On Fri, Jul 10, 2015 at 07:06:55PM +0200, Lars-Peter Clausen wrote: > On 07/10/2015 07:01 PM, Vinod Koul wrote: > [...] > >>>To handle the video case something else is needed. The current dmaengine > >>>semantics are that once a descriptor is passed to dma_engine_submit() it is > >>>owned by the DMAengine driver and the client must never ever touch it > >>>again. > >> > >>>Commit b9855f03d560 overloaded DMA_CTRL_ACK to mean that the should be > >>>re-usable by the client. But this breaks existing users of DMA_CTRL_ACK since > >>>its a completely new and different meaning to the existing one. For marking a > >>>descriptor as reusable for the client a new flag should probably be > >>>introduced. > >Lars, > >IIRC the usage in sound/ was to ensure we dont clean up the descriptor once > >the dma driver completes it so that we keep on using the same descriptor in > >cyclic fashion, right? > > No, this has nothing to do with cyclic DMA. The flag needs to be set > otherwise drivers with descriptor pools will consider the descriptor > to be in use, even if we are done using it and hence the it will run > out of descriptors eventually. > > > > >>Maybe yes. > >>The problem will be to list the existing users of DMA_CTRL_ACK (dmaengine slave > >>API side), and see what is their expectation from this bit. > >> > >> From the discussion we had with Vinod, there was no clear definition so far for > >>DMA_CTRL_ACK (still for dmaengine slave API). I might have overlooked something > >>here, so any input will be valuable. > >I should have realised sound/ uses it and check this bit. Worst case we need > >another flag > > It's not just sound. The recommendation has always been set this > flag unless you know why you are not setting it. So pretty much > every DMAengine client driver sets it. Actually there has never been a recommendation for slave cases. It came from async API and slave users should never have used it, untill now. Now to solve the mess, either we can revert this patch and find a new one for actual reuse of descriptors. Alternate would be fix users I am leaning towards former then follow up by fixing the users who dont want this. Since this is going to be a tree wide update it will need one merge cycle to complete. Once done we can bring back the usage required for actual descriptor reuse case. This will lead only one broken driver from Robert. Is the client for your driver merged? -- ~Vinod -- 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
