove
unnecessary entries in a disc-based /dev, the number of conceivable
entries remains the same: you just limit yourself in order to save
space.

Devfs provides a fast connection between a VFS node and the device
driver, in a scalable way.

/dev as a system administration tool

Right now /dev contains a list of conceivable devices, most of which I
don't have. Devfs only shows those devices available on my
system. This means that listing /dev is a handy way of checking what
devices are available.

Major&minor size

Existing major and minor numbers are limited to 8 bits each. This is
now a limiting factor for some drivers, particularly the SCSI disc
driver, which consumes a single major number. Only 16 discs are
supported, and each disc may have only 15 partitions. Maybe this isn't
a problem for you, but some of us are building huge Linux systems with
disc arrays. With devfs an arbitrary pointer can be associated with
each device entry, which can be used to give an effective 32 bit
device identifier (i.e. that's like having a 32 bit minor
number). Since this is private to the kernel, there are no C library
compatibility issues which you would have with increasing major and
minor number sizes. See the section on "Allocation of Device Numbers"
for details on maintaining compatibility with userspace.

Solving this requires a kernel change.

Since writing this, the kernel has been modified so that the SCSI disc
driver has more major numbers allocated to it and now supports up to
128 discs. Since these major numbers are non-contiguous (a result of
unplanned expansion), the implementation is a little more cumbersome
than originally.

Just like the changes to IPv4 to fix impending limitations in the
address space, people find ways around the limitations. In the long
run, however, solutions like IPv6 or devfs can't be put off forever.

Read-only root filesystem

Having your device nodes on the root filesystem means that you can't
operate properly with a read-only root filesystem. This is because you
want to change ownerships and protections of tty devices. Existing
practice prevents you using a CD-ROM as your root filesystem for a
*real* system. Sure, you can boot off a CD-ROM, but you can't change
tty ownerships, so it's only good for installing.

Also, you can't use a shared NFS root filesystem for a cluster of
discless Linux machines (having tty ownerships changed on a common
/dev is not good). Nor can you embed your root filesystem in a
ROM-FS.

You can get around this by creating a RAMDISC at boot time, making
an ext2 filesystem in it, mounting it somewhere and copying the
contents of /dev into it, then unmounting it and mounting it over
/dev.

A devfs is a cleaner way of solving this.

Non-Unix root filesystem

Non-Unix filesystems (such as NTFS) can't be used for a root
filesystem because they variously don't support character and block
special files or symbolic links. You can't have a separate disc-based
or RAMDISC-based filesystem mounted on /dev because you need device
nodes before you can mount these. Devfs can be mounted without any
device nodes. Devlinks won't work because symlinks aren't supported.
An alternative solution is to use initrd to mount a RAMDISC initial
root filesystem (which is populated with a minimal set of device
nodes), and then construct a new /dev in another RAMDISC, and finally
switch to your non-Unix root filesystem. This requires clever boot
scripts and a fragile and conceptually complex boot procedure.

Devfs solves this in a robust and conceptually simple way.

PTY security

Current pseudo-tty (pty) devices are owned by root and read-writable
by everyone. The user of a pty-pair cannot change
ownership/protections without being suid-root.

This could be solved with a secure user-space daemon which runs as
root and does the actual creation of pty-pairs. Such a daemon would
require modification to *every* programme that wants to use this new
mechanism. It also slows down creation of pty-pairs.

An alternative is to create a new open_pty() syscall which does much
the same thing as the user-space daemon. Once again, this requires
modifications to pty-handling programmes.

The devfs solution allows a device driver to "tag" certain device
files so that when an unopened device is opened, the ownerships are
changed to the current euid and egid of the opening process, and the
protections are changed to the default registered by the driver. When
the device is closed ownership is set back to root and protections are
set back to read-write for everybody. No programme need be changed.
The devpts filesystem provides this auto-ownership feature for Unix98
ptys. It doesn't support old-style pty devices, nor does it have all
the other features of devfs.

Intelligent device management

Devfs implements a simple yet powerful protocol for communication with
a device management daemon (devfsd) which runs in user space. It is
possible to send a message (either synchronously or asynchronously) to
devfsd on any event, such as registration/unregistration of device
entries, opening and closing devices, looking up inodes, scanning
directories and more. This has many possibilities. Some of these are
already implemented. See:


http://www.atnf.csiro.au/~rgooch/linux/

Device entry registration events can be used by devfsd to change
permissions of newly-created device nodes. This is one mechanism to
control device permissions.

Device entry registration/unregistration events can be used to run
programmes or scripts. This can be used to provide automatic mounting
of filesystems when a new block device media is inserted into the
drive.

Asynchronous device open and close events can be used to implement
clever permissions management. For example, the default permissions on
/dev/dsp do not allow everybody to read from the device. This is
sensible, as you don't want some remote user recording what you say at
your console. However, the console user is also prevented from
recording. This behaviour is not desirable. With asynchronous device
open and close events, you can have devfsd run a programme or script
when console devices are opened to change the ownerships for *other*
device nodes (such as /dev/dsp). On closure, you can run a different
script to restore permissions. An advantage of this scheme over
modifying the C library tty handling is that this works even if your
programme crashes (how many times have you seen the utmp database with
lingering entries for non-existent logins?).

Synchronous device open events can be used to perform intelligent
device access protections. Before the device driver open() method is
called, the daemon must first validate the open attempt, by running an
external programme or script. This is far more flexible than access
control lists, as access can be determined on the basis of other
system conditions instead of just the UID and GID.

Inode lookup events can be used to authenticate module autoload
requests. Instead of using kmod directly, the event is sent to
devfsd which can implement an arbitrary authentication before loading
the module itself.

Inode lookup events can also be used to construct arbitrary
namespaces, without having to resort to populating devfs with symlinks
to devices that don't exist.

Speculative Device Scanning

Consider an application (like cdparanoia) that wants to find all
CD-ROM devices on the system (SCSI, IDE and other types), whether or
not their respective modules are loaded. The application must
speculatively open certain device nodes (such as /dev/sr0 for the SCSI
CD-ROMs) in order to make sure the module is loaded. This requires
that all Linux distributions follow the standard device naming scheme
(last time I looked RedHat did things differently). Devfs solves the
naming problem.

