Dne 22. 12. 21 v 13:26 Pavel Hofman napsal(a):

Dne 21. 12. 21 v 13:18 John Keeping napsal(a):

quoted

On Mon, Dec 20, 2021 at 10:11:26PM +0100, Pavel Hofman wrote:

quoted

When the USB host stops capture/playback, the corresponding PCM
substream (if activated and running) is stopped and drained.

Signed-off-by: Pavel Hofman <redacted>
---
  drivers/usb/gadget/function/u_audio.c | 16 ++++++++++++++++
  1 file changed, 16 insertions(+)

diff --git a/drivers/usb/gadget/function/u_audio.c 

b/drivers/usb/gadget/function/u_audio.c
index a6293415c071..9dbce51c2eb7 100644

--- a/drivers/usb/gadget/function/u_audio.c
+++ b/drivers/usb/gadget/function/u_audio.c

@@ -544,6 +544,20 @@ static void set_reported_srate(struct 

uac_rtd_params *prm, int srate)
      }
  }
+static void stop_substream(struct uac_rtd_params *prm)
+{
+    unsigned long _flags;
+    struct snd_pcm_substream *substream;
+
+    substream = prm->ss;
+    if (substream) {
+        snd_pcm_stream_lock_irqsave(substream, _flags);
+        if (snd_pcm_running(substream))
+            snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);

I'm not sure if this is right (and the series should probably be CC'd to
alsa-devel to check the audio side of this).

DRAINING seems to be right for capture, but for playback should this end
up in state SETUP?  Does this need to handle resuming a paused stream
like snd_pcm_drain() does?

Honestly, I do not know. This code comes from a SPDIF receiver driver 
where it handles interrupted incoming SPDIF stream. You are right it is 
related to capture. I will ask the alsa devs about the playback solution 
specifically.

Yes I will CC the next version to alsa-dev just in case.

I have not received a response from the alsa devels yet, however based 
on existing variants in the other drivers in the kernel there are two 
options:

if (substream) {
   snd_pcm_stream_lock_irqsave(substream, _flags);
   if (snd_pcm_running(substream)) {
     if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
       snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP);
     else
       snd_pcm_stop(substream, SNDRV_PCM_STATE_DRAINING);
   }
   snd_pcm_stream_unlock_irqrestore(substream, _flags);
}


When testing playback/capture, sox does not exit with this version (only 
reporting an error), recovering if playback/capture resume within 
standard alsa blocking read/write timeout. Aplay/arecord exit.

Another option is

if (substream) {
   snd_pcm_stream_lock_irqsave(substream, _flags);
   if (snd_pcm_running(substream))
     snd_pcm_stop(substream, SNDRV_PCM_STATE_DISCONNECTED);
   snd_pcm_stream_unlock_irqrestore(substream, _flags);
}

Here both sox and aplay/arecord exit when the pcm stream is stopped. 
 From my use case I like this option better.

Or the gadget can use SETUP/DRAINING for host-side playback/capture stop 
and DISCONNECTED for USB cable unplugging. But I am not sure whether 
from the gadget side POW there is a any real difference between the host 
stopping playback/capture and cable disconnection.

Also this patch would change behaviour of the gadget for existing 
installations, as the existing version just stops data 
delivery/consumption. Maybe some installations already count on this 
behaviour. Perhaps the snd_pcm_stop call should be an opt-in via a new 
configfs parameter?

Please can existing gadget users comment on this?

Thanks a lot,

Pavel.