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

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Message-ID: <48ABF6DC.8070305@keyaccess.nl>
Date:	Wed, 20 Aug 2008 12:50:04 +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 12:04, 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.

In the short run and if I'm not just mistaken, the best fix might be to 
make PAT dependent on not having a dumb AGP driver (but as said, still 
looking).

Note that my chipset is capable of a 2G AGP aperture. That's 512K pages 
if fully used, 256K for 1G, 128K for 512M, ...

Rene.
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