linux-kernel - Re: CMA alignment question

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Date:	Tue, 4 Nov 2014 20:18:58 -0800
From:	Gregory Fong <gregory.0xf0@...il.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	linux-mm@...ck.org, Laura Abbott <lauraa@...eaurora.org>,
	iamjoonsoo.kim@....com,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Brian Norris <computersforpeace@...il.com>
Subject: Re: CMA alignment question

On Tue, Nov 4, 2014 at 2:27 PM, Michal Nazarewicz <mina86@...a86.com> wrote:
> On Tue, Nov 04 2014, Gregory Fong wrote:
>> The alignment in cma_alloc() is done w.r.t. the bitmap.  This is a
>> problem when, for example:
>>
>> - a device requires 16M (order 12) alignment
>> - the CMA region is not 16 M aligned
>>
>> In such a case, can result with the CMA region starting at, say,
>> 0x2f800000 but any allocation you make from there will be aligned from
>> there.  Requesting an allocation of 32 M with 16 M alignment, will
>> result in an allocation from 0x2f800000 to 0x31800000, which doesn't
>> work very well if your strange device requires 16M alignment.
>>
>> This doesn't have the behavior I would expect, which would be for the
>> allocation to be aligned w.r.t. the start of memory.  I realize that
>> aligning the CMA region is an option, but don't see why cma_alloc()
>> aligns to the start of the CMA region.  Is there a good reason for
>> having cma_alloc() alignment work this way?
>
> No, it's a bug.  The alignment should indicate alignment of physical
> address not position in CMA region.
>

Ah, now I see that Marek submitted this patch from you back in 2011
that would have allowed the bitmap lib to support an alignment offset:
http://thread.gmane.org/gmane.linux.kernel/1121103/focus=1121100

Any idea why this didn't make it into the later changesets?  If not,
I'll resubmit it and to use it to fix this bug.

Thanks,
Gregory
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