The same application also wants to see which devices are actually
available on the system. With the existing system it needs to read the
/dev directory and speculatively open each /dev/sr* device to
determine if the device exists or not. With a large /dev this is an
inefficient operation, especially if there are many /dev/sr* nodes. A
solution like scsidev could reduce the number of /dev/sr* entries (but
of course that also requires all that inefficient directory scanning).

With devfs, the application can open the /dev/sr directory
(which triggers the module autoloading if required), and proceed to
read /dev/sr. Since only the available devices will have
entries, there are no inefficencies in directory scanning or device
openings.

-----------------------------------------------------------------------------

Who else does it?

FreeBSD has a devfs implementation. Solaris and AIX each have a
pseudo-devfs (something akin to scsidev but for all devices, with some
unspecified kernel support). BeOS, Plan9 and QNX also have it. SGI's
IRIX 6.4 and above also have a device filesystem.

While we shouldn't just automatically do something because others do
it, we should not ignore the work of others either. FreeBSD has a lot
of competent people working on it, so their opinion should not be
blithely ignored.

-----------------------------------------------------------------------------


How it works

Registering device entries

For every entry (device node) in a devfs-based /dev a driver must call
devfs_register(). This adds the name of the device entry, the
file_operations structure pointer and a few other things to an
internal table. Device entries may be added and removed at any
time. When a device entry is registered, it automagically appears in
any mounted devfs'.

Inode lookup

When a lookup operation on an entry is performed and if there is no
driver information for that entry devfs will attempt to call
devfsd. If still no driver information can be found then a negative
dentry is yielded and the next stage operation will be called by the
VFS (such as create() or mknod() inode methods). If driver information
can be found, an inode is created (if one does not exist already) and
all is well.

Manually creating device nodes

The mknod() method allows you to create an ordinary named pipe in the
devfs, or you can create a character or block special inode if one
does not already exist. You may wish to create a character or block
special inode so that you can set permissions and ownership. Later, if
a device driver registers an entry with the same name, the
permissions, ownership and times are retained. This is how you can set
the protections on a device even before the driver is loaded. Once you
create an inode it appears in the directory listing.

Unregistering device entries

A device driver calls devfs_unregister() to unregister an entry.

Chroot() gaols

2.2.x kernels

The semantics of inode creation are different when devfs is mounted
with the "explicit" option. Now, when a device entry is registered, it
will not appear until you use mknod() to create the device. It doesn't
matter if you mknod() before or after the device is registered with
devfs_register(). The purpose of this behaviour is to support
chroot(2) gaols, where you want to mount a minimal devfs inside the
gaol. Only the devices you specifically want to be available (through
your mknod() setup) will be accessible.

2.4.x kernels

As of kernel 2.3.99, the VFS has had the ability to rebind parts of
the global filesystem namespace into another part of the namespace.
This now works even at the leaf-node level, which means that
individual files and device nodes may be bound into other parts of the
namespace. This is like making links, but better, because it works
across filesystems (unlike hard links) and works through chroot()
gaols (unlike symbolic links).

Because of these improvements to the VFS, the multi-mount capability
in devfs is no longer needed. The administrator may create a minimal
device tree inside a chroot(2) gaol by using VFS bindings. As this
provides most of the features of the devfs multi-mount capability, I
removed the multi-mount support code (after issuing an RFC). This
yielded code size reductions and simplifications.

If you want to construct a minimal chroot() gaol, the following
command should suffice:

mount --bind /dev/null /gaol/dev/null


Repeat for other device nodes you want to expose. Simple!

-----------------------------------------------------------------------------


Operational issues


Instructions for the impatient

Nobody likes reading documentation. People just want to get in there
and play. So this section tells you quickly the steps you need to take
to run with devfs mounted over /dev. Skip these steps and you will end
up with a nearly unbootable system. Subsequent sections describe the
issues in more detail, and discuss non-essential configuration
options.

Devfsd
OK, if you're reading this, I assume you want to play with
devfs. First you should ensure that /usr/src/linux contains a
recent kernel source tree. Then you need to compile devfsd, the device
management daemon, available at

http://www.atnf.csiro.au/~rgooch/linux/.
Because the kernel has a naming scheme
which is quite different from the old naming scheme, you need to
install devfsd so that software and configuration files that use the
old naming scheme will not break.

Compile and install devfsd. You will be provided with a default
configuration file /etc/devfsd.conf which will provide
compatibility symlinks for the old naming scheme. Don't change this
config file unless you know what you're doing. Even if you think you
do know what you're doing, don't change it until you've followed all
the steps below and booted a devfs-enabled system and verified that it
works.

Now edit your main system boot script so that devfsd is started at the
very beginning (before any filesystem
checks). /etc/rc.d/rc.sysinit is often the main boot script
on systems with SysV-style boot scripts. On systems with BSD-style
boot scripts it is often /etc/rc. Also check
/sbin/rc.

NOTE that the line you put into the boot
script should be exactly:

/sbin/devfsd /dev

DO NOT use some special daemon-launching
programme, otherwise the boot script may not wait for devfsd to finish
initialising.

System Libraries
There may still be some problems because of broken software making
assumptions about device names. In particular, some software does not
handle devices which are symbolic links. If you are running a libc 5
based system, install libc 5.4.44 (if you have libc 5.4.46, go back to
libc 5.4.44, which is actually correct). If you are running a glibc
based system, make sure you have glibc 2.1.3 or later.

/etc/securetty
PAM (Pluggable Authentication Modules) is supposed to be a flexible
mechanism for providing better user authentication and access to
services. Unfortunately, it's also fragile, complex and undocumented
(check out RedHat 6.1, and probably other distributions as well). PAM
has problems with symbolic links. Append the following lines to your
/etc/securetty file:

vc/1
vc/2
vc/3
vc/4
vc/5
vc/6
vc/7
vc/8

This will not weaken security. If you have a version of util-linux
earlier than 2.10.h, please upgrade to 2.10.h or later. If you
absolutely cannot upgrade, then also append the following lines to
your /etc/securetty file:

1
2
3
4
5
6
7
8

This may potentially weaken security by allowing root logins over the
network (a password is still required, though). However, since there
are problems with dealing with symlinks, I'm suspicious of the level
of security offered in any case.

XFree86
While not essential, it's probably a good idea to upgrade to XFree86
4.0, as patches went in to make it more devfs-friendly. If you don't,
you'll probably need to apply the following patch to
/etc/security/console.perms so that ordinary users can run
startx. Note that not all distributions have this file (e.g. Debian),
so if it's not present, don't worry about it.

