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

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Date:	Tue, 19 Jul 2016 22:23:12 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Russell King - ARM Linux <linux@...linux.org.uk>
CC:	<vinod.koul@...el.com>, <linux-kernel@...r.kernel.org>,
	<dmaengine@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-omap@...r.kernel.org>, <tony@...mide.com>
Subject: Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on
 transfer completion

On 07/19/2016 07:20 PM, Russell King - ARM Linux wrote:
>> vchan_cookie_complete() will only mark the cookie completed, adds the vd to
>> the desc_completed list (it was deleted from desc_issued list when it was
>> started by omap_dma_start_desc) and schedule the tasklet to deal with the real
>> completion later.
>> Marking the just finished descriptor/cookie done first then looking for
>> possible descriptors in the queue to start feels like a better sequence.
> 
> I deliberately arranged the code in the original order so that the next
> transfer was started on the hardware with the least amount of work by
> the CPU.  Yes, there may not be much in it, but everything you mention
> above adds to the number of CPU cycles that need to be executed before
> the next transfer can be started.
> 
> More CPU cycles wasted means higher latency between transfers, which
> means lower performance.

OK. I will drop this patch in v2.

>> After a quick grep in the kernel source: only omap-dma.c was starting the
>> next transfer before marking the current completed descriptor/cookie done.
> 
> Right, because I've thought about the issue, having been the author of
> both virt-dma and omap-dma.
> 


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