MTRR setting failure

[Thomas Hellström]

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