linux-kernel - Re: Linux 2.6.39-rc3

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Date:	Wed, 13 Apr 2011 15:01:10 -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 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.  512ULL<<21 is of
course a particularly moronic way to write 1 GiB, but it was a debug patch.

The value 512 MiB apparently comes from
7677b2ef6c0c4fddc84f6473f3863f40eb71821b, which is apparently totally ad
hoc; effectively it tries to prevent a collision with kexec by
hardcoding the kdump allocation as it sat at that point in time in the
GART assignment rules.

Yeah.  Brilliant.

	-hpa

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