--- /etc/security/console.perms.orig    Sat Apr 17 16:26:47 1999 
+++ /etc/security/console.perms Fri Feb 25 23:53:55 2000 
@@ -14,7 +14,7 @@ 
 # man 5 console.perms 

 # file classes -- these are regular expressions 
-<console>=tty[0-9][0-9]* :[0-9]\.[0-9] :[0-9] 
+<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9] 

 # device classes -- these are shell-style globs 
 <floppy>=/dev/fd[0-1]* 

If the patch does not apply, then change the line:

<console>=tty[0-9][0-9]* :[0-9]\.[0-9] :[0-9]

with:

<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9]


Disable devpts
I've had a report of devpts mounted on /dev/pts not working
correctly. Since devfs will also manage /dev/pts, there is no
need to mount devpts as well. You should either edit your
/etc/fstab so devpts is not mounted, or disable devpts from
your kernel configuration.

Unsupported drivers
Not all drivers have devfs support. If you depend on one of these
drivers, you will need to create a script or tarfile that you can use
at boot time to create device nodes as appropriate. There is a
section which describes this. Another
section lists the drivers which have
devfs support.

/dev/mouse

Many disributions configure /dev/mouse to be the mouse device
for XFree86 and GPM. I actually think this is a bad idea, because it
adds another level of indirection. When looking at a config file, if
you see /dev/mouse you're left wondering which mouse
is being referred to. Hence I recommend putting the actual mouse
device (for example /dev/psaux) into your
/etc/X11/XF86Config file (and similarly for the GPM
configuration file).

Alternatively, use the same technique used for unsupported drivers
described above.

The Kernel
Finally, you need to make sure devfs is compiled into your kernel. Set
CONFIG_EXPERIMENTAL=y, CONFIG_DEVFS_FS=y and CONFIG_DEVFS_MOUNT=y by
using favourite configuration tool (i.e. make config or
make xconfig) and then make clean and then recompile your kernel and 
modules. At boot, devfs will be mounted onto /dev.

If you encounter problems booting (for example if you forgot a
configuration step), you can pass devfs=nomount at the kernel
boot command line. This will prevent the kernel from mounting devfs at
boot time onto /dev.

In general, a kernel built with CONFIG_DEVFS_FS=y but without mounting
devfs onto /dev is completely safe, and requires no
configuration changes. One exception to take note of is when
LABEL= directives are used in /etc/fstab. In this
case you will be unable to boot properly. This is because the
mount(8) programme uses /proc/partitions as part of
the volume label search process, and the device names it finds are not
available, because setting CONFIG_DEVFS_FS=y changes the names in
/proc/partitions, irrespective of whether devfs is mounted.

Now you've finished all the steps required. You're now ready to boot
your shiny new kernel. Enjoy.

Changing the configuration

OK, you've now booted a devfs-enabled system, and everything works.
Now you may feel like changing the configuration (common targets are
/etc/fstab and /etc/devfsd.conf). Since you have a
system that works, if you make any changes and it doesn't work, you
now know that you only have to restore your configuration files to the
default and it will work again.


Permissions persistence across reboots

If you don't use mknod(2) to create a device file, nor use chmod(2) or
chown(2) to change the ownerships/permissions, the inode ctime will
remain at 0 (the epoch, 12 am, 1-JAN-1970, GMT). Anything with a ctime
later than this has had it's ownership/permissions changed. Hence, a
simple script or programme may be used to tar up all changed inodes,
prior to shutdown. Although effective, many consider this approach a
kludge.

A much better approach is to use devfsd to save and restore
permissions. It may be configured to record changes in permissions and
will save them in a database (in fact a directory tree), and restore
these upon boot. This is an efficient method and results in immediate
saving of current permissions (unlike the tar approach, which saves
permissions at some unspecified future time).

The default configuration file supplied with devfsd has config entries
which you may uncomment to enable persistence management.

If you decide to use the tar approach anyway, be aware that tar will
first unlink(2) an inode before creating a new device node. The
unlink(2) has the effect of breaking the connection between a devfs
entry and the device driver. If you use the "devfs=only" boot option,
you lose access to the device driver, requiring you to reload the
module. I consider this a bug in tar (there is no real need to
unlink(2) the inode first).

Alternatively, you can use devfsd to provide more sophisticated
management of device permissions. You can use devfsd to store
permissions for whole groups of devices with a single configuration
entry, rather than the conventional single entry per device entry.

Permissions database stored in mounted-over /dev

If you wish to save and restore your device permissions into the
disc-based /dev while still mounting devfs onto /dev
you may do so. This requires a 2.4.x kernel (in fact, 2.3.99 or
later), which has the VFS binding facility. You need to do the
following to set this up:



make sure the kernel does not mount devfs at boot time


make sure you have a correct /dev/console entry in your
root file-system (where your disc-based /dev lives)

create the /dev-state directory


add the following lines near the very beginning of your boot
scripts:

mount --bind /dev /dev-state
mount -t devfs none /dev
devfsd /dev




add the following lines to your /etc/devfsd.conf file:

REGISTER	^pt[sy]		IGNORE
CREATE		^pt[sy]		IGNORE
CHANGE		^pt[sy]		IGNORE
DELETE		^pt[sy]		IGNORE
REGISTER	.*		COPY	/dev-state/$devname $devpath
CREATE		.*		COPY	$devpath /dev-state/$devname
CHANGE		.*		COPY	$devpath /dev-state/$devname
DELETE		.*		CFUNCTION GLOBAL unlink /dev-state/$devname
RESTORE		/dev-state

Note that the sample devfsd.conf file contains these lines,
as well as other sample configurations you may find useful. See the
devfsd distribution


reboot.




Permissions database stored in normal directory

If you are using an older kernel which doesn't support VFS binding,
then you won't be able to have the permissions database in a
mounted-over /dev. However, you can still use a regular
directory to store the database. The sample /etc/devfsd.conf
file above may still be used. You will need to create the
/dev-state directory prior to installing devfsd. If you have
old permissions in /dev, then just copy (or move) the device
nodes over to the new directory.

Which method is better?

