linux-kernel - Re: [PATCH] ep93xx: Implement double buffering for M2M DMA channels

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Date:	Thu, 22 Mar 2012 11:00:37 +0100
From:	Rafal Prylowski <prylowski@...asoft.pl>
To:	Ryan Mallon <rmallon@...il.com>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	Mika Westerberg <mika.westerberg@....fi>,
	H Hartley Sweeten <hartleys@...ionengravers.com>
Subject: Re: [PATCH] ep93xx: Implement double buffering for M2M DMA channels

On 2012-03-22 01:57, Ryan Mallon wrote:
> I haven't looked through the patch yet, since I'm waiting on more
> information from Mika and Hartley's testing.
> 
> However, the commit log doesn't tell me why we want this change. Is it a
> performance improvement? If so, do you have some numbers that we can
> paste into the commit log?

In principle, using double buffering should be faster than using only
one buffer and disabling/enabling channel each time. But my measurements
doesn't show any significant change.

The real reason for this change is that current code is not 100% reliable
in IDE-DMA (I'm planning to submit ep93xx ide driver to linux-ide soon).
Although using only one buffer and DONE interrupt is simple, surprisingly
we can get interrupt when M2M DMA is in DMA_BUF_ON and DMA_STALL state.
If we disable the channel at this moment, we end with ata timeout.
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