ALSA: usb-audio: Reduce latency at playback start

USB-audio driver behaves a bit strangely for the playback stream --
namely, it starts sending silent packets at PCM prepare state while
the actual data is submitted at first when the trigger START is kicked
off.  This is a workaround for the behavior where URBs are processed
too quickly at the beginning.  That is, if we start submitting URBs at
trigger START, the first few URBs will be immediately completed, and
this would result in the immediate period-elapsed calls right after
the start, which may confuse applications.

OTOH, submitting the data after silent URBs would, of course, result
in a certain delay of the actual data processing, and this is rather
more serious problem on modern systems, in practice.

This patch tries to revert the workaround and lets the URB submission
starting at PCM trigger for the playback again.  As far as I've tested
with various backends (native ALSA, PA, JACK, PW), I haven't seen any
problems (famous last words :)

Note that the capture stream handling needs no such workaround, since
the capture is driven per received URB.

Link: https://lore.kernel.org/r/20210601162457.4877-6-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -613,11 +613,6 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
 	subs->last_frame_number = 0;
 	runtime->delay = 0;
 
-	/* for playback, submit the URBs now; otherwise, the first hwptr_done
-	 * updates for all URBs would happen at the same time when starting */
-	if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK)
-		ret = start_endpoints(subs);
-
  unlock:
 	snd_usb_unlock_shutdown(chip);
 	return ret;
@@ -1430,6 +1425,7 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea
 					      int cmd)
 {
 	struct snd_usb_substream *subs = substream->runtime->private_data;
+	int err;
 
 	switch (cmd) {
 	case SNDRV_PCM_TRIGGER_START:
@@ -1440,6 +1436,14 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea
 					      prepare_playback_urb,
 					      retire_playback_urb,
 					      subs);
+		if (cmd == SNDRV_PCM_TRIGGER_START) {
+			err = start_endpoints(subs);
+			if (err < 0) {
+				snd_usb_endpoint_set_callback(subs->data_endpoint,
+							      NULL, NULL, NULL);
+				return err;
+			}
+		}
 		subs->running = 1;
 		dev_dbg(&subs->dev->dev, "%d:%d Start Playback PCM\n",
 			subs->cur_audiofmt->iface,