The best method is to have the permissions database stored in the
mounted-over /dev. This is because you will not need to copy
device nodes over to /dev-state, and because it allows you to
switch between devfs and non-devfs kernels, without requiring you to
copy permissions between /dev-state (for devfs) and
/dev (for non-devfs).


Dealing with drivers without devfs support

Currently, not all device drivers in the kernel have been modified to
use devfs. Device drivers which do not yet have devfs support will not
automagically appear in devfs. The simplest way to create device nodes
for these drivers is to unpack a tarfile containing the required
device nodes. You can do this in your boot scripts. All your drivers
will now work as before.

Hopefully for most people devfs will have enough support so that they
can mount devfs directly over /dev without losing most functionality
(i.e. losing access to various devices). As of 22-JAN-1998 (devfs
patch version 10) I am now running this way. All the devices I have
are available in devfs, so I don't lose anything.

WARNING: if your configuration requires the old-style device names
(i.e. /dev/hda1 or /dev/sda1), you must install devfsd and configure
it to maintain compatibility entries. It is almost certain that you
will require this. Note that the kernel creates a compatibility entry
for the root device, so you don't need initrd.

Note that you no longer need to mount devpts if you use Unix98 PTYs,
as devfs can manage /dev/pts itself. This saves you some RAM, as you
don't need to compile and install devpts. Note that some versions of
glibc have a bug with Unix98 pty handling on devfs systems. Contact
the glibc maintainers for a fix. Glibc 2.1.3 has the fix.

Note also that apart from editing /etc/fstab, other things will need
to be changed if you *don't* install devfsd. Some software (like the X
server) hard-wire device names in their source. It really is much
easier to install devfsd so that compatibility entries are created.
You can then slowly migrate your system to using the new device names
(for example, by starting with /etc/fstab), and then limiting the
compatibility entries that devfsd creates.

IF YOU CONFIGURE TO MOUNT DEVFS AT BOOT, MAKE SURE YOU INSTALL DEVFSD
BEFORE YOU BOOT A DEVFS-ENABLED KERNEL!

Now that devfs has gone into the 2.3.46 kernel, I'm getting a lot of
reports back. Many of these are because people are trying to run
without devfsd, and hence some things break. Please just run devfsd if
things break. I want to concentrate on real bugs rather than
misconfiguration problems at the moment. If people are willing to fix
bugs/false assumptions in other code (i.e. glibc, X server) and submit
that to the respective maintainers, that would be great.


All the way with Devfs

The devfs kernel patch creates a rationalised device tree. As stated
above, if you want to keep using the old /dev naming scheme,
you just need to configure devfsd appopriately (see the man
page). People who prefer the old names can ignore this section. For
those of us who like the rationalised names and an uncluttered
/dev, read on.

If you don't run devfsd, or don't enable compatibility entry
management, then you will have to configure your system to use the new
names. For example, you will then need to edit your
/etc/fstab to use the new disc naming scheme. If you want to
be able to boot non-devfs kernels, you will need compatibility
symlinks in the underlying disc-based /dev pointing back to
the old-style names for when you boot a kernel without devfs.

You can selectively decide which devices you want compatibility
entries for. For example, you may only want compatibility entries for
BSD pseudo-terminal devices (otherwise you'll have to patch you C
library or use Unix98 ptys instead). It's just a matter of putting in
the correct regular expression into /dev/devfsd.conf.

There are other choices of naming schemes that you may prefer. For
example, I don't use the kernel-supplied
names, because they are too verbose. A common misconception is
that the kernel-supplied names are meant to be used directly in
configuration files. This is not the case. They are designed to
reflect the layout of the devices attached and to provide easy
classification.

If you like the kernel-supplied names, that's fine. If you don't then
you should be using devfsd to construct a namespace more to your
liking. Devfsd has built-in code to construct a
namespace that is both logical and easy to
manage. In essence, it creates a convenient abbreviation of the
kernel-supplied namespace.

You are of course free to build your own namespace. Devfsd has all the
infrastructure required to make this easy for you. All you need do is
write a script. You can even write some C code and devfsd can load the
shared object as a callable extension.


Other Issues

The init programme
Another thing to take note of is whether your init programme
creates a Unix socket /dev/telinit. Some versions of init
create /dev/telinit so that the telinit programme can
communicate with the init process. If you have such a system you need
to make sure that devfs is mounted over /dev *before* init
starts. In other words, you can't leave the mounting of devfs to
/etc/rc, since this is executed after init. Other
versions of init require a named pipe /dev/initctl
which must exist *before* init starts. Once again, you need to
mount devfs and then create the named pipe *before* init
starts.

The default behaviour now is not to mount devfs onto /dev at
boot time for 2.3.x and later kernels. You can correct this with the
"devfs=mount" boot option. This solves any problems with init,
and also prevents the dreaded:

Cannot open initial console

message. For 2.2.x kernels where you need to apply the devfs patch,
the default is to mount.

If you have automatic mounting of devfs onto /dev then you
may need to create /dev/initctl in your boot scripts. The
following lines should suffice:

mknod /dev/initctl p
kill -SIGUSR1 1       # tell init that /dev/initctl now exists

Alternatively, if you don't want the kernel to mount devfs onto
/dev then you could use the following procedure is a
guideline for how to get around /dev/initctl problems:

# cd /sbin
# mv init init.real
# cat > init
#! /bin/sh
mount -n -t devfs none /dev
mknod /dev/initctl p
exec /sbin/init.real $*
[control-D]
# chmod a+x init

Note that newer versions of init create /dev/initctl
automatically, so you don't have to worry about this.

Module autoloading
You will need to configure devfsd to enable module
autoloading. The following lines should be placed in your
/etc/devfsd.conf file:

LOOKUP	.*		MODLOAD


As of devfsd-v1.3.10, a generic /etc/modules.devfs
configuration file is installed, which is used by the MODLOAD
action. This should be sufficient for most configurations. If you
require further configuration, edit your /etc/modules.conf
file. The way module autoloading work with devfs is:


a process attempts to lookup a device node (e.g. /dev/fred)


if that device node does not exist, the full pathname is passed to
devfsd as a string


devfsd will pass the string to the modprobe programme (provided the
configuration line shown above is present), and specifies that
/etc/modules.devfs is the configuration file


/etc/modules.devfs includes /etc/modules.conf to
access local configurations

modprobe will search it's configuration files, looking for an alias
that translates the pathname into a module name


the translated pathname is then used to load the module.


