linux-kernel - Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on transfer completion

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Message-ID: <a4b6fc99-13a9-b78f-b395-8b3e5eb30642@ti.com>
Date:	Fri, 22 Jul 2016 14:00:33 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Russell King - ARM Linux <linux@...linux.org.uk>
CC:	Robert Jarzmik <robert.jarzmik@...e.fr>, <vinod.koul@...el.com>,
	<linux-kernel@...r.kernel.org>, <tony@...mide.com>,
	<dmaengine@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on
 transfer completion

On 07/21/16 12:47, Russell King - ARM Linux wrote:
> On Thu, Jul 21, 2016 at 12:33:12PM +0300, Peter Ujfalusi wrote:
>> On 07/20/16 09:26, Robert Jarzmik wrote:
>>> Speaking of which, from a purely design point of view, as long as you think
>>> beforehand what is your sequence, ie. what is the sequence of your link
>>> chaining, completion handling, etc ..., both marking before or after next tx
>>> start should be fine IMHO.
>>
>> Yes, it might be a bit better from performance point of view if we first start
>> the pending descriptor (if there is one) then do the vchan_cookie_complete().
>> On the other hand if we care more about latency and accuracy we should
>> complete the transfer first then look for pending descriptors. But since
>> virt_dma is using a tasklet for the real completion, the latency is always
>> going to be when the tasklet is given the chance to execute.
> 
> I think this shows a slight misunderstanding of the DMA engine API.  The
> DMA completion is defined by the API to always happen in tasklet context,
> which is why the virt-dma stuff does it that way - and all other DMA
> engine drivers.  It's one of the fundamentals of the API.
> 
> As it happens in tasklet context, tasklets can be scheduled to run with
> variable latency, so any use of the DMA engine API which has a predictable
> latency around the completion handling is going to be unreliable.
> 
> Remember also that with circular buffers, there's no guarantee of getting
> period-based completion callbacks - several periods can complete and you
> are only guaranteed to get one completion callback.
> 
> So, the idea that completion callbacks can have anything to do with low
> latency or accuracy is totally incorrect.

Thanks for refreshing my memory, you are absolutely right.

-- 
Péter