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

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Message-ID: <48AC89C8.1040804@keyaccess.nl>
Date:	Wed, 20 Aug 2008 23:16:56 +0200
From:	Rene Herman <rene.herman@...access.nl>
To:	Dave Airlie <airlied@...il.com>
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 20-08-08 23:02, Dave Airlie wrote:

> 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.

[ ... ]

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

The underlying problem is the order 0 allocations (agp_allocate_memory 
--> agp_generic_allocate_page) where each single page is set uncached 
individually, creating a PAT entry.

Non order 0 allocations generally would ofcourse help. That's very much 
AGP internal -- do you feel that's the way to go?

All the current AGP drivers except sgi-agp use agp_generic_alloc_page().

Doing a quick local hack to collect pages in agp_allocate_memory() into 
regions and set the regions (generally 1) UC in one fell swoop, but I 
don't know if that's safe (and it feels like a rather poor hack anyway).

(not to mention that it's time for bed again).

Rene.
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