If you wanted a lookup of /dev/fred to load the
mymod module, you would require the following configuration
line in /etc/modules.conf:

alias    /dev/fred    mymod

The /etc/modules.devfs configuration file provides many such
aliases for standard device names. If you look closely at this file,
you will note that some modules require multiple alias configuration
lines. This is required to support module autoloading for old and new
device names.

Mounting root off a devfs device
If you wish to mount root off a devfs device when you pass the
"devfs=only" boot option, then you need to pass in the
"root=<device>" option to the kernel when booting. If you use
LILO, then you must have this in lilo.conf:

append = "root=<device>"

Surprised? Yep, so was I. It turns out if you have (as most people
do):

root = <device>


then LILO will determine the device number of <device> and will
write that device number into a special place in the kernel image
before starting the kernel, and the kernel will use that device number
to mount the root filesystem. So, using the "append" variety ensures
that LILO passes the root filesystem device as a string, which devfs
can then use.

Note that this isn't an issue if you don't pass "devfs=only".

TTY issues
The ttyname(3) function in some versions of the C library makes
false assumptions about device entries which are symbolic links.  The
tty(1) programme is one that depends on this function.  I've
written a patch to libc 5.4.43 which fixes this. This has been
included in libc 5.4.44 and a similar fix is in glibc 2.1.3.


Kernel Naming Scheme

The kernel provides a default naming scheme. This scheme is designed
to make it easy to search for specific devices or device types, and to
view the available devices. Some device types (such as hard discs),
have a directory of entries, making it easy to see what devices of
that class are available. Often, the entries are symbolic links into a
directory tree that reflects the topology of available devices. The
topological tree is useful for finding how your devices are arranged.

Below is a list of the naming schemes for the most common drivers. A
list of reserved device names is
available for reference. Please send email to
rgooch@atnf.csiro.au to obtain an allocation. Please be
patient (the maintainer is busy). An alternative name may be allocated
instead of the requested name, at the discretion of the maintainer.

Disc Devices

All discs, whether SCSI, IDE or whatever, are placed under the
/dev/discs hierarchy:

	/dev/discs/disc0	first disc
	/dev/discs/disc1	second disc


Each of these entries is a symbolic link to the directory for that
device. The device directory contains:

	disc	for the whole disc
	part*	for individual partitions


CD-ROM Devices

All CD-ROMs, whether SCSI, IDE or whatever, are placed under the
/dev/cdroms hierarchy:

	/dev/cdroms/cdrom0	first CD-ROM
	/dev/cdroms/cdrom1	second CD-ROM


Each of these entries is a symbolic link to the real device entry for
that device.

Tape Devices

All tapes, whether SCSI, IDE or whatever, are placed under the
/dev/tapes hierarchy:

	/dev/tapes/tape0	first tape
	/dev/tapes/tape1	second tape


Each of these entries is a symbolic link to the directory for that
device. The device directory contains:

	mt			for mode 0
	mtl			for mode 1
	mtm			for mode 2
	mta			for mode 3
	mtn			for mode 0, no rewind
	mtln			for mode 1, no rewind
	mtmn			for mode 2, no rewind
	mtan			for mode 3, no rewind


SCSI Devices

To uniquely identify any SCSI device requires the following
information:

  controller	(host adapter)
  bus		(SCSI channel)
  target	(SCSI ID)
  unit		(Logical Unit Number)


All SCSI devices are placed under /dev/scsi (assuming devfs
is mounted on /dev). Hence, a SCSI device with the following
parameters: c=1,b=2,t=3,u=4 would appear as:

	/dev/scsi/host1/bus2/target3/lun4	device directory


Inside this directory, a number of device entries may be created,
depending on which SCSI device-type drivers were installed.

See the section on the disc naming scheme to see what entries the SCSI
disc driver creates.

See the section on the tape naming scheme to see what entries the SCSI
tape driver creates.

The SCSI CD-ROM driver creates:

	cd


The SCSI generic driver creates:

	generic


IDE Devices

To uniquely identify any IDE device requires the following
information:

  controller
  bus		(aka. primary/secondary)
  target	(aka. master/slave)
  unit


All IDE devices are placed under /dev/ide, and uses a similar
naming scheme to the SCSI subsystem.

XT Hard Discs

All XT discs are placed under /dev/xd. The first XT disc has
the directory /dev/xd/disc0.

TTY devices

The tty devices now appear as:

  New name                   Old-name                   Device Type
  --------                   --------                   -----------
  /dev/tts/{0,1,...}         /dev/ttyS{0,1,...}         Serial ports
  /dev/cua/{0,1,...}         /dev/cua{0,1,...}          Call out devices
  /dev/vc/0                  /dev/tty                   Current virtual console
  /dev/vc/{1,2,...}          /dev/tty{1...63}           Virtual consoles
  /dev/vcc/{0,1,...}         /dev/vcs{1...63}           Virtual consoles
  /dev/pty/m{0,1,...}        /dev/ptyp??                PTY masters
  /dev/pty/s{0,1,...}        /dev/ttyp??                PTY slaves


RAMDISCS

The RAMDISCS are placed in their own directory, and are named thus:

  /dev/rd/{0,1,2,...}


Meta Devices

The meta devices are placed in their own directory, and are named
thus:

  /dev/md/{0,1,2,...}


Floppy discs

Floppy discs are placed in the /dev/floppy directory.

Loop devices

Loop devices are placed in the /dev/loop directory.

Sound devices

Sound devices are placed in the /dev/sound directory
(audio, sequencer, ...).


Devfsd Naming Scheme

Devfsd provides a naming scheme which is a convenient abbreviation of
the kernel-supplied namespace. In some
cases, the kernel-supplied naming scheme is quite convenient, so
devfsd does not provide another naming scheme. The convenience names
that devfsd creates are in fact the same names as the original devfs
kernel patch created (before Linus mandated the Big Name
Change). These are referred to as "new compatibility entries".

In order to configure devfsd to create these convenience names, the
following lines should be placed in your /etc/devfsd.conf:

REGISTER	.*		MKNEWCOMPAT
UNREGISTER	.*		RMNEWCOMPAT

This will cause devfsd to create (and destroy) symbolic links which
point to the kernel-supplied names.

