linux-kernel - Re: [PATCH] dmaengine: Add API documentation for slave dma usage

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Date:	Tue, 24 May 2011 14:03:30 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	"Koul, Vinod" <vinod.koul@...el.com>
Cc:	Per Forlin <per.forlin@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] dmaengine: Add API documentation for slave dma usage

On Tue, May 24, 2011 at 9:06 AM, Koul, Vinod <vinod.koul@...el.com> wrote:
> On Tue, 2011-05-24 at 17:40 +0200, Per Forlin wrote:
>> Hi Vinod,
>>
>> On Tue, May 24, 2011 at 2:04 PM, Koul, Vinod <vinod.koul@...el.com> wrote:
>> > From: Vinod Koul <vinod.koul@...el.com>
>> >
>> > Signed-off-by: Vinod Koul <vinod.koul@...el.com>
>> > ---
>> >  Documentation/dma-slave-api.txt |   74 +++++++++++++++++++++++++++++++++++++++
>> >  1 files changed, 74 insertions(+), 0 deletions(-)
>> >  create mode 100644 Documentation/dma-slave-api.txt
>> I suggest putting this in subsection of dmaengine.txt instead.
>> dmaengine.txt would be the natural place to look at.
> Agreed, that would make it easier for people to find
>
>> > +4. Submit the transaction(s) and wait for callback notification when slave API
>> > +is 3 above returns, the non NULL value would imply a "descriptor" for the
>> > +transaction. These transaction(s) would need to be submitted which pushes them
>> > +into queue in DMA driver. If DMA is idle then the first descriptor submit will
>> > +be pushed to DMA and subsequent ones will be queued. On completion of the DMA
>> > +operation the next in queue is submitted and a tasklet triggered. The tasklet
>> > +would then call the client driver completion callback routine for notification,
>> > +if set.
>> > +
>> Does submit really start the transfer as well? My interpretation of
>> submit is that is only adds desc to a pending queue. When calling
>> issue_pending all these descs will be schedule for DMA transfer. Calls
>> to submit after this point will added to the pending queue again and
>> not be issued until calling issue_pending once more.
> For slave dma devices, submit() is used to start the transaction if the
> channel is idle. If its already doing a transaction then it will queue
> it up and submit once cureent excuting one is completed. It is not
> required to call issue_pending once more.
> I am not sure if this is true for non slave usage, Dan would that be
> correct for you as well?

No, ->submit() is just an "add this descriptor to the chain"
operation, and ->issue_pending() is always required to make sure the
everything submitted previously is actively executing.  This was a
holdover from the very first dmaengine implementation where, for
efficiency reasons, it could save mmio writes by batching the issuing
of requests.

--
Dan
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