[PATCH] Add duplex sound support for USB devices using implicit feedback

[Takashi Iwai]

On Sat, 23 May 2020 20:09:31 +0200,
Erwin Burema wrote:
> 
> On zaterdag 23 mei 2020 19:53:49 CEST Alexander Tsoy wrote:
> > В Вс, 10/05/2020 в 20:29 +0200, Erwin Burema пишет:
> > > For USB sound devices using implicit feedback the endpoint used for
> > > this feedback should be able to be opened twice, once for required
> > > feedback and second time for audio data. This way these devices can
> > > be put in duplex audio mode. Since this only works if the settings of
> > > the endpoint don't change a check is included for this.
> > > 
> > > This fixes bug 207023 ("MOTU M2 regression on duplex audio") and
> > > should also fix bug 103751 ("M-Audio Fast Track Ultra usb audio
> > > device will not operate full-duplex")
> > > 
> > > Signed-off-by: Erwin Burema <e.burema at gmail.com>
> > > ---
> > 
> > This patch seems to cause kernel panic on my system. This happens
> > during boot when gdm (with pulseaudio) is starting up.
> >
> 
> That's interesting, not running gnome (and thus also not running gdm, 
> currently KDE with SDDM) here so would need to take some time troubleshooting. 
> Suspect I missed something in the check if both input and output are similarly 
> configured.
> 
> Will see if I can reproduce it and where exactly it goes wrong, in the 
> meantime would it be possible to test if 5.6 or later show the same issue 
> since I intially developed the patch against that release?

Judging from the point triggering the bug (memset()), this can be a
leftover URB handling that is performed even after the capture stream
is closed.  If so, the procedure would be:
 - start capture
 - start playback
 - stop capture while keeping playback running

If my guess is correct, can the patch below work around the issue?


thanks,

Takashi

---
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -1782,6 +1782,7 @@ static int snd_usb_substream_capture_trigger(struct snd_pcm_substream *substream
 		return 0;
 	case SNDRV_PCM_TRIGGER_STOP:
 		stop_endpoints(subs);
+		subs->data_endpoint->retire_data_urb = NULL;
 		subs->running = 0;
 		return 0;
 	case SNDRV_PCM_TRIGGER_PAUSE_PUSH: