ALSA: usb-audio: Avoid killing in-flight URBs during draining

While draining a stream, ALSA PCM core stops the stream by issuing
snd_pcm_stop() after all data has been sent out.  And, at PCM trigger
stop, currently USB-audio driver kills the in-flight URBs explicitly,
then at sync-stop ops, sync with the finish of all remaining URBs.
This might result in a drop of the drained samples as most of
USB-audio devices / hosts allow relatively long in-flight samples (as
a sort of FIFO).

For avoiding the trimming, this patch changes the stream-stop behavior
during PCM draining state.  Under that condition, the pending URBs
won't be killed.  The leftover in-flight URBs are caught by the
sync-stop operation that shall be performed after the trigger-stop
operation.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

sound/usb/endpoint.c

@@ -955,7 +955,7 @@ void snd_usb_endpoint_sync_pending_stop(struct snd_usb_endpoint *ep)
  *
  * This function moves the EP to STOPPING state if it's being RUNNING.
  */
-static int stop_urbs(struct snd_usb_endpoint *ep, bool force)
+static int stop_urbs(struct snd_usb_endpoint *ep, bool force, bool keep_pending)
 {
 	unsigned int i;
 	unsigned long flags;
@@ -972,6 +972,9 @@ static int stop_urbs(struct snd_usb_endpoint *ep, bool force)
 	ep->next_packet_queued = 0;
 	spin_unlock_irqrestore(&ep->lock, flags);
 
+	if (keep_pending)
+		return 0;
+
 	for (i = 0; i < ep->nurbs; i++) {
 		if (test_bit(i, &ep->active_mask)) {
 			if (!test_and_set_bit(i, &ep->unlink_mask)) {
@@ -995,7 +998,7 @@ static int release_urbs(struct snd_usb_endpoint *ep, bool force)
 	snd_usb_endpoint_set_callback(ep, NULL, NULL, NULL);
 
 	/* stop and unlink urbs */
-	err = stop_urbs(ep, force);
+	err = stop_urbs(ep, force, false);
 	if (err)
 		return err;
 
@@ -1527,14 +1530,15 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
 	return 0;
 
 __error:
-	snd_usb_endpoint_stop(ep);
+	snd_usb_endpoint_stop(ep, false);
 	return -EPIPE;
 }
 
 /**
  * snd_usb_endpoint_stop: stop an snd_usb_endpoint
  *
  * @ep: the endpoint to stop (may be NULL)
+ * @keep_pending: keep in-flight URBs
  *
  * A call to this function will decrement the running count of the endpoint.
  * In case the last user has requested the endpoint stop, the URBs will
@@ -1545,7 +1549,7 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep)
  * The caller needs to synchronize the pending stop operation via
  * snd_usb_endpoint_sync_pending_stop().
  */
-void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
+void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep, bool keep_pending)
 {
 	if (!ep)
 		return;
@@ -1560,7 +1564,7 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
 	if (!atomic_dec_return(&ep->running)) {
 		if (ep->sync_source)
 			WRITE_ONCE(ep->sync_source->sync_sink, NULL);
-		stop_urbs(ep, false);
+		stop_urbs(ep, false, keep_pending);
 	}
 }
 

sound/usb/endpoint.h

@@ -38,7 +38,7 @@ void snd_usb_endpoint_set_callback(struct snd_usb_endpoint *ep,
 				   struct snd_usb_substream *data_subs);
 
 int snd_usb_endpoint_start(struct snd_usb_endpoint *ep);
-void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep);
+void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep, bool keep_pending);
 void snd_usb_endpoint_sync_pending_stop(struct snd_usb_endpoint *ep);
 void snd_usb_endpoint_suspend(struct snd_usb_endpoint *ep);
 int  snd_usb_endpoint_activate(struct snd_usb_endpoint *ep);

sound/usb/pcm.c

@@ -219,16 +219,16 @@ int snd_usb_init_pitch(struct snd_usb_audio *chip,
 	return 0;
 }
 
-static bool stop_endpoints(struct snd_usb_substream *subs)
+static bool stop_endpoints(struct snd_usb_substream *subs, bool keep_pending)
 {
 	bool stopped = 0;
 
 	if (test_and_clear_bit(SUBSTREAM_FLAG_SYNC_EP_STARTED, &subs->flags)) {
-		snd_usb_endpoint_stop(subs->sync_endpoint);
+		snd_usb_endpoint_stop(subs->sync_endpoint, keep_pending);
 		stopped = true;
 	}
 	if (test_and_clear_bit(SUBSTREAM_FLAG_DATA_EP_STARTED, &subs->flags)) {
-		snd_usb_endpoint_stop(subs->data_endpoint);
+		snd_usb_endpoint_stop(subs->data_endpoint, keep_pending);
 		stopped = true;
 	}
 	return stopped;
@@ -261,7 +261,7 @@ static int start_endpoints(struct snd_usb_substream *subs)
 	return 0;
 
  error:
-	stop_endpoints(subs);
+	stop_endpoints(subs, false);
 	return err;
 }
 
@@ -437,7 +437,7 @@ static int configure_endpoints(struct snd_usb_audio *chip,
 
 	if (subs->data_endpoint->need_setup) {
 		/* stop any running stream beforehand */
-		if (stop_endpoints(subs))
+		if (stop_endpoints(subs, false))
 			sync_pending_stops(subs);
 		err = snd_usb_endpoint_configure(chip, subs->data_endpoint);
 		if (err < 0)
@@ -572,7 +572,7 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream)
 	subs->cur_audiofmt = NULL;
 	mutex_unlock(&chip->mutex);
 	if (!snd_usb_lock_shutdown(chip)) {
-		if (stop_endpoints(subs))
+		if (stop_endpoints(subs, false))
 			sync_pending_stops(subs);
 		close_endpoints(chip, subs);
 		snd_usb_unlock_shutdown(chip);
@@ -1559,7 +1559,7 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea
 		return 0;
 	case SNDRV_PCM_TRIGGER_SUSPEND:
 	case SNDRV_PCM_TRIGGER_STOP:
-		stop_endpoints(subs);
+		stop_endpoints(subs, substream->runtime->status->state == SNDRV_PCM_STATE_DRAINING);
 		snd_usb_endpoint_set_callback(subs->data_endpoint,
 					      NULL, NULL, NULL);
 		subs->running = 0;
@@ -1607,7 +1607,7 @@ static int snd_usb_substream_capture_trigger(struct snd_pcm_substream *substream
 		return 0;
 	case SNDRV_PCM_TRIGGER_SUSPEND:
 	case SNDRV_PCM_TRIGGER_STOP:
-		stop_endpoints(subs);
+		stop_endpoints(subs, false);
 		fallthrough;
 	case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
 		snd_usb_endpoint_set_callback(subs->data_endpoint,