On Thu, Mar 23, 2023 at 07:19:45AM +0100, Takashi Iwai wrote:
On Tue, 21 Mar 2023 05:02:58 +0100, Takashi Sakamoto wrote:
Hi,
On Mon, Mar 20, 2023 at 03:28:38PM +0100, Takashi Iwai wrote:
The recent support of low latency playback in USB-audio driver made the snd_usb_queue_pending_output_urbs() function to be called via PCM ack ops. In the new code path, the function is performed alread in
's/alread/already/' or slang.
the PCM stream lock. The problem is that, when an XRUN is detected, the function calls snd_pcm_xrun() to notify, but snd_pcm_xrun() is
's/the function/the function/'
Corrected at applying.
supposed to be called only outside the stream lock. As a result, it leads to a deadlock of PCM stream locking.
For avoiding such a recursive locking, this patch adds an additional check to the code paths in PCM core that call the ack callback; now it checks the error code from the callback, and if it's -EPIPE, the XRUN is handled in the PCM core side gracefully. Along with it, the USB-audio driver code is changed to follow that, i.e. -EPIPE is returned instead of the explicit snd_pcm_xrun() call when the function is performed already in the stream lock.
Practically, the implementation of 'pcm_hw' in alsa-lib never evaluates the return value (see 'snd_pcm_hw_mmap_commit()' and the others). I guess that it is inconvenient for the low-latency mode of USB Audio device class driver for the case of failure.
My additional concern is PCM indirect layer, since typically the layer is used by drivers with SNDRV_PCM_INFO_SYNC_APPLPTR. But as long as I read, the change does not matter to them.
I find rather that's an extra bonus :) It allows the existing code to give a more proper error handling. For example, the indirect PCM helper returned -EINVAL, but this can be switched to -EPIPE for stopping the stream.
I'm going to submit the patch together with the documentation updates.
In my opinion, extra care is required for the simple replacement of -EINVAL with -EPIPE, since it may not come from any operation relevant to hardware. In my understanding, it comes from just careless implementation of user space PCM application, thus it is still reasonable to return -EINVAL.
... However, you are the best person who has enough knowledge about the layer. I would not mind the replacement if you did.
Regards
Takashi Sakamoto