On 2020/08/17 15:29 Benjamin Bara - SKIDATA Benjamin.Bara@skidata.com wrote:
We think this is not an i.MX6-specific problem, but a problem of the DMAengine usage from the PCM. In case of a XRUN, the DMA channel is never closed but first a SNDRV_PCM_TRIGGER_STOP next a SNDRV_PCM_TRIGGER_START is triggered. The SNDRV_PCM_TRIGGER_STOP simply executes a dmaengine_terminate_async() [1] but does not await the termination by calling dmaengine_synchronize(), which is required as stated by the docu [2]. Anyways, we are not able to fix it in the pcm_dmaengine layer either at the end of SNDRV_PCM_TRIGGER_STOP (called from the DMA on complete interrupt handler) or at the beginning of SNDRV_PCM_TRIGGER_START (called from a PCM ioctl), since the dmaengine_synchronize() requires a non-atomic context.
Based on my understanding, most of the DMA implementations don't even implement device_synchronize and if they do, it might not really be necessary since the terminate_all operation is synchron.
With the i.MX6, it looks a bit different: Since [4], the terminate_all operation really schedules a worker which waits the required ~1ms and then does the context freeing. Now, the ioctl(SNDRV_PCM_IOCTL_PREPARE) and the following ioctl(SNDRV_PCM_IOCTL_READI_FRAMES), which are called from US to handle/recover from a XRUN, are in a race with the terminate_worker. If the terminate_worker finishes earlier, everything is fine. Otherwise, the sdma_prep_dma_cyclic() is called, sets up everything and as soon as it is scheduled out to wait for data, the terminate_worker is scheduled and kills it. In this case, we wait in [5] until the timeout is reached and return with -EIO.
Based on our understanding, there exist two different fixing approaches: We thought that the pcm_dmaengine should handle this by either synchronizing the DMA on a trigger or terminating it synchronously. However, as we are in an atomic context, we either have to give up the atomic context of the PCM to finish the termination or we have to design a synchronous terminate variant which is callable from an atomic context.
For the first option, which is potentially more performant, we have to leave the atomic PCM context and we are not sure if we are allowed to. For the second option, we would have to divide the dma_device terminate_all into an atomic sync and an async one, which would align with the dmaengine API, giving it the option to ensure termination in an atomic context. Based on my understanding, most of them are synchronous anyways, for the currently async ones we would have to implement busy waits. However, with this approach, we reach the WARN_ON [6] inside of an atomic context, indicating we might not do the right thing.
Ad Failure Log (at bottom): I haven't added the ioctl syscalls, but this is basically the output with additional prints to be able to follow the code execution path. A XRUN (buffer size is 480 but 960 available) leads to a SNDRV_PCM_TRIGGER_STOP. This leads to terminate_async, starting the terminate_worker. Next, the XRUN recovery triggers SNDRV_PCM_TRIGGER_START, calling sdma_prep_dma_cyclic and then waits for the DMA in wait_for_avail(). Next we see the two freeings, first the old, then the newly added one; so the terminate_worker is back at work. Now the DMA is terminated, while we are still waiting on data from it.
What do you think about it? Is any of the provided solutions practicable? If you need further information or additional logging, feel free to ask.
busy_wait is not good I think, would you please have a try with the attached patch which is based on https://lkml.org/lkml/2020/8/11/111? The basic idea is to keep the freed descriptor into another list for freeing in later terminate_worker instead of freeing directly all in terminate_worker by vchan_get_all_descriptors which may break next descriptor coming soon