linux-kernel - Re: [PATCHv2 2/4] mm: cma: Contiguous Memory Allocator added

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Message-ID: <20100727065842.40ae76c8@bike.lwn.net>
Date:	Tue, 27 Jul 2010 06:58:42 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	'Russell King - ARM Linux' <linux@....linux.org.uk>,
	Michal Nazarewicz <m.nazarewicz@...sung.com>,
	linux-mm@...ck.org, 'Daniel Walker' <dwalker@...eaurora.org>,
	Pawel Osciak <p.osciak@...sung.com>,
	'Mark Brown' <broonie@...nsource.wolfsonmicro.com>,
	linux-kernel@...r.kernel.org, 'Hiremath Vaibhav' <hvaibhav@...com>,
	'FUJITA Tomonori' <fujita.tomonori@....ntt.co.jp>,
	'Kyungmin Park' <kyungmin.park@...sung.com>,
	'Zach Pfeffer' <zpfeffer@...eaurora.org>,
	linux-media@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2 2/4] mm: cma: Contiguous Memory Allocator added

On Tue, 27 Jul 2010 14:45:58 +0200
Marek Szyprowski <m.szyprowski@...sung.com> wrote:

> > How does one obtain the CPU address of this memory in order for the CPU
> > to access it?  
> 
> Right, we did not cover such case. In CMA approach we tried to separate
> memory allocation from the memory mapping into user/kernel space. Mapping
> a buffer is much more complicated process that cannot be handled in a
> generic way, so we decided to leave this for the device drivers. Usually
> video processing devices also don't need in-kernel mapping for such
> buffers at all.

Still...that *is* why I suggested an interface which would return both
the DMA address and a kernel-space virtual address, just like the DMA
API does...  Either that, or just return the void * kernel address and
let drivers do the DMA mapping themselves.  Returning only the
dma_addr_t address will make the interface difficult to use in many
situations.

jon
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