SCSI Hard Discs

All SCSI discs are placed under /dev/sd (assuming devfs is
mounted on /dev). Hence, a SCSI disc with the following
parameters: c=1,b=2,t=3,u=4 would appear as:

	/dev/sd/c1b2t3u4	for the whole disc
	/dev/sd/c1b2t3u4p5	for the 5th partition
	/dev/sd/c1b2t3u4p5s6	for the 6th slice in the 5th partition


SCSI Tapes

All SCSI tapes are placed under /dev/st. A similar naming
scheme is used as for SCSI discs. A SCSI tape with the
parameters:c=1,b=2,t=3,u=4 would appear as:

	/dev/st/c1b2t3u4m0	for mode 0
	/dev/st/c1b2t3u4m1	for mode 1
	/dev/st/c1b2t3u4m2	for mode 2
	/dev/st/c1b2t3u4m3	for mode 3
	/dev/st/c1b2t3u4m0n	for mode 0, no rewind
	/dev/st/c1b2t3u4m1n	for mode 1, no rewind
	/dev/st/c1b2t3u4m2n	for mode 2, no rewind
	/dev/st/c1b2t3u4m3n	for mode 3, no rewind


SCSI CD-ROMs

All SCSI CD-ROMs are placed under /dev/sr. A similar naming
scheme is used as for SCSI discs. A SCSI CD-ROM with the
parameters:c=1,b=2,t=3,u=4 would appear as:

	/dev/sr/c1b2t3u4


SCSI Generic Devices

The generic (aka. raw) interface for all SCSI devices are placed under
/dev/sg. A similar naming scheme is used as for SCSI discs. A
SCSI generic device with the parameters:c=1,b=2,t=3,u=4 would appear
as:

	/dev/sg/c1b2t3u4


IDE Hard Discs

All IDE discs are placed under /dev/ide/hd, using a similar
convention to SCSI discs. The following mappings exist between the new
and the old names:

	/dev/hda	/dev/ide/hd/c0b0t0u0
	/dev/hdb	/dev/ide/hd/c0b0t1u0
	/dev/hdc	/dev/ide/hd/c0b1t0u0
	/dev/hdd	/dev/ide/hd/c0b1t1u0


IDE Tapes

A similar naming scheme is used as for IDE discs. The entries will
appear in the /dev/ide/mt directory.

IDE CD-ROM

A similar naming scheme is used as for IDE discs. The entries will
appear in the /dev/ide/cd directory.

IDE Floppies

A similar naming scheme is used as for IDE discs. The entries will
appear in the /dev/ide/fd directory.

XT Hard Discs

All XT discs are placed under /dev/xd. The first XT disc
would appear as /dev/xd/c0t0.


Old Compatibility Names

The old compatibility names are the legacy device names, such as
/dev/hda, /dev/sda, /dev/rtc and so on.
Devfsd can be configured to create compatibility symlinks so that you
may continue to use the old names in your configuration files and so
that old applications will continue to function correctly.

In order to configure devfsd to create these legacy names, the
following lines should be placed in your /etc/devfsd.conf:

REGISTER	.*		MKOLDCOMPAT
UNREGISTER	.*		RMOLDCOMPAT

This will cause devfsd to create (and destroy) symbolic links which
point to the kernel-supplied names.


-----------------------------------------------------------------------------


Device drivers currently ported

- All miscellaneous character devices support devfs (this is done
  transparently through misc_register())

- SCSI discs and generic hard discs

- Character memory devices (null, zero, full and so on)
  Thanks to C. Scott Ananian <cananian@alumni.princeton.edu>

- Loop devices (/dev/loop?)
 
- TTY devices (console, serial ports, terminals and pseudo-terminals)
  Thanks to C. Scott Ananian <cananian@alumni.princeton.edu>

- SCSI tapes (/dev/scsi and /dev/tapes)

- SCSI CD-ROMs (/dev/scsi and /dev/cdroms)

- SCSI generic devices (/dev/scsi)

- RAMDISCS (/dev/ram?)

- Meta Devices (/dev/md*)

- Floppy discs (/dev/floppy)

- Parallel port printers (/dev/printers)

- Sound devices (/dev/sound)
  Thanks to Eric Dumas <dumas@linux.eu.org> and
  C. Scott Ananian <cananian@alumni.princeton.edu>

- Joysticks (/dev/joysticks)

- Sparc keyboard (/dev/kbd)

- DSP56001 digital signal processor (/dev/dsp56k)

- Apple Desktop Bus (/dev/adb)

- Coda network file system (/dev/cfs*)

- Virtual console capture devices (/dev/vcc)
  Thanks to Dennis Hou <smilax@mindmeld.yi.org>

- Frame buffer devices (/dev/fb)

- Video capture devices (/dev/v4l)


-----------------------------------------------------------------------------


Allocation of Device Numbers

Devfs allows you to write a driver which doesn't need to allocate a
device number (major&minor numbers) for the internal operation of the
kernel. However, there are a number of userspace programmes that use
the device number as a unique handle for a device. An example is the
find programme, which uses device numbers to determine whether
an inode is on a different filesystem than another inode. The device
number used is the one for the block device which a filesystem is
using. To preserve compatibility with userspace programmes, block
devices using devfs need to have unique device numbers allocated to
them. Furthermore, POSIX specifies device numbers, so some kind of
device number needs to be presented to userspace.

The simplest option (especially when porting drivers to devfs) is to
keep using the old major and minor numbers. Devfs will take whatever
values are given for major&minor and pass them onto userspace.

This device number is a 16 bit number, so this leaves plenty of space
for large numbers of discs and partitions. This scheme can also be
used for character devices, in particular the tty devices, which are
currently limited to 256 pseudo-ttys (this limits the total number of
simultaneous xterms and remote logins).  Note that the device number
is limited to the range 36864-61439 (majors 144-239), in order to
avoid any possible conflicts with existing official allocations.

Please note that using dynamically allocated block device numbers may
break the NFS daemons (both user and kernel mode), which expect dev_t
for a given device to be constant over the lifetime of remote mounts.

A final note on this scheme: since it doesn't increase the size of
device numbers, there are no compatibility issues with userspace.

-----------------------------------------------------------------------------


Questions and Answers


Making things work
Alternatives to devfs
What I don't like about devfs
How to report bugs
Strange kernel messages
Compilation problems with devfsd



Making things work

Here are some common questions and answers.



