linux-kernel - Re: [PATCH v2 1/4] dma-mapping: Add devm_ interface for dma_map

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Date:	Wed, 4 Jun 2014 23:25:25 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Eli Billauer <eli.billauer@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, Shuah Khan <shuah.kh@...sung.com>,
	devel@...verdev.osuosl.org, gregkh@...uxfoundation.org,
	bhelgaas@...gle.com, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, iommu@...ts.linux-foundation.org,
	discuss@...-64.org
Subject: Re: [PATCH v2 1/4] dma-mapping: Add devm_ interface for
 dma_map_single()

Hi,

On Wed, Jun 04, 2014 at 06:03:36PM +0300, Eli Billauer wrote:
> I believe that I need a managed dma_map_single() my own driver,
> which doesn't fall in the case of a single use: The driver allocates
> its buffers with __get_free_pages() (or the to-be managed version of
> it). Then it cuts the allocated memory into smaller buffers (in some
> cases, and with certain alignment rules), and  then calls
> dma_map_single() to do the DMA mapping for each. The buffers are
> held along the driver's lifetime, with DMA sync API juggling control
> back and forth to the hardware. Note that the DMA is noncoherent.

What you are trying to do should work with dma_alloc_noncoherent(). The
API allows partial syncs on this memory, so you should be fine.

The problem with a devm variant of dma_map_* is that it is too easy to
misuse or to use it wrong so that a driver could eat up all available
DMA handles on some platforms.


	Joerg


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