Re: [gentoo-user] [OT?] MTRR and PAT

[pk <peterk2@coolmail.se>]

Andrey Falko wrote:

> Ahh I see. I don't know much about MTRR...all I know is that you can
> adjust them via grub:
> http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?part=1&chap=10

Hmm... I missed that one. Thanks.

My grub kernel command line (which I haven't given much thought to since 
it's inception many years ago):
root=/dev/sda2 vga=804 video=vesafb:1280x1024-32@85,mtrr:3,ywrap

This should tell vesafb to set MTRR to use write-combining (for which 
memory region, I don't know), but it doesn't work for me, for some 
reason. All of my mem (under X) is set to write-back with the exception 
of 1Mb set to uncacheable. My other computer has even stranger settings 
and the kernel log complains about MTRR settings not correctly setup (it 
states that the BIOS probably doesn't init's both cores).

> . You probably read this already:
> http://en.wikipedia.org/wiki/Memory_Type_Range_Registers

Yep. Here's a few more if you (or anyone else) find this interesting:
http://en.wikipedia.org/wiki/Write-combining
http://en.wikipedia.org/wiki/Cache

Basically the MTRR (and PAT) determines how the processor (and it's 
cache(s)) accesses the memory regions (in the MTRR). Xorg supposedly 
should set up the MTRR when it starts.

> Yeh, I have Nvidia cards, so I guess ATI is different.

Perhaps.

Thanks again! It's always good to weather one's thoughts on a subject...

This just got me thinking. I'll investigate what the MTRR settings look 
like before the display manager starts...

Best regards

Peter K
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