fix BIOS PCI config cycle buglet causing ACPI boot regression

fix BIOS PCI config cycle buglet causing ACPI boot regression

I figured out another ACPI related regression today.

randconfig testing triggered an early boot-time hang on a laptop of mine
(32-bit x86, config attached) - the screen was scrolling ACPI AML
exceptions [with no serial port and no early debugging available].

v2.6.24 works fine on that laptop with the same .config, so after a few
hours of bisection (had to restart it 3 times - other regressions
interacted), it honed in on this commit:

| acebf2c242b890c743dda3e35866c1a12b60e821 is first bad commit
|
| Author: Linus Torvalds <torvalds@woody.linux-foundation.org>
| Date:   Wed Feb 13 09:56:14 2008 -0800
|
|     acpi: fix acpi_os_read_pci_configuration() misuse of raw_pci_read()

reverting this commit ontop of -rc5 gave a correctly booting kernel.

But this commit fixes a real bug so the real question is, why did it
break the bootup?

After quite some head-scratching, the following change stood out:

-                               pci_id->bus = tu8;
+                               pci_id->bus = val;

pci_id->bus is defined as u16:

   struct acpi_pci_id {
           u16 segment;
           u16 bus;
   ...

and 'tu8' changed from u8 to u32. So previously we'd unconditionally
mask the return value of acpi_os_read_pci_configuration()
(raw_pci_read()) to 8 bits, but now we just trust whatever comes back
from the PCI access routines and only crop it to 16 bits.

But if the high 8 bits of that result contains any noise then we'll
write that into ACPI's PCI ID descriptor and confuse the heck out of the
rest of ACPI.

So lets check the PCI-BIOS code on that theory. We have this codepath
for 8-bit accesses (arch/x86/pci/pcbios.c:pci_bios_read()):

        switch (len) {
        case 1:
                __asm__("lcall *(%%esi); cld\n\t"
                        "jc 1f\n\t"
                        "xor %%ah, %%ah\n"
                        "1:"
                        : "=c" (*value),
                          "=a" (result)
                        : "1" (PCIBIOS_READ_CONFIG_BYTE),
                          "b" (bx),
                          "D" ((long)reg),
                          "S" (&pci_indirect));

Aha! The "=a" output constraint puts the full 32 bits of EAX into
*value. But if the BIOS's routines set any of the high bits to nonzero,
we'll return a value with more set in it than intended.

The other, more common PCI access methods (v1 and v2 PCI reads) clear
out the high bits already, for example pci_conf1_read() does:

        switch (len) {
        case 1:
                *value = inb(0xCFC + (reg & 3));

which explicitly converts the return byte up to 32 bits and zero-extends
it.

So zero-extending the result in the PCI-BIOS read routine fixes the
regression on my laptop. ( It might fix some other long-standing issues
we had with PCI-BIOS during the past decade ... ) Both 8-bit and 16-bit
accesses were buggy.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>