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2009-02-23x86: remove the Voyager 32-bit subarchIngo Molnar
Impact: remove unused/broken code The Voyager subarch last built successfully on the v2.6.26 kernel and has been stale since then and does not build on the v2.6.27, v2.6.28 and v2.6.29-rc5 kernels. No actual users beyond the maintainer reported this breakage. Patches were sent and most of the fixes were accepted but the discussion around how to do a few remaining issues cleanly fizzled out with no resolution and the code remained broken. In the v2.6.30 x86 tree development cycle 32-bit subarch support has been reworked and removed - and the Voyager code, beyond the build problems already known, needs serious and significant changes and probably a rewrite to support it. CONFIG_X86_VOYAGER has been marked BROKEN then. The maintainer has been notified but no patches have been sent so far to fix it. While all other subarchs have been converted to the new scheme, voyager is still broken. We'd prefer to receive patches which clean up the current situation in a constructive way, but even in case of removal there is no obstacle to add that support back after the issues have been sorted out in a mutually acceptable fashion. So remove this inactive code for now. Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-02-02x86 setup: a20: early timeout for a nonexistent keyboard controllerH. Peter Anvin
When probing the keyboard controller to enable A20, if we get FF back (which is *possible* as a valid status word, but is extremely unlikely) then bail after much fewer iterations than we otherwise would, and abort the attempt to access the KBC. This hopefully should make it work a lot better for embedded platforms which don't have a KBC and where the BIOS doesn't implement INT 15h AX=2401h (and doesn't boot with A20 already enabled.) If this works, it will be the one remaining use of CONFIG_X86_ELAN as anything other than a processor type optimization option. Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
2009-01-27x86 setup: fix asm constraints in vesa_store_edidAndreas Schwab
Impact: fix potential miscompile (currently believed non-manifest) As the comment explains, the VBE DDC call can clobber any register. Tell the compiler about that fact. Signed-off-by: Andreas Schwab <schwab@suse.de> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
2008-12-23Merge branches 'x86/apic', 'x86/cleanups', 'x86/cpufeature', ↵Ingo Molnar
'x86/crashdump', 'x86/debug', 'x86/defconfig', 'x86/detect-hyper', 'x86/doc', 'x86/dumpstack', 'x86/early-printk', 'x86/fpu', 'x86/idle', 'x86/io', 'x86/memory-corruption-check', 'x86/microcode', 'x86/mm', 'x86/mtrr', 'x86/nmi-watchdog', 'x86/pat2', 'x86/pci-ioapic-boot-irq-quirks', 'x86/ptrace', 'x86/quirks', 'x86/reboot', 'x86/setup-memory', 'x86/signal', 'x86/sparse-fixes', 'x86/time', 'x86/uv' and 'x86/xen' into x86/core
2008-11-25x86: use limited register constraint for setnzSteven Rostedt
Impact: build fix with certain compilers GCC can decide to use %dil when "r" is used, which is not valid for setnz. This bug was brought out by Stephen Rothwell's merging of the branch tracer into linux-next. [ Thanks to Uros Bizjak for recommending 'q' over 'Q' ] Signed-off-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-11-23x86: boot - fix sparse warningsHannes Eder
Impact: make global variables static Fix these sparse warnings: arch/x86/boot/video.c:233:3: warning: symbol 'saved' was not declared. Should it be static? arch/x86/boot/video-vga.c:37:13: warning: symbol 'video_vga' was not declared. Should it be static? Signed-off-by: Hannes Eder <hannes@hanneseder.net> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-10-30.gitignore updatesAlexey Dobriyan
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-10-23Merge branch 'x86-fixes-for-linus' of ↵Linus Torvalds
git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip * 'x86-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip: x86: fix section mismatch warning - apic_x2apic_phys x86: fix section mismatch warning - apic_x2apic_cluster x86: fix section mismatch warning - apic_x2apic_uv_x x86: fix section mismatch warning - apic_physflat x86: fix section mismatch warning - apic_flat x86: memtest fix use of reserve_early() x86 syscall.h: fix argument order x86/tlb_uv: remove strange mc146818rtc include x86: remove redundant KERN_DEBUG on pr_debug x86: do_boot_cpu - check if we have ESR register x86: MAINTAINERS change for AMD microcode patch loader x86/proc: fix /proc/cpuinfo cpu offline bug x86: call dmi-quirks for HP Laptops after early-quirks are executed x86, kexec: fix hang on i386 when panic occurs while console_sem is held MCE: Don't run 32bit machine checks with interrupts on x86: SB600: skip IRQ0 override if it is not routed to INT2 of IOAPIC x86: make variables static
