linux-kernel - Re: CMA alignment question

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Message-ID: <xa1tlhnq7ga7.fsf@mina86.com>
Date:	Tue, 04 Nov 2014 23:27:28 +0100
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Gregory Fong <gregory.0xf0@...il.com>, linux-mm@...ck.org
Cc:	lauraa@...eaurora.org, iamjoonsoo.kim@....com,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"linux-kernel\@vger.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 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.

-- 
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