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SubjectRe: another Cyrix/mtrr problem?
Date
davej@suse.de wrote:
> Normally the answer would be "Closed driver, complain to nVidia",
> but just in case..

Glad you were open-minded enough to consider that it *might* be
"our" code.

> Can you verify that..
>
> a. You have MTRR support compiled into the kernel.

yes

> b. You have a /proc/mtrr file

yes

> c. You can add/delete ranges using /proc/mtrr
> (See Documentation/mtrr.txt for info on how to do this)

yes, BUT...

The "Documentation/mtrr.txt" file says "... 4 megabytes, which is
0x400000 bytes (in hexadecimal)." The math is correct: 1MB == 2**20
== 0x100000 the last time I checked. Unfortunately, when I execute

echo "base=0xd8000000 size=0x100000 type=write-combining" >| /proc/mtrr

I get a 2MB region instead of the 1MB region I expected...

reg00: base=0xd8000000 (3456MB), size= 2MB: write-combining, count=1
reg01: base=0x000c0000 ( 0MB), size= 512KB: uncachable, count=1
reg07: base=0x00000000 ( 0MB), size= 256MB: write-through, count=1

Similarly, the NVIDIA driver attempts to create a 32MB write-combining
region by passing a size argument of 0x2000000, and fails because the
underlying mtrr code tries to carve out a 64MB region whereas my video
card has only 32MB of RAM.

Looks like an off-by-one (bit) error to me. So... Is this a legitimate
bug sighting, or is my analysis faulty?

--Bob Tracy
rct@frus.com
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