linux-kernel - Re: Linux 2.6.39-rc3

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Message-ID: <4DA6242D.90408@zytor.com>
Date:	Wed, 13 Apr 2011 15:31:09 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Joerg Roedel <joro@...tes.org>
CC:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Alex Deucher <alexdeucher@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.freedesktop.org,
	Thomas Gleixner <tglx@...utronix.de>, Tejun Heo <tj@...nel.org>
Subject: Re: Linux 2.6.39-rc3

On 04/13/2011 03:22 PM, Joerg Roedel wrote:
> On Wed, Apr 13, 2011 at 03:01:10PM -0700, H. Peter Anvin wrote:
>> On 04/13/2011 02:50 PM, Joerg Roedel wrote:
>>> On Wed, Apr 13, 2011 at 01:48:48PM -0700, Yinghai Lu wrote:
>>>> -	addr = memblock_find_in_range(0, 1ULL<<32, aper_size, 512ULL<<20);
>>>> +	addr = memblock_find_in_range(0, 1ULL<<32, aper_size, 512ULL<<21);
>>>
>>> Btw, while looking at this code I wondered why the 512M goal is enforced
>>> by the alignment. Start could be set to 512M instead and the alignment
>>> can be aper_size as it should. Any reason for such a big alignment?
>>>
>>> 	Joerg
>>>
>>> P.S.: The box is still in the office, I will try this debug-patch
>>>       tomorrow.
>>
>> The only reason that I can think of is that the aperture itself can be
>> huge, and perhaps 512 MiB is the biggest such known. 
> 
> Well, that would work as well by just using aper_size as alignment, the
> aperture needs to be aligned on its size anyway. This code only runs
> when Linux allocates the aperture itself and if I am mistaken is uses
> always 64MB when doing this.

Yes, I would agree with that.  The sane thing would be to set the base
to whatever address needs to be guarded against (WHICH SHOULD BE
MOTIVATED), and use aper_size as alignment, *unless* we are only using
the initial portion of a much larger hardware structure that needs
natural alignment (which isn't clear to me, I do know we sometimes use
only a fraction of the GART, but that doesn't mean we need to
naturally-align the entire thing, nor that 512 MiB is sufficient to do so.)

	-hpa


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