AGP and PAT (induced?) problem (on AMD family 6)

[Rene Herman <rene.herman@keyaccess.nl>]

Hi Dave.

A while ago I sent a message about long AGP delays upon starting and 
exiting X:

http://marc.info/?l=linux-kernel&m=121647129632110&w=2

There was no reply (if that was due to the linux.ie address, could you 
perhaps update it in MAINTAINERS?) but today Shaohua Li posted a patch 
that made me wonder about PAT in this context:

http://marc.info/?l=linux-kernel&m=121783222306075&w=2
http://marc.info/?l=linux-kernel&m=121783222406078&w=2
http://marc.info/?l=linux-kernel&m=121783222406081&w=2

His patch does not solve anything appreciable for me -- the delays are 
still as described in that previous post, with an exception for (with 
Option "AGSize" "64") delays upon exiting X that are now sometimes as 
bad as a full 12 seconds.

What _does_ solve this though is booting with the "nopat" command line 
parameter. I'm on 2.6.26.1 and have enabled PAT for my AMD Duron myself. 
With "nopat", there's no problem to be seen anymore -- exiting X 
specifically is instantaneous.

With or without PAT, my /proc/mtrr is always:

reg00: base=0x00000000 (   0MB), size= 512MB: write-back, count=1
reg01: base=0x20000000 ( 512MB), size= 256MB: write-back, count=1
reg02: base=0xe8000000 (3712MB), size=  64MB: write-combining, count=1

under X joined by:

reg03: base=0xe4000000 (3648MB), size=  32MB: write-combining, count=2

This is a machine with 768M, the AGP aperture set to 64MB and a 32MB 
Matrox Millenium G550 AGP card. More detail in previous post.

Is this something inherent to PAT? Inherent to PAT on AMD family 6? 
Inherent to DRM/AGP with PAT? On AMD family 6?

This is probably fairly important to get sorted because although I don't 
know what's where at the moment, last I saw was a patch in x86/tip that 
enabled PAT on many more models including all of AMD.

For reference, /proc/cpuinfo:

processor	: 0
vendor_id	: AuthenticAMD
cpu family	: 6
model		: 7
model name	: AMD Duron(tm) Processor
stepping	: 1
cpu MHz		: 1313.094
cache size	: 64 KB
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 1
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov 
pat pse36 mmx fxsr sse syscall mmxext 3dnowext 3dnow
bogomips	: 2628.89
clflush size	: 32
power management: ts

and the PAT enabler patch that I apply locally to 2.6.26:

diff --git a/arch/x86/kernel/cpu/addon_cpuid_features.c 
b/arch/x86/kernel/cpu/addon_cpuid_features.c
index c2e1ce3..8992282 100644
--- a/arch/x86/kernel/cpu/addon_cpuid_features.c
+++ b/arch/x86/kernel/cpu/addon_cpuid_features.c
@@ -55,7 +55,7 @@ void __cpuinit validate_pat_support(struct cpuinfo_x86 *c)
  {
         switch (c->x86_vendor) {
         case X86_VENDOR_AMD:
-               if (c->x86 >= 0xf && c->x86 <= 0x11)
+               if (c->x86 == 6 || c->x86 >= 0xf)
                         return;
                 break;
         case X86_VENDOR_INTEL:

Rene.