On 10/08/2013 04:13 PM, Bjorn Helgaas wrote:
On Mon, Oct 7, 2013 at 3:14 PM, Deng-Cheng Zhu <dengcheng.zhu@xxxxxxxxxx> wrote:
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On 10/04/2013 09:37 PM, Bjorn Helgaas wrote:
On Fri, Oct 4, 2013 at 9:42 PM, DengCheng Zhu <DengCheng.Zhu@xxxxxxxxxx>
wrote:
Does this fix a user-visible problem? If so, what does it look like
when the problem occurs?
No, I found the problem while debugging another issue and trying to
understand this piece of code and after looking into different versions
of
PIIX4 datasheets. But I don't think it should prevent such a fix because
the code is straightforward and the spec is clear enough. If the existing
encoding was intentionally made like this by the code author due to the
inaccuracy of the spec, then it's very likely some code comments were
placed here. There are 2 possibilities:
- This fix breaks something. People should have to bisect the problem.
- This fix is valid. Some day people need PIIX4 mem quirks and they don't
have to run into a possibly well-hidden issue.
What do you think?
Don't worry, I'm willing to fix it even if nobody has actually
reported a problem. It's just nice to include the symptoms if
somebody *has* reported it, so when other people see the same symptom,
they can more easily find the fix.
Ah, I see. Thanks.
So far I didn't see them on any branch in kernel/git/helgaas/pci.git
<https://git.kernel.org/cgit/linux/kernel/git/helgaas/pci.git/>
This change definitely makes the code match the specs you mentioned.
But we don't have any problem reports,
It's very likely that nobody really used PIIX4 ACPI devices 12 and 13.
Maybe some day they'll be used, or maybe they can be deleted from the code.
But if they'll be kept in the code, had better fix it IMO.
and when Linus added this code
(6693e74a "PCI: be more verbose about resource quirks), he presumably
was looking at a spec, too. It's possible he transcribed the masks
incorrectly, but twice in one patch?
The PIIX4 is an old part, and for something like that I would
generally say "don't touch it" because the risk of breakage outweighs
a possible fix for machines nobody uses anymore.
However, I see that the PIIX4 is emulated in, e.g., qemu, so it's
still relevant. But before I apply this, can you please research qemu
and how it uses these registers? For example, if you can show that
qemu emulates the registers with the additional bits you add to the
masks here, then we should be able to make linux act incorrectly by
setting those bits in a qemu BIOS.
QEMU doesn't emulate PIIX4 ACPI devices 12 & 13, so these registers are not
used in it.
Deng-Cheng
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