MTRRs not working in 2.6 linux kernels?

[Jonathan Thambidurai]

When I try to use the application tvtime under Xorg 6.8.1 with Linux
kernel 2.6.7, I get very choppy framerates; tvtime indicates that it is
only getting about 50 MB/s for blitting to the screen.  With the same
version of Xorg and Linux kernel 2.4.22, I get smooth play, and tvtime
indicates that it gets about 150 MB/s for blitting to screen.  I made
sure DRI wasn't playing a role in this by rmmod'ing the agpgart module
before starting X.  

Of note is the difference in /proc/mtrr between 2.6 and 2.4
kernels:

in 2.4, it contains only

reg00: base=0x00000000 (   0MB), size= 256MB: write-back, count=1
reg01: base=0x0ff80000 ( 255MB), size= 512KB: uncachable, count=1

while in 2.6 it contains

reg00: base=0x00000000 (   0MB), size= 256MB: write-back, count=1
reg01: base=0x0ff80000 ( 255MB), size= 512KB: uncachable, count=1
reg02: base=0xe8000000 (3712MB), size=   2MB: write-combining, count=1

Lastly, in the X.org log when running a 2.6 kernel the following line is
displayed:

(WW) RADEON(0): Failed to set up write-combining range (0xe8000000,0x1000000)

while under 2.4 the following line is displayed:

(==) RADEON(0): Write-combining range (0xe8000000,0x1000000)

I am running a P4 at 1.8 Ghz Laptop with an AGP 4x Radeon 7500 and 16 MB
of VRAM; it has 256 MB of system memory, which makes the third line of 
/proc/mtrr in the 2.6 kernels look wrong to my untrained eyes.

I just tried this on kernel 2.6.9-rc2 with the same results
as 2.6.7 and saw that dmesg complained about some mtrr overlapping with 
something else; if this is important, I can report the exact message.

Perhaps this is a kernel issue and should be posted to the kernel list? 
Any help would be appreciated.

--Jonathan Thambidurai