ALSA: usb-audio: Notify xrun for low-latency mode

The low-latency mode of USB-audio driver uses a similar approach like
the implicit feedback mode but it has an explicit queuing at the
trigger start time.  The difference is, however, that no packet will
be handled any longer after all queued packets are handled but no
enough data is fed.  In the case of implicit feedback mode, the
capture-side packet handling triggers the re-queuing, and this checks
the XRUN.  OTOH, in the low-latency mode, it just stops without XRUN
notification unless any new action is taken from user-space via ack
callback.  For example, when you stop the stream in aplay, no XRUN is
reported.

This patch adds the XRUN check at the packet complete callback in the
case all pending URBs are exhausted.  Strictly speaking, this state
doesn't match really with XRUN; in theory the application may queue
immediately after this happens.  But such behavior is only for
1-period configuration, which the USB-audio driver doesn't support.
So we may conclude that this situation leads certainly to XRUN.

A caveat is that the XRUN should be triggered only for the PCM RUNNING
state, and not during DRAINING.  This additional state check is put in
notify_xrun(), too.

Fixes: d5f871f ("ALSA: usb-audio: Improved lowlatency playback support")
Reported-by: Leonard Crestez <cdleonard@gmail.com>
Link: https://lore.kernel.org/25d5b0d8-4efd-4630-9d33-7a9e3fa9dc2b@gmail.com
Link: https://patch.msgid.link/20241128080446.1181-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/endpoint.c

@@ -403,10 +403,15 @@ static int prepare_inbound_urb(struct snd_usb_endpoint *ep,
 static void notify_xrun(struct snd_usb_endpoint *ep)
 {
 	struct snd_usb_substream *data_subs;
+	struct snd_pcm_substream *psubs;
 
 	data_subs = READ_ONCE(ep->data_subs);
-	if (data_subs && data_subs->pcm_substream)
-		snd_pcm_stop_xrun(data_subs->pcm_substream);
+	if (!data_subs)
+		return;
+	psubs = data_subs->pcm_substream;
+	if (psubs && psubs->runtime &&
+	    psubs->runtime->state == SNDRV_PCM_STATE_RUNNING)
+		snd_pcm_stop_xrun(psubs);
 }
 
 static struct snd_usb_packet_info *
@@ -562,7 +567,10 @@ static void snd_complete_urb(struct urb *urb)
 			push_back_to_ready_list(ep, ctx);
 			clear_bit(ctx->index, &ep->active_mask);
 			snd_usb_queue_pending_output_urbs(ep, false);
-			atomic_dec(&ep->submitted_urbs); /* decrement at last */
+			/* decrement at last, and check xrun */
+			if (atomic_dec_and_test(&ep->submitted_urbs) &&
+			    !snd_usb_endpoint_implicit_feedback_sink(ep))
+				notify_xrun(ep);
 			return;
 		}