MTRR use in drivers

[Dave Airlie]

>>> Why do you care about performance when PAT is disabled?

breaking old boxes just because, is just going to get reverted when I
get the first regression report that you broke old boxes.

Andy Lutomirski just submitted a bunch of patches to clean up the DRM
usage of mtrrs, they are in drm-next, afaik we no longer add them on
PAT systems.

Dave.

>>
>> It will regress already slow boxes.  We blacklist a LOT of P4s, PMs, etc and
>> nobody ever took the pain to track down which ones of those actually have
>> PAT+MTRR aliasing bugs.
>>
>> These boxes have boards like the Radeon X300, which needs either PAT or MTRR
>> to not become unusable...
>>
>
> We're talking hardware which is now many years old, but this is causing
> very serious problems on real, modern hardware.  As far as I understand
> it, too, the blacklisting was precautionary (the only bug that I
> personally know about is a performance bug, where WC would be
> incorrectly converted to UC.)
>
> We need a way forward here.  If it is the only way I think we would have
> to sacrifice the old machines, but perhaps something can be worked out
> (e.g. if PAT is disabled, fall back to MTRRs if available for ioremap_wc()).
>
>         -hpa
>
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