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 -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