2008-10-22x86: Fix ASM_X86__ header guardsH. Peter Anvin
Change header guards named "ASM_X86__*" to "_ASM_X86_*" since: a. the double underscore is ugly and pointless. b. no leading underscore violates namespace constraints. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-10-22x86: make variables staticroel kluin
These variables are only used in their source files, so make them static. Signed-off-by: Roel Kluin <roel.kluin@gmail.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-10-16fbdev: ignore VESA modes if framebuffer does not support themMichal Januszewski
Currently, it is possible to set a graphics VESA mode at boot time via the vga= parameter even when no framebuffer driver supporting this is configured. This could lead to the system booting with a black screen, without a usable console. Fix this problem by only allowing to set graphics modes at boot time if a supporting framebuffer driver is configured. Signed-off-by: Michal Januszewski <spock@gentoo.org> Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl> Cc: "H. Peter Anvin" <hpa@zytor.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-10-12Merge branches 'x86/xen', 'x86/build', 'x86/microcode', 'x86/mm-debug-v2', ↵Ingo Molnar
'x86/memory-corruption-check', 'x86/early-printk', 'x86/xsave', 'x86/ptrace-v2', 'x86/quirks', 'x86/setup', 'x86/spinlocks' and 'x86/signal' into x86/core-v2
2008-10-11Merge branch 'x86/unify-cpu-detect' into x86-v28-for-linus-phase4-DIngo Molnar
Conflicts: arch/x86/kernel/cpu/common.c arch/x86/kernel/signal_64.c include/asm-x86/cpufeature.h
2008-10-06Merge branches 'x86/alternatives', 'x86/cleanups', 'x86/commandline', ↵Ingo Molnar
'x86/crashdump', 'x86/debug', 'x86/defconfig', 'x86/doc', 'x86/exports', 'x86/fpu', 'x86/gart', 'x86/idle', 'x86/mm', 'x86/mtrr', 'x86/nmi-watchdog', 'x86/oprofile', 'x86/paravirt', 'x86/reboot', 'x86/sparse-fixes', 'x86/tsc', 'x86/urgent' and 'x86/vmalloc' into x86-v28-for-linus-phase1
2008-10-06Merge branch 'x86/header-guards' into x86-v28-for-linus-phase1Ingo Molnar
Conflicts: include/asm-x86/dma-mapping.h include/asm-x86/gpio.h include/asm-x86/idle.h include/asm-x86/kvm_host.h include/asm-x86/namei.h include/asm-x86/uaccess.h Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-10-05x86: replace a magic number with a named constant in the VESA boot codeMichal Januszewski
Replace a magic number with a named constant in the VESA boot code. Signed-off-by: Michal Januszewski <spock@gentoo.org> Cc: linux-fbdev-devel@lists.sourceforge.net Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-10-04x86 setup: remove IMAGE_OFFSETPaul Bolle
After commit 968de4f ("i386: Relocatable kernel support") IMAGE_OFFSET wasn't actually used anymore in the (current) X86 build system. Now remove its last traces. Signed-off-by: Paul Bolle <pebolle@tiscali.nl> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-10-03x86 setup: correct segfault in generation of 32-bit reloc kernelH. Peter Anvin
Impact: segfault on build of a 32-bit relocatable kernel When converting arch/x86/boot/compressed/relocs.c to support unlimited sections, the computation of sym_strtab in walk_relocs() was done incorrectly. This causes a segfault for some people when building the relocatable 32-bit kernel. Pointed out by Anonymous <pageexec@freemail.hu>. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-10-03Merge commit 'v2.6.27-rc8' into x86/setupIngo Molnar
2008-10-03Revert "x86: fix ghost EDD devices in /sys again"H. Peter Anvin
This reverts commit 464f04c9e9b3b1c4f5ffb89c51d8ba2a2034c846. Obsoleted by commit 6cdcdb99cf7c2e1835fc5b471864d21161c3e679. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-10-03x86 setup: fix ghost entries under /sys/firmware/edd take 3Andrey Borzenkov
Some BIOSes do not indicate error when trying to read from non- existing device. Zero buffer before reading and check that we possibly have valid MBR by looking for MBR magic. This was fixed in different way for edd.S in http://marc.info/?l=linux-kernel&m=114087765422490&w=2, but lost again when edd.S was rewritten in C. Signed-off-by: Andrey Borzenkov < arvidjaar@mail.ru> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-09-19Merge commit 'v2.6.27-rc6' into x86/cleanupsIngo Molnar
2008-09-16x86 setup: handle more than 8 CPU flag wordsH. Peter Anvin
Checkin e38e05a85828dac23540cd007df5f20985388afc added a 9th CPU flag word, but didn't adjust the boot code to match. This patch adds the necessary boot code support. Note: due to a typo in an #if statement, it didn't trigger the #error this was supposed to do. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-09-16x86 setup: drop SWAP_DEVPaul Bolle
