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It is VERY recommended to check if the MTRR registers are set up properly, because they can give a big performance boost.
Do a cat /proc/mtrr:
--($:~)-- cat /proc/mtrr
reg00: base=0xe4000000 (3648MB), size= 16MB: write-combining, count=9
reg01: base=0xd8000000 (3456MB), size= 128MB: write-combining, count=1
It's right, shows my Matrox G400 with 16MB memory. I did this from XFree 4.x.x , which sets up MTRR registers automatically.
If nothing worked, you have to do it manually. First, you have to find the base address. You have 3 ways to find it:
from X11 startup messages, for example:
(--) SVGA: PCI: Matrox MGA G400 AGP rev 4, Memory @ 0xd8000000, 0xd4000000 (--) SVGA: Linear framebuffer at 0xD8000000
from /proc/pci (use lspci -v command):
01:00.0 VGA compatible controller: Matrox Graphics, Inc.: Unknown device 0525 Memory at d8000000 (32-bit, prefetchable)
from mga_vid kernel driver messages (use dmesg):
mga_mem_base = d8000000
Then let's find the memory size. This is very easy, just convert video RAM size to hexadecimal, or use this table:
| 1 MB | 0x100000 |
| 2 MB | 0x200000 |
| 4 MB | 0x400000 |
| 8 MB | 0x800000 |
| 16 MB | 0x1000000 |
| 32 MB | 0x2000000 |
You know base address and memory size, let's setup MTRR registers!
For example, for the Matrox card above (base=0xd8000000)
with 32MB ram (size=0x2000000) just execute:
echo "base=0xd8000000 size=0x2000000 type=write-combining" >| /proc/mtrr
Not all CPUs have MTRRs. For example older K6-2 (around 266MHz, stepping 0) CPUs don't have MTRRs, but stepping 12 does (execute cat /proc/cpuinfo to check it).