linux-kernel - Re: AGP and PAT (induced?) problem (on AMD family 6)

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Date:	Wed, 20 Aug 2008 16:27:22 +0200
From:	Rene Herman <rene.herman@...access.nl>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	Dave Airlie <airlied@...il.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Arjan van de Ven <arjan@...radead.org>,
	"dri-users@...ts.sourceforge.net" <dri-users@...ts.sourceforge.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Jones <davej@...emonkey.org.uk>
Subject: Re: AGP and PAT (induced?) problem (on AMD family 6)

On 20-08-08 12:50, Rene Herman wrote:

> On 20-08-08 12:04, Ingo Molnar wrote:
> 
>> * Venki Pallipadi <venkatesh.pallipadi@...el.com> wrote:
>>
>>>> I'd really like a reply from the AGP or PAT side right about now.
>>> Hmm. Looks like there are more than 16000 entries in the PAT list!
>>
>> hm, btw., why is that?
> 
> Because 64M of AGP memory divided by 4K pages is 16K. That is, the 
> underlying problem seems to be AGP drivers using order 0 allocations. 
> I'm looking.
> 
> Do note also that this means that Venki's change would not constitite a 
> correct/final fix. Sure, caching the last entry speeds up traversing a 
> 16K entry list but the issue is that there shouldn't be a 16K entry 
> list. Through AGP, or maybe even by coalescing entries in the PAT list 
> if that's at all possible (I guess it's not really).
> 
> Even if such a more fundamental fix isn't (easily) available, the PAT 
> code already comments that the list, which is sorted by ->start value, 
> is expected to be short, and should be turned into an rbtree if it isn't 
> which might be slightly less of a bandaid.
> 
> Dave Airlie (as the MAINTAINERS entry) can't be arsed to answer email it 
> seems so I've added Dave Jones for a possible comment from the AGP side.
> If I'm reading this right upto now, still many AGP driver (among which 
> my amd-k7-agp) are affected.

This was based on a wrong reading; I was looking at the GATT allocation.

I'm giving up looking until someone can tell me whether or not those 16K 
entries are expected though. I have just one AGP card in a PAT capable 
machine.

How many entries in /debug/x86/pat_memtype_list are there on other AGP 
systems with Option "AGPSize" "64" in their xorg.conf:"Device" section 
(and their AGP aperture set to 64M or bigger in the BIOS)?

Rene.
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