linux-kernel - Re: [PATCH 7/8] dmaengine/dw_dmac: Unmap all memory buffers after completion of slave transfers

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Date:	Mon, 16 Jan 2012 10:54:48 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Viresh Kumar <viresh.kumar@...com>
Cc:	vinod.koul@...el.com, dan.j.williams@...el.com,
	pratyush.anand@...com, rajeev-dlh.kumar@...com,
	bhupesh.sharma@...com, armando.visconti@...com,
	linus.walleij@...aro.org, mirko.gardi@...com,
	linux-kernel@...r.kernel.org, vipin.kumar@...com,
	shiraz.hashim@...com, amit.virdi@...com, vipulkumar.samar@...com,
	viresh.linux@...il.com, deepak.sikri@...com, bhavna.yadav@...com,
	linux-arm-kernel@...ts.infradead.org, vincenzo.frascino@...com
Subject: Re: [PATCH 7/8] dmaengine/dw_dmac: Unmap all memory buffers after
	completion of slave transfers

On Mon, Jan 16, 2012 at 03:20:35PM +0530, Viresh Kumar wrote:
> Currently, after completion of transfer, source address or destination address
> of only the first LLI descriptor is unmapped. And length passed for unmap is
> total length of all descriptors in the list. Which means unmapping code assumed
> that the memory buffers pointed to by the descriptors will be physically
> contiguous, which might not be the case. It is possible for other drivers to
> pass sglist to slave_sg(), in which all buffers are scattered throughout the
> memory.
> 
> This patch intends to fix this wrong expectation of dw_dmac. Now, first desc
> will not contain total length of transfer. But individual descriptors will
> contain their individual lengths. Finally, we will call unmap for all
> descriptors.

Note that DMA engine drivers are not responsible for unmapping the buffers
when the transfer completes - that is the responsibility of the caller.

The automatic buffer unmapping is required for the async_tx APIs and
offload APIs.
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