linux-kernel - CMA alignment question

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Date:	Tue, 4 Nov 2014 13:03:22 -0800
From:	Gregory Fong <gregory.0xf0@...il.com>
To:	linux-mm@...ck.org
Cc:	lauraa@...eaurora.org, iamjoonsoo.kim@....com, mina86@...a86.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: CMA alignment question

Hi all,

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?

Thanks and regards,
Gregory
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