mtrr and ati-radeon

[Jim McCloskey <mcclosk@ucsc.edu>]

Hello. My question concerns xserver-common 4.3.0.dfsg.1-4 from Debian
testing.

The graphics-card in my laptop is an ATI Radeon Mobility M6:

01:00.0 VGA compatible controller: ATI Technologies Inc Radeon
Mobility M6 LY (prog-if 00 [VGA])
        Subsystem: Mitac: Unknown device 8170
        Flags: bus master, stepping, 66Mhz, medium devsel, latency 64,
	IRQ 11
        Memory at 90000000 (32-bit, prefetchable) [size=128M]
        I/O ports at c000 [size=256]
        Memory at e0000000 (32-bit, non-prefetchable) [size=64K]
        Expansion ROM at <unassigned> [disabled] [size=128K]
        Capabilities: [58] AGP version 2.0
        Capabilities: [50] Power Management version 2

For some time now, the following warning has turned up in my X
log-file:

(WW) RADEON(0): Failed to set up write-combining range (0x90000000,0x1000000)

Searching on the web suggests that this warning derives from some
mis-configuration of MTRR support in the kernel, and indeed that is
confirmed by the presence of the following in /var/log/messages:

kernel: mtrr: 0x90000000,0x1000000 overlaps existing 0x90000000,0x200000

Cat /proc/mtrr gives:

reg00: base=0x00000000 (   0MB), size= 512MB: write-back, count=1
reg01: base=0x90000000 (2304MB), size=   2MB: write-combining, count=1
reg02: base=0xa0000000 (2560MB), size=  64MB: write-combining, count=1

And from the X log-file:

(--) RADEON(0): Chipset: "ATI Radeon Mobility M6 LY (AGP)" (ChipID =0x4c59)
(--) RADEON(0): Linear framebuffer at 0x90000000
(--) RADEON(0): VideoRAM: 16384 kByte (64 bit DDR SDRAM)

X is perfectly usable despite this warning, but my understanding is
that MTRR properly configured can give really substantial performance
gains under X and of course I'd like to be able to take advantage of
those.

I've read mtrr.txt in the kernel Documentation directory, and so I
know in principle how to create MTRRs from the shell, using
/proc/mtrr. However, I don't understand the principles behind this
well enough to know what ranges to set up.

If there were someone here who could advise me about this, or who could
point me to something useful to read, I'd be very grateful.

Thanks for all your work,

Jim