Write combining failure.

[Thomas Hellström]

This mail seems to have not reached the list, so I'll repost it.

Since this is a bit out of my "territory" I'd hoped for a brief
review before committing.

Regards
Thomas



Thomas Hellström wrote:

> Hi!
>
> It seems that the linux MTRR setting code both in Xorg and in kernels 
> >=2.6 is not doing it's job properly:
>
> The background is that some BIOSes do not set up MTRR correctly, but 
> just sets up write-combining in a small part of the framebuffer 
> memory. When the X server tries to set it's MTRR this fails because 
> there are overlapping regions of different sizes. I've seen this in 
> most VIA bioses and also newer DELL workstations with ATI Radeons.
>
> In the via/unichrome driver we have an ugly workaround which first 
> maps the framebuffer as MMIO (uncached) to get rid of any offending 
> MTRR region, then unmaps it and remaps it as write-combining.
>
> On 2.6 series kernels, removing a wc region does sometimes not work 
> the first time it is tried. You have to give the command twice to make 
> this work. Don't know why. I never saw this in the 2.4 series.
>
> This makes the code even more ugly, having to map framebuffer as MMIO 
> twice.
>
> I suggest to
>
> 1. Change the linux X framebuffer mapping code to remove any offending 
> MTRR regions, possibly saving them for a clean exit.
> 2. Have the code double check if the removal really went through. If 
> not, do it again, repeat a limited number of times.
> 3. Maybe inplement the newer IOCTL removal instead of 
> fprintf(mtrrfile, "disable=%d\n", region); ?
>
> Failure to properly set up MTRR can be such a performance degrador. 
> Particularly when using Xv, and the user will rarely find out the 
> cause of the slowness.
>
> /Thomas
>
>
I've posted a patch that fixes this for review on Bugzilla Bug 2750
https://bugs.freedesktop.org/show_bug.cgi?id=2750.


/Thomas



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