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authorBoaz Harrosh <bharrosh@panasas.com>2009-03-24 12:23:40 +0100
committerJens Axboe <jens.axboe@oracle.com>2009-03-24 12:35:17 +0100
commit05378940caf979a8655c18b18a17213dcfa52412 (patch)
treeb72d29396fd6aa6f4485af638090066c7280325b /include/linux
parent0061d38642244892e17156f005bd7055fe744644 (diff)
bsg: add support for tail queuing
Currently inherited from sg.c bsg will submit asynchronous request at the head-of-the-queue, (using "at_head" set in the call to blk_execute_rq_nowait()). This is bad in situation where the queues are full, requests will execute out of order, and can cause starvation of the first submitted requests. The sg_io_v4->flags member is used and a bit is allocated to denote the Q_AT_TAIL. Zero is to queue at_head as before, to be compatible with old code at the write/read path. SG_IO code path behavior was changed so to be the same as write/read behavior. SG_IO was very rarely used and breaking compatibility with it is OK at this stage. sg_io_hdr at sg.h also has a flags member and uses 3 bits from the first nibble and one bit from the last nibble. Even though none of these bits are supported by bsg, The second nibble is allocated for use by bsg. Just in case. Signed-off-by: Boaz Harrosh <bharrosh@panasas.com> CC: Douglas Gilbert <dgilbert@interlog.com> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/bsg.h8
1 files changed, 8 insertions, 0 deletions
diff --git a/include/linux/bsg.h b/include/linux/bsg.h
index cf0303a6061..3f0c64ace42 100644
--- a/include/linux/bsg.h
+++ b/include/linux/bsg.h
@@ -7,6 +7,14 @@
#define BSG_SUB_PROTOCOL_SCSI_TMF 1
#define BSG_SUB_PROTOCOL_SCSI_TRANSPORT 2
+/*
+ * For flags member below
+ * sg.h sg_io_hdr also has bits defined for it's flags member. However
+ * none of these bits are implemented/used by bsg. The bits below are
+ * allocated to not conflict with sg.h ones anyway.
+ */
+#define BSG_FLAG_Q_AT_TAIL 0x10 /* default, == 0 at this bit, is Q_AT_HEAD */
+
struct sg_io_v4 {
__s32 guard; /* [i] 'Q' to differentiate from v3 */
__u32 protocol; /* [i] 0 -> SCSI , .... */