Impact: None (cleanup) SWAP_DEV is unused since 2.6.23-rc1. The comment was already incorrect since (at least) 2.6.12. Signed-off-by: Paul Bolle <pebolle@tiscali.nl> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-09-06x86: fix ghost EDD devices in /sys againAndrey Borzenkov
> This is regression but old enough. Apparently I had for whatever reasons > EDD turned off till recently. This is 2.6.27-rc5 just in case. > > In 2006 I fixed ghost devices due to buggy BIOS: > > http://marc.info/?l=linux-kernel&m=114087765422490&w=2 > > Later edd.S has been rewritten in C, and apparently this patch has been > lost: > > {pts/1}% ls /sys/firmware/edd > int13_dev80/ int13_dev84/ int13_dev88/ int13_dev8c/ > int13_dev81/ int13_dev85/ int13_dev89/ int13_dev8d/ > int13_dev82/ int13_dev86/ int13_dev8a/ int13_dev8e/ > int13_dev83/ int13_dev87/ int13_dev8b/ int13_dev8f/ > > But I have just a single disk. This is the same system BTW. Some BIOSes do not always set CF on error before return from int13. The patch adds additional check for status being zero (AH == 0). This was fixed for edd.S in http://marc.info/?l=linux-kernel&m=114087765422490&w=2, but lost again when edd.S was rewritten in C. Signed-off-by: Andrey Borzenkov <arvidjaar@mail.ru> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-09-05x86: when building image.iso, use isohybrid if it existsH. Peter Anvin
When building image.iso (make isoimage), use the isohybrid tool if it exists. isohybrid is a script included with Syslinux 3.72 and higher, which creates an image that can be booted either as a hard disk (including removable, e.g. USB disk) or as a CD-ROM. If isohybrid doesn't exist, then this has no effect. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-09-05x86: boot: stub out unimplemented CPU feature wordsH. Peter Anvin
The CPU feature detection code in the boot code is somewhat minimal, and doesn't include all possible CPUID words. In particular, it doesn't contain the code for CPU feature words 2 (Transmeta), 3 (Linux-specific), 5 (VIA), or 7 (scattered). Zero them out, so we can still set those bits as known at compile time; in particular, this allows creating a Linux-specific NOPL flag and have it required (and therefore resolvable at compile time) in 64-bit mode. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-09-04Merge branch 'x86/cpu' into x86/coreH. Peter Anvin
Conflicts: arch/x86/kernel/cpu/feature_names.c include/asm-x86/cpufeature.h
2008-09-04x86: boot/compressed/Makefile: fix "make clean"H. Peter Anvin
The Kbuild variable "targets" is supposed to be configuration-independent and reflect "all possible targets". This is required to make "make clean" work properly. Therefore, move all manipulation of "targets" as well as custom rules out of the x86-32 ifdef statement. Only leave inside the ifdefs the things that are genuinely configuration-dependent. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-08-27x86: generate names for /proc/cpuinfo from <asm/cpufeature.h>H. Peter Anvin
We have had a number of cases where <asm/cpufeature.h> (and its predecessors) have diverged substantially from the names list in /proc/cpuinfo. This patch generates the latter from the former. It retains the option for explicitly overriding the strings, but by making that require a separate action it should at least be less likely to happen. It would be good to do a future pass and rename strings that are gratuituously different in the kernel (/proc/cpuinfo is a userspace interface and must remain constant.) Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-08-27Merge branch 'x86/urgent' into x86/cpuH. Peter Anvin
Conflicts: arch/x86/kernel/cpu/cyrix.c
2008-08-20Merge branch 'linus' into x86/cleanupsIngo Molnar
2008-08-18x86: boot: stub out unimplemented CPU feature wordsH. Peter Anvin
The CPU feature detection code in the boot code is somewhat minimal, and doesn't include all possible CPUID words. In particular, it doesn't contain the code for CPU feature words 2 (Transmeta), 3 (Linux-specific), 5 (VIA), or 7 (scattered). Zero them out, so we can still set those bits as known at compile time; in particular, this allows creating a Linux-specific NOPL flag and have it required (and therefore resolvable at compile time) in 64-bit mode. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-08-18x86: fix build warnings in real mode codeAndi Kleen
