> Hallo Ivanko,
>
>> BTW, You should make sure You kernel does NOT use the PAT (MTRR
>> replacement) kernel options. This feature is very problematic if
>> activates.
>
> Did so, didn't help, at least not much, if at all.
> (Cost me a reboot, since not switchable at run time, only kernel
> parameter.)
>

Do You mean :

http://www.v13.gr/blog/?p=8

? In this advice You also should make sure using true MTRR.
===============
fglrx (Catalyst 8.8) + kernel 2.6.26 + PAT
15 September 2008, 1:29 am


It seems that PAT support that is included in linux kernel 2.6.26 has
some problems. If you are using a recent ATI Catalst (fglrx) driver
(at the time of this writting, the lattest was 8.8) with 2.6.26 kernel
with PAT support you may have problems.

Problems may include freezes every couple of seconds when playing
videos or full-screen 3D apps/games.

To solve this problem you will have to disable PAT support in kernel.
Fortunately there is a kernel boot parameter that does with without
requiring recompilation. Just add “nopat” to kernel boot parameters
and you should be OK. Debian users should not worry since PAT is
disabled in the default kernel.

The above is half the truth. When disabling PAT you should make sure
that MTRR is being set-up correctly. Start the X server and look at
Xorg.0.log for the string “Linear”:

 $ grep Linear /var/log/Xorg.0.log
 (--) fglrx(0): Linear framebuffer (phys) at 0xc0000000

 Then look at lspci -v outpout:

 01:00.0 VGA compatible controller: ATI Technologies Inc Mobilitiy
Radeon HD 3600 Series
 Subsystem: ASUSTeK Computer Inc. Device 01e4
 Flags: bus master, fast devsel, latency 0, IRQ 16
 Memory at c0000000 (64-bit, prefetchable) [size=256M]
 Memory at ff6f0000 (64-bit, non-prefetchable) [size=64K]
 [...]

 (“size=256M” is what we need). Don’t worry if you have more than
256MB of memory in your graphics card but it only shows 256MB. This is
expected behaviour.

Then look at /proc/mtrr for a line like this one:

 reg04: base=0xc0000000 (3072MB), size= 256MB: write-combining, count=1

 The base address is from the Xorg.0.log file and the size is from
lspci output (0×10000000 is 256MB (=256*1024*1024 bytes)). For most
cases this should be taken care of by the X server

If you don’t see it then you’ll have to setup MTRR by yourself. There
are many documents on the web but at the end you’ll be running
something like this:

 echo "base=0xc0000000 size=0x10000000 type=write-combining" > /proc/mtrr

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