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

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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

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[PATCH 0/7] dmaengine:omap-dma: Linked List transfer for slave_sg Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-14 14:50 +0200
  [PATCH 1/7] dmaengine: omap-dma: Simplify omap_dma_start_sg parameter list Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-14 14:50 +0200
  [PATCH 7/7] dmaengine: omap-dma: Support for LinkedList transfer of slave_sg Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-14 14:50 +0200
    Re: [PATCH 7/7] dmaengine: omap-dma: Support for LinkedList transfer  of slave_sg Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-07-18 12:50 +0200
      Re: [PATCH 7/7] dmaengine: omap-dma: Support for LinkedList transfer  of slave_sg Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-18 13:20 +0200
  [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on transfer completion Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-14 14:50 +0200
    Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-07-18 12:40 +0200
      Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-19 14:40 +0200
        Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-07-19 18:30 +0200
          Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-19 21:30 +0200
          Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Vinod Koul <vinod.koul@intel.com> - 2016-07-24 09:40 +0200
        Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on transfer completion Robert Jarzmik <robert.jarzmik@free.fr> - 2016-07-20 08:30 +0200
          Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-21 11:40 +0200
          Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-21 11:40 +0200
            Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-07-21 11:50 +0200
              Re: [PATCH 2/7] dmaengine: omap-dma: Complete the cookie first on  transfer completion Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-22 13:10 +0200
  [PATCH 4/7] dmaengine: omap-dma: Dynamically allocate memory for lch_map Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-14 14:50 +0200
  [PATCH 6/7] dmaengine: omap-dma: Use pointer to omap_sg in slave_sg setup's loop Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-14 14:50 +0200
  Re: [PATCH 0/7] dmaengine:omap-dma: Linked List transfer for slave_sg Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-07-18 12:40 +0200
    Re: [PATCH 0/7] dmaengine:omap-dma: Linked List transfer for slave_sg Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-18 14:10 +0200
      Re: [PATCH 0/7] dmaengine:omap-dma: Linked List transfer for slave_sg Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-07-18 14:30 +0200
        Re: [PATCH 0/7] dmaengine:omap-dma: Linked List transfer for slave_sg Peter Ujfalusi <peter.ujfalusi@ti.com> - 2016-07-18 14:40 +0200

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