Re: [PATCH v1 2/3] x86/PCI: trim _CRS windows when they conflict with previous reservations

[Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>]

Bjorn Helgaas wrote:
> On Wed, 2010-03-17 at 12:25 +0900, Kenji Kaneshige wrote:
>> Bjorn Helgaas wrote:
>>> Yanko's GA-MA78GM-S2H (BIOS F11) reports a host bridge window that overlaps
>>> system memory:
>>>
>>>     PCI window: [mem 0xcff00000-0x10ed0ffff]
>>>     System RAM: [mem 0x100000000-0x22fffffff]
>>>
>>> We can be pretty confident that the System RAM region is correct (if it
>>> were wrong, we'd crash as soon as we tried to use any memory in that area),
>>> so this patch tries to correct the PCI window by trimming it so it doesn't
>>> conflict with any previous reservations.
>> Though I might misunderstand something, it looks Yanko's machine specific
>> workaround. I'm wondering if trimming _CRS is a generic workaround for
>> broken _CRS machine.
>>
>> How about doing this when GA-MA78GM-S2H (BIOS F11) (and known machines
>> that have the same problem) is detected? Or how about switching nocrs
>> mode if the problem (resource conflict) is detected?
> 
> It's certainly a possibility to do this only for specific machines, but
> I'd like to avoid tripping over issues one-by-one.
> 
> I think there are three ways to address BIOS _CRS defects:
> 
>   1) Ship an OEM-specific host bridge driver
>   2) Put a platform- or BIOS-specific quirk into Windows

"into Linux"?

>   3) Change the BIOS
> 
> The first two sound like such a hassle to me that I doubt they would be
> practical.

I agree.
For 1), we need OEM-specific driver, not chipset specific driver.

For 2), I thought it depends on how many machines with broken _CRS are
there, and I didn't think there are so many, but...

> 
> But it's clear that there are systems like this with what appear to be
> _CRS defects.  It's quite possible that it's not really a defect, and we
> just don't understand how to parse _CRS correctly yet.  Or, Windows
> might have a few heuristics to clean up obvious errors.

Indeed, it might be true. Now I realize I need to change my opinion
about 2).

> 
> For example, I think Windows aligns host bridge windows, as documented
> here: http://bugzilla.kernel.org/show_bug.cgi?id=14337
> 
> I think Windows also knows to ignore the Consumer/Producer bit in
> Address Space Descriptors, and assume that all resources on bridges are
> Producers.
> 
> Hmm, what we really need is a way to run Windows in a virtualized
> environment where we could manipulate the _CRS method and see what
> Windows does with it...
> 
> Anyway, I'd like to make Linux behave as much like Windows as possible
> in this area so we can take advantage of all the testing that's done
> with Windows.

Okey, thank you very much for explanation. I understood the background
of your change.

Thanks,
Kenji Kaneshige