Devfsd doesn't start

Make sure you have compiled and installed devfsd
Make sure devfsd is being started from your boot
scripts
Make sure you have configured your kernel to enable devfs (see
below)
Make sure devfs is mounted (see below)


Devfsd is not managing all my permissions

Make sure you are capturing the appropriate events. For example,
device entries created by the kernel generate REGISTER events,
but those created by devfsd generate CREATE events.


Devfsd is not capturing all REGISTER events

See the previous entry: you may need to capture CREATE events.


X will not start

Make sure you followed the steps 
outlined above.


Why don't my network devices appear in devfs?

This is not a bug. Network devices have their own, completely separate
namespace. They are accessed via socket(2) and
setsockopt(2) calls, and thus require no device nodes. I have
raised the possibilty of moving network devices into the device
namespace, but have had no response.


How can I test if I have devfs compiled into my kernel?

All filesystems built-in or currently loaded are listed in
/proc/filesystems. If you see a devfs entry, then
you know that devfs was compiled into your kernel. If you have
correctly configured and rebuilt your kernel, then devfs will be
built-in. If you think you've configured it in, but
/proc/filesystems doesn't show it, you've made a mistake.
Common mistakes include:

Using a 2.2.x kernel without applying the devfs patch (if you
don't know how to patch your kernel, use 2.4.x instead, don't bother
asking me how to patch)
Forgetting to set CONFIG_EXPERIMENTAL=y
Forgetting to set CONFIG_DEVFS_FS=y
Forgetting to set CONFIG_DEVFS_MOUNT=y (if you want devfs
to be automatically mounted at boot)
Editing your .config manually, instead of using make
config or make xconfig
Forgetting to run make dep; make clean after changing the
configuration and before compiling
Forgetting to compile your kernel and modules
Forgetting to install your kernel
Forgetting to install your modules

Please check twice that you've done all these steps before sending in
a bug report.



How can I test if devfs is mounted on /dev?

The device filesystem will always create an entry called
".devfsd", which is used to communicate with the daemon. Even
if the daemon is not running, this entry will exist. Testing for the
existence of this entry is the approved method of determining if devfs
is mounted or not. Note that the type of entry (i.e. regular file,
character device, named pipe, etc.) may change without notice. Only
the existence of the entry should be relied upon.


When I start devfsd, I see the error:
Error opening file: ".devfsd"   No such file or directory?

This means that devfs is not mounted. Make sure you have devfs mounted.


How do I mount devfs?

First make sure you have devfs compiled into your kernel (see
above). Then you will either need to:

set CONFIG_DEVFS_MOUNT=y in your kernel config
pass devfs=mount to your boot loader
mount devfs manually in your boot scripts with:
mount -t none devfs /dev



Mount by volume LABEL=<label> doesn't work with
devfs

Most probably you are not mounting devfs onto /dev. What
happens is that if your kernel config has CONFIG_DEVFS_FS=y
then the contents of /proc/partitions will have the devfs
names (such as scsi/host0/bus0/target0/lun0/part1). The
contents of /proc/partitions are used by mount(8) when
mounting by volume label. If devfs is not mounted on /dev,
then mount(8) will fail to find devices. The solution is to
make sure that devfs is mounted on /dev. See above for how to
do that.


I have extra or incorrect entries in /dev

You may have stale entries in your dev-state area. Check for a
RESTORE configuration line in your devfsd configuration
(typically /etc/devfsd.conf). If you have this line, check
the contents of the specified directory for stale entries. Remove
any entries which are incorrect, then reboot.


I get "Unable to open initial console" messages at boot

This usually happens when you don't have devfs automounted onto
/dev at boot time, and there is no valid
/dev/console entry on your root file-system. Create a valid
/dev/console device node.





Alternatives to devfs

I've attempted to collate all the anti-devfs proposals and explain
their limitations. Under construction.


Why not just pass device create/remove events to a daemon?

Here the suggestion is to develop an API in the kernel so that devices
can register create and remove events, and a daemon listens for those
events. The daemon would then populate/depopulate /dev (which
resides on disc).

This has several limitations:


it only works for modules loaded and unloaded (or devices inserted
and removed) after the kernel has finished booting. Without a database
of events, there is no way the daemon could fully populate
/dev


if you add a database to this scheme, the question is then how to
present that database to user-space. If you make it a list of strings
with embedded event codes which are passed through a pipe to the
daemon, then this is only of use to the daemon. I would argue that the
natural way to present this data is via a filesystem (since many of
the events will be of a hierarchical nature), such as devfs.
Presenting the data as a filesystem makes it easy for the user to see
what is available and also makes it easy to write scripts to scan the
"database"


the tight binding between device nodes and drivers is no longer
possible (requiring the otherwise perfectly avoidable
table lookups)


you cannot catch inode lookup events on /dev which means
that module autoloading requires device nodes to be created. This is a
problem, particularly for drivers where only a few inodes are created
from a potentially large set


this technique can't be used when the root FS is mounted
read-only




Just implement a better scsidev

This suggestion involves taking the scsidev programme and
extending it to scan for all devices, not just SCSI devices. The
scsidev programme works by scanning /proc/scsi

Problems:


the kernel does not currently provide a list of all devices
available. Not all drivers register entries in /proc or
generate kernel messages


there is no uniform mechanism to register devices other than the
devfs API


implementing such an API is then the same as the
proposal above




Put /dev on a ramdisc

This suggestion involves creating a ramdisc and populating it with
device nodes and then mounting it over /dev.

Problems:



this doesn't help when mounting the root filesystem, since you
still need a device node to do that


if you want to use this technique for the root device node as
well, you need to use initrd. This complicates the booting sequence
and makes it significantly harder to administer and configure. The
initrd is essentially opaque, robbing the system administrator of easy
configuration


insufficient information is available to correctly populate the
ramdisc. So we come back to the
proposal above to "solve" this


a ramdisc-based solution would take more kernel memory, since the
backing store would be (at best) normal VFS inodes and dentries, which
take 284 bytes and 112 bytes, respectively, for each entry. Compare
that to 72 bytes for devfs




Do nothing: there's no problem

Sometimes people can be heard to claim that the existing scheme is
fine. This is what they're ignoring:


device number size (8 bits each for major and minor) is a real
limitation, and must be fixed somehow. Systems with large numbers of
SCSI devices, for example, will continue to consume the remaining
unallocated major numbers. USB will also need to push beyond the 8 bit
minor limitation


simply increasing the device number size is insufficient. Apart
from causing a lot of pain, it doesn't solve the management issues
of a /dev with thousands or more device nodes


ignoring the problem of a huge /dev will not make it go
away, and dismisses the legitimacy of a large number of people who
want a dynamic /dev


the standard response then becomes: "write a device management
daemon", which brings us back to the
proposal above




What I don't like about devfs

Here are some common complaints about devfs, and some suggestions and
solutions that may make it more palatable for you. I can't please
everybody, but I do try :-)

I hate the naming scheme

First, remember that no naming scheme will please everybody. You hate
the scheme, others love it. Who's to say who's right and who's wrong?
Ultimately, the person who writes the code gets to choose, and what
exists now is a combination of the choices made by the
devfs author and the
kernel maintainer (Linus).

