Serverworks LE, 4GB RAM, and MTRR

[Marvin Justice <mjustice@austin.rr.com>]

Slow performance on Serverworks LE boards with 4GB of RAM seem to be related 
to mtrr misconfiguration. Here is the /proc/mtrr for a Tyan 2510 ( 2.4.7-ac2):

reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
reg01: base=0x80000000 (2048MB), size=1024MB: write-back, count=1
reg02: base=0xc0000000 (3072MB), size= 512MB: write-back, count=1
reg03: base=0xe0000000 (3584MB), size= 256MB: write-back, count=1
reg04: base=0xf0000000 (3840MB), size= 128MB: write-back, count=1
reg05: base=0xf8000000 (3968MB), size=  64MB: write-back, count=1
reg06: base=0xfc000000 (4032MB), size=  64MB: uncachable, count=1

Also, the framebuffer is 4MB starting at 0xfd000000 (4048MB) on this system.
The last entry seems to be the culprit. Why should there be 64MB uncachable
starting at 4032?

Back in April there was a thread concering the mtrr setup for the LE chipset.
A patch for mtrr.c was submitted (but never accepted, apparently) that allows 
write-combining (which is currently disabled for all Serverworks LE) for 
revisions >5.  If I modify mtrr.c to allow write-combining the system works 
normally with 4G. /proc/mtrr is unchanged but the following line shows up in 
the syslog when the X-server is started:

mtrr: type mismatch for fd000000,400000 old: uncachable new: write-combining

The slowness of the system without write-combining is independent of whether 
X is started.

Marvin Justice