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

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Date:	Thu, 21 Aug 2008 07:02:00 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"Rene Herman" <rene.herman@...access.nl>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Venki Pallipadi" <venkatesh.pallipadi@...el.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 Wed, Aug 20, 2008 at 8:50 PM, Rene Herman <rene.herman@...access.nl> 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.

I haven't anything to add, I'm the maintainer not the author, all the
people who wrote the offending code were
already involved.

Dave.
>
> 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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