However, not all is lost. If you want to create your own naming
scheme, it is a simple matter to write a standalone script, hack
devfsd, or write a script called by devfsd. You can create whatever
naming scheme you like.

Further, if you want to remove all traces of the devfs naming scheme
from /dev, you can mount devfs elsewhere (say
/devfs) and populate /dev with links into
/devfs. This population can be automated using devfsd if you
wish.

You can even use the VFS binding facility to make the links, rather
than using symbolic links. This way, you don't even have to see the
"destination" of these symbolic links.

Devfs puts policy into the kernel

There's already policy in the kernel. Device numbers are in fact
policy (why should the kernel dictate what device numbers I use?).
Face it, some policy has to be in the kernel. The real difference
between device names as policy and device numbers as policy is that
no one will use device numbers directly, because device
numbers are devoid of meaning to humans and are ugly. At least with
the devfs device names, (even though you can add your own naming
scheme) some people will use the devfs-supplied names directly. This
offends some people :-)

Devfs is bloatware

This is not even remotely true. As shown above,
both code and data size are quite modest.


How to report bugs

If you have (or think you have) a bug with devfs, please follow the
steps below:



make sure you have enabled debugging output when configuring your
kernel. You will need to set (at least) the following config options:

CONFIG_DEVFS_DEBUG=y
CONFIG_DEBUG_KERNEL=y
CONFIG_DEBUG_SLAB=y



please make sure you have the latest devfs patches applied. The
latest kernel version might not have the latest devfs patches applied
yet (Linus is very busy)


save a copy of your complete kernel logs (preferably by
using the dmesg programme) for later inclusion in your bug
report. You may need to use the -s switch to increase the
internal buffer size so you can capture all the boot messages.
Don't edit or trim the dmesg output




try booting with devfs=dall passed to the kernel boot
command line (read the documentation on your bootloader on how to do
this), and save the result to a file. This may be quite verbose, and
it may overflow the messages buffer, but try to get as much of it as
you can


if you get an Oops, run ksymoops to decode it so that the
names of the offending functions are provided. A non-decoded Oops is
pretty useless


send a copy of your devfsd configuration file(s)

send the bug report to me first.
Don't expect that I will see it if you post it to the linux-kernel
mailing list. Include all the information listed above, plus
anything else that you think might be relevant. Put the string
devfs somewhere in the subject line, so my mail filters mark
it as urgent




Here is a general guide on how to ask questions in a way that greatly
improves your chances of getting a reply:

http://www.tuxedo.org/~esr/faqs/smart-questions.html. If you have
a bug to report, you should also read

http://www.chiark.greenend.org.uk/~sgtatham/bugs.html.


Strange kernel messages

You may see devfs-related messages in your kernel logs. Below are some
messages and what they mean (and what you should do about them, if
anything).



devfs_register(fred): could not append to parent, err: -17

You need to check what the error code means, but usually 17 means
EEXIST. This means that a driver attempted to create an entry
fred in a directory, but there already was an entry with that
name. This is often caused by flawed boot scripts which untar a bunch
of inodes into /dev, as a way to restore permissions. This
message is harmless, as the device nodes will still
provide access to the driver (unless you use the devfs=only
boot option, which is only for dedicated souls:-). If you want to get
rid of these annoying messages, upgrade to devfsd-v1.3.20 and use the
recommended RESTORE directive to restore permissions.


devfs_mk_dir(bill): using old entry in dir: c1808724 ""

This is similar to the message above, except that a driver attempted
to create a directory named bill, and the parent directory
has an entry with the same name. In this case, to ensure that drivers
continue to work properly, the old entry is re-used and given to the
driver. In 2.5 kernels, the driver is given a NULL entry, and thus,
under rare circumstances, may not create the require device nodes.
The solution is the same as above.





Compilation problems with devfsd

Usually, you can compile devfsd just by typing in
make in the source directory, followed by a make
install (as root). Sometimes, you may have problems, particularly
on broken configurations.



error messages relating to DEVFSD_NOTIFY_DELETE

This happened because you have an ancient set of kernel headers
installed in /usr/include/linux or /usr/src/linux.
Install kernel 2.4.10 or later. You may need to pass the
KERNEL_DIR variable to make (if you did not install
the new kernel sources as /usr/src/linux), or you may copy
the devfs_fs.h file in the kernel source tree into
/usr/include/linux.




-----------------------------------------------------------------------------


Other resources



Douglas Gilbert has written a useful document at

http://www.torque.net/sg/devfs_scsi.html which
explores the SCSI subsystem and how it interacts with devfs


Douglas Gilbert has written another useful document at

http://www.torque.net/scsi/SCSI-2.4-HOWTO/ which
discusses the Linux SCSI subsystem in 2.4.


Johannes Erdfelt has started a discussion paper on Linux and
hot-swap devices, describing what the requirements are for a scalable
solution and how and why he's used devfs+devfsd. Note that this is an
early draft only, available in plain text form at:

http://johannes.erdfelt.com/hotswap.txt.
Johannes has promised a HTML version will follow.


I presented an invited 
paper
at the

2nd Annual Storage Management Workshop held in Miamia, Florida,
U.S.A. in October 2000.




-----------------------------------------------------------------------------


Translations of this document

This document has been translated into other languages.




The document master (in English) by rgooch@atnf.csiro.au is
available at

http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html



A Korean translation by viatoris@nownuri.net is available at

http://your.destiny.pe.kr/devfs/devfs.html




-----------------------------------------------------------------------------
Most flags courtesy of ITA's 
Flags of All Countries
used with permission.