On 07/21/16 12:33, Peter Ujfalusi wrote: > On 07/20/16 09:26, Robert Jarzmik wrote: >> Peter Ujfalusi <peter.ujfalusi@xxxxxx> writes: >> >>> On 07/18/16 13:34, Russell King - ARM Linux wrote: >>>> On Thu, Jul 14, 2016 at 03:42:37PM +0300, Peter Ujfalusi wrote: >>>>> Before looking for the next descriptor to start, complete the just finished >>>>> cookie. >>>> >>>> This change will reduce performance as we no longer have an overlap >>>> between the next request starting to be dealt with in the hardware >>>> vs the previous request being completed. >>> >>> 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. >>> >>> 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. >> >> Euh actually I think it's done in other drivers as well : >> - Documentation/dmaengine/pxa_dma.txt (chapter "Transfers hot-chaining) >> - drivers/dma/pxa_dma.c >> => look for pxad_try_hotchain() and it's impact on pxad_chan_handler() which >> will mark the completion while the next transfer is already pumped by the >> hardware. > > The 'hot-chaining' is a bit different then what omap-dma is doing. s/then/than > If I got it > right. When the DMA is running and a new request comes the driver will append > the new transfer to the list used by the HW. This way there will be no stop > and restart needed, the DMA is running w/o interruption. > >> 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. > >> So in your quest for the "better sequence" the pxa driver's one might give you >> some perspective :) > > I did thought about similar 'hot-chaining' for TI's eDMA and sDMA. Especially > eDMA would benefit from it, but so far I see too many race conditions to > overcome to be brave enough to write something to test it. and I don't have > time for it atm ;) > -- 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