This recent patch commit c3965bd15118742d72b4bc1a290d37b3f081eb98 Author: Paul Jackson <pj@sgi.com> Date: Wed May 14 08:15:34 2008 -0700 x86 boot: proper use of ARRAY_SIZE instead of repeated E820MAX constant caused these new warnings during a normal build: In file included from linux-2.6/arch/x86/boot/memory.c:17: linux-2.6/include/linux/log2.h: In function '__ilog2_u32': linux-2.6/include/linux/log2.h:34: warning: implicit declaration of function 'fls' linux-2.6/include/linux/log2.h: In function '__ilog2_u64': linux-2.6/include/linux/log2.h:42: warning: implicit declaration of function 'fls64' linux-2.6/include/linux/log2.h: In function '__roundup_pow_of_two ': linux-2.6/include/linux/log2.h:63: warning: implicit declaration of function 'fls_long' I tried to fix them in log2.h, but it's difficult because the real mode environment is completely different from a normal kernel environment. Instead define an own ARRAY_SIZE macro in boot.h, similar to the other private macros there. Signed-off-by: Andi Kleen <ak@linux.intel.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-08-15x86: coding style fixes to arch/x86/boot/compressed/misc.cPaolo Ciarrocchi
Before: total: 4 errors, 6 warnings, 439 lines checked After: total: 1 errors, 5 warnings, 441 lines checked Before -#include <asm/io.h> +#include <linux/io.h> paolo@paolo-desktop:~/linux.trees.git$ md5sum /tmp/misc.o.* 8b2394e1fe519a9542e9a7e3e7b69c39 /tmp/misc.o.after 8b2394e1fe519a9542e9a7e3e7b69c39 /tmp/misc.o.before After -#include <asm/io.h> +#include <linux/io.h> paolo@paolo-desktop:~/linux.trees.git$ md5sum /tmp/misc.o.* 59a2d264284be5e72b5af4f3a8ccfb47 /tmp/misc.o.after 8b2394e1fe519a9542e9a7e3e7b69c39 /tmp/misc.o.before Signed-off-by: Paolo Ciarrocchi <paolo.ciarrocchi@gmail.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-08-14x86: cleanup for setup code crashes during IST probeH. Peter Anvin
Clean up the code for crashes during SpeedStep probing on older machines. Signed-off-by: H. Peter Anvin <hpa@zytor.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-08-13x86: fix setup code crashes on my old 486 boxJoerg Roedel
yesterday I tried to reactivate my old 486 box and wanted to install a current Linux with latest kernel on it. But it turned out that the latest kernel does not boot because the machine crashes early in the setup code. After some debugging it turned out that the problem is the query_ist() function. If this interrupt with that function is called the machine simply locks up. It looks like a BIOS bug. Looking for a workaround for this problem I wrote the attached patch. It checks for the CPUID instruction and if it is not implemented it does not call the speedstep BIOS function. As far as I know speedstep should be available since some Pentium earliest. Alan Cox observed that it's available since the Pentium II, so cpuid levels 4 and 5 can be excluded altogether. H. Peter Anvin cleaned up the code some more: > Right in concept, but I dislike the implementation (duplication of the > CPU detect code we already have). Could you try this patch and see if > it works for you? which, with a small modification to fix a build error with it the resulting kernel boots on my machine. Signed-off-by: Joerg Roedel <joro@8bytes.org> Signed-off-by: "H. Peter Anvin" <hpa@zytor.com> Cc: <stable@kernel.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-08-11x86: fix comment in protected mode headerPhilipp Kohlbecher
Comments in arch/x86/boot/compressed/head_32.S erroneously refer to the real mode pointer as the second and the heap area as the third argument to decompress_kernel(). In fact, these have been the first and second argument, respectively, since v2.6.20. This patch corrects the comments. It introduces no code changes. Signed-off-by: Philipp Kohlbecher <xt28@gmx.de> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-07-26Merge branch 'linus' into x86/header-guardsIngo Molnar
Conflicts: include/asm-x86/gpio.h include/asm-x86/ide.h Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-07-25inflate: refactor inflate malloc codeThomas Petazzoni
Inflate requires some dynamic memory allocation very early in the boot process and this is provided with a set of four functions: malloc/free/gzip_mark/gzip_release. The old inflate code used a mark/release strategy rather than implement free. This new version instead keeps a count on the number of outstanding allocations and when it hits zero, it resets the malloc arena. This allows removing all the mark and release implementations and unifying all the malloc/free implementations. The architecture-dependent code must define two addresses: - free_mem_ptr, the address of the beginning of the area in which allocations should be made - free_mem_end_ptr, the address of the end of the area in which allocations should be made. If set to 0, then no check is made on the number