On 24/01/2020 9.20, Peter Ujfalusi wrote: > Hi Laurent, > > On 23/01/2020 14.23, Laurent Pinchart wrote: >>>>> I think capture (camera) is another potential beneficiary of this. >> >> Possibly, although in the camera case I'd rather have the hardware stop >> if there's no more buffer. Requiring a buffer to always be present is >> annoying from a userspace point of view. For display it's different, if >> userspace doesn't submit a new frame, the same frame should keep being >> displayed on the screen. >> >>>>> So you don't need to terminate the running interleaved_cyclic and start >>>>> a new one, but prepare and issue a new one, which would >>>>> terminate/replace the currently running cyclic interleaved DMA? >> >> Correct. >> >>>> Why not explicitly terminate the transfer and start when a new one is >>>> issued. That can be common usage for audio and display.. >>> >>> Yes, this is what I'm asking. The cyclic transfer is running and in >>> order to start the new transfer, the previous should stop. But in cyclic >>> case it is not going to happen unless it is terminated. >>> >>> When one would want to have different interleaved transfer the display >>> (or capture )IP needs to be reconfigured as well. The the would need to >>> be terminated anyways to avoid interpreting data in a wrong way. >> >> The use case here is not to switch to a new configuration, but to switch >> to a new buffer. If the transfer had to be terminated manually first, >> the DMA engine would potentially miss a frame, which is not acceptable. >> We need an atomic way to switch to the next transfer. > > You have a special hardware in hand, most DMAs can not just replace a > cyclic transfer in-flight and it also kind of violates the DMAengine > principles. Is there any specific reason why you need DMAengine driver for a display DMA? Usually the drm drivers handle their DMA internally. > If cyclic transfer is started then it is expected to run forever until > it is terminated. Preparing and issuing a new transfer will not get > executed when there is already a cyclic transfer in flight as your only > option is to terminate_all, which will kill the running cyclic _and_ > will discard the issued and pending transfers. > > So the use case is page flip when you have multiple framebuffers and you > switch them to show the updated one, right? > > There are things missing in DMAengine in API level for sure to do this, > imho. > The issue is that cyclic transfers will never complete, they run until > terminated, but you want to replace the currently executing one with a > another cyclic transfer without actually terminating the other. > > It is like pause the 1st cyclic and continue with the 2nd one. Then at > some point you pause the 2nd one and restart the 1st one. > It is also crucial that the pause /switch happens when the executing one > finished the interleaved round and not in the middle somewhere, right? > > If you: > desc_1 = dmaengine_prep_interleaved_cyclic(chan, ); > cookie_1 = dmaengine_submit(desc_1); > desc_2 = dmaengine_prep_interleaved_cyclic(chan, ); > cookie_2 = dmaengine_submit(desc_1); > > /* cookie_1/desc_1 is started */ > dma_async_issue_pending(chan); > > /* When need to switch to cookie_2 */ > dmaengine_cyclic_set_active_cookie(chan, cookie_2); > /* > * cookie_1 execution is suspended after it finished the running > * dma_interleaved_template or buffer in normal cyclic and cookie_2 > * is replacing it. > */ > > /* Switch back to cookie_1 */ > dmaengine_cyclic_set_active_cookie(chan, cookie_1); > /* > * cookie_2 execution is suspended after it finished the running > * dma_interleaved_template or buffer in normal cyclic and cookie_1 > * is replacing it. > */ > > There should be a (yet another) capabilities flag got > cyclic_set_active_cookie and the documentation should be strict on what > is the expected behavior. > > You can kill everything with terminate_all. > There is another thing which is missing imho from DMAengine: to > terminate a specific cookie, not the entire channel, which might be a > good addition as you might spawn framebuffers and then delete them and > you might want to release the corresponding cookie/descriptor as well. This is a bit trickier as DMAengine's cookie is s32 and internally treated as a running number and cookie status is checked against s32 numbers with < >, I think this will not like when someone kills a cookie in the middle. > > What do you think? > > - Péter > > Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. > Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki > - Péter Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
