From: robert.jarzmik@free•fr (Robert Jarzmik)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v4 3/4] dma: mmp_pdma: add support for residue reporting
Date: Mon, 26 Aug 2013 08:07:34 +0200 [thread overview]
Message-ID: <87vc2touhl.fsf@free.fr> (raw)
In-Reply-To: 20130825180615.GX6617@n2100.arm.linux.org.uk
Hi Russell,
Russell King - ARM Linux <linux@arm•linux.org.uk> writes:
> Even when there is a callback set, there's no requirement for the DMA
> engine driver to keep the descriptor around after it has been submitted -
> it is free to copy that data into whatever internal representation it
> requires and then discard the descriptor if it so wishes.
That means that a driver which wished to "resubmit" the descriptor has to
"rebuild it" again, no ?
I was under the impression that a descriptor had the guarantee to survive even
after completion. This comes from include/linux/dmaengine.h:168, in enum
dma_ctrl_flags comment :
* @DMA_CTRL_ACK - if clear, the descriptor cannot be reused until the client
* acknowledges receipt, i.e. has has a chance to establish any dependency
* chains
The sentence "cannot be reused until" meant that after it could be reused.
And there are usecases for that, for example a video capture buffer. The source
for DMA is always the same (camera FIFO), the destination is always the same
(ie. the same scatter-gather). Why rebuild all the info to resubmit the same
descriptor ?
So is my understand incorrect, and in that case is there a way with dmaengine to
"resubmit" a descriptor, without going through the whole preparation ?
Cheers.
--
Robert
next prev parent reply other threads:[~2013-08-26 6:07 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-16 17:05 [PATCH v4 0/4] dma: pdma: cyclic, residue, DMA_PRIVATE Daniel Mack
2013-08-16 17:05 ` [PATCH v4 1/4] dma: mmp_pdma: only complete one transaction from dma_do_tasklet() Daniel Mack
2013-08-20 2:27 ` Xiang Wang
2013-08-20 7:44 ` Daniel Mack
2013-08-25 1:15 ` Robert Jarzmik
2013-08-16 17:05 ` [PATCH v4 2/4] dma: mmp_pdma: add support for cyclic DMA descriptors Daniel Mack
2013-08-16 17:05 ` [PATCH v4 3/4] dma: mmp_pdma: add support for residue reporting Daniel Mack
2013-08-21 11:35 ` Xiang Wang
2013-08-21 11:52 ` Andy Shevchenko
2013-08-21 12:03 ` Daniel Mack
2013-08-21 12:08 ` Andy Shevchenko
2013-08-25 16:11 ` Vinod Koul
2013-08-25 17:01 ` Russell King - ARM Linux
2013-08-25 16:48 ` Vinod Koul
2013-08-25 18:06 ` Russell King - ARM Linux
2013-08-26 6:07 ` Robert Jarzmik [this message]
2013-08-21 11:55 ` Daniel Mack
2013-08-16 17:05 ` [PATCH v4 4/4] dma: mmp_pdma: set DMA_PRIVATE Daniel Mack
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