of allocations, it just grows as much as needed The architecture-dependent code can also provide an arch_decomp_wdog() function call. This function will be called several times during the decompression process, and allow to notify the watchdog that the system is still running. If an architecture provides such a call, then it must define ARCH_HAS_DECOMP_WDOG so that the generic inflate code calls arch_decomp_wdog(). Work initially done by Matt Mackall, updated to a recent version of the kernel and improved by me. [akpm@linux-foundation.org: coding-style fixes] Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> Cc: Matt Mackall <mpm@selenic.com> Cc: Richard Henderson <rth@twiddle.net> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: Mikael Starvik <mikael.starvik@axis.com> Cc: Jesper Nilsson <jesper.nilsson@axis.com> Cc: Haavard Skinnemoen <hskinnemoen@atmel.com> Cc: David Howells <dhowells@redhat.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Andi Kleen <andi@firstfloor.org> Cc: "H. Peter Anvin" <hpa@zytor.com> Acked-by: Paul Mundt <lethal@linux-sh.org> Acked-by: Yoshinori Sato <ysato@users.sourceforge.jp> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-07-22x86: consolidate header guardsVegard Nossum
This patch consolidates the header guard names which are also used externally, i.e. in .c files. Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
2008-07-18x86: check function status in EDD boot codeJan Beulich
Without checking the return value of get_edd_info() and adding the entry only in the success case, 6 devices show up under /sys/firmware/edd/, no matter how many devices are actually present. Signed-off-by: Jan Beulich <jbeulich@novell.com> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-07-17x86: unify and correct the GDT_ENTRY() macroH. Peter Anvin
Merge the GDT_ENTRY() macro between arch/x86/boot/pm.c and arch/x86/kernel/acpi/sleep.c and put the new one in <asm-x86/segment.h>. While we're at it, correct the bitmasks for the limit and flags. The new version relies on using ULL constants in order to cause type promotion rather than explicit casts; this avoids having to include <linux/types.h> in <asm-x86/segments.h>. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
2008-07-08Merge branch 'x86/mpparse' into x86/develIngo Molnar
Conflicts: arch/x86/Kconfig arch/x86/kernel/io_apic_32.c arch/x86/kernel/setup_64.c arch/x86/mm/init_32.c Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-07-08Merge branch 'linus' into tmp.x86.mpparse.newIngo Molnar
2008-07-08Merge branch 'x86/setup' into x86/develIngo Molnar
2008-07-03x86: config option to disable info from decompression of the kernelBen Collins
This patch allows the disabling of decompression messages during x86 bootup. Signed-off-by: Ben Collins <ben.collins@canonical.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-07-01x86: fix regression: boot failure on AMD Elan TS-5500H. Peter Anvin
Jeremy Fitzhardinge wrote: > > Maybe it really does require the far jump immediately after setting PE > in cr0... > > Hm, I don't remember this paragraph being in vol 3a, section 8.9.1 > before. Is it a recent addition? > > Random failures can occur if other instructions exist between steps > 3 and 4 above. Failures will be readily seen in some situations, > such as when instructions that reference memory are inserted between > steps 3 and 4 while in system management mode. > I don't remember that, either. Signed-off-by: Ingo Molnar <mingo@elte.hu>
2008-06-30x86: remove arbitrary ELF section limit in i386 relocatable kernelH. Peter Anvin
Impact: build failure in maximal configurations The 32-bit x86 relocatable kernel requires an auxilliary host program to process the relocations. This program had a hard-coded arbitrary limit of a 100 ELF sections. Instead of a hard-coded limit, allocate the structures dynamically. Signed-off-by: H. Peter Anvin <hpa@zytor.com> Acked-by: Vivek Goyal <vgoyal@redhat.com>
2008-06-27x86: setup: issue a null command after enabling A20 via KBCH. Peter Anvin
Apparently, DOS and possibly other legacy operating systems issued a null command to the keyboard controller after toggling A20, specifically "pulse output pins" with no output pins specified. This was presumably done for synchronization reasons. This has made it into at least the UHCI spec, and it has been found to cause compatibility problems when "legacy USB" is enabled (which it almost always is) to not have this byte sent. It is *NOT* clear if any of these compatibility problems has any effect on Linux. However, for maximum compatibility, issue this null command after togging A20 through the KBC. Signed-off-by: H. Peter Anvin <hpa@zytor.com>