ALSA: usb-audio: Rename early_playback_start flag with lowlatency_playback

This is a preparation patch for the upcoming low-latency improvement
changes.

Rename early_playback_start flag with lowlatency_playback as it's more
intuitive.  The new flag is basically a reverse meaning.

Along with the rename, factor out the code to set the flag to a
function.  This makes the complex condition checks simpler.

Also, the same flag is introduced to snd_usb_endpoint, too, that is
carried from the snd_usb_substream flag.  Currently the endpoint flag
isn't still referred, but will be used in later patches.

Link: https://lore.kernel.org/r/20210929080844.11583-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.h

@@ -126,6 +126,7 @@ struct snd_usb_endpoint {
 	int skip_packets;		/* quirks for devices to ignore the first n packets
 					   in a stream */
 	bool implicit_fb_sync;		/* syncs with implicit feedback */
+	bool lowlatency_playback;	/* low-latency playback mode */
 	bool need_setup;		/* (re-)need for configure? */
 
 	/* for hw constraints */
@@ -190,7 +191,7 @@ struct snd_usb_substream {
 	} dsd_dop;
 
 	bool trigger_tstamp_pending_update; /* trigger timestamp being updated from initial estimate */
-	bool early_playback_start;	/* early start needed for playback? */
+	bool lowlatency_playback;	/* low-latency playback mode */
 	struct media_ctl *media_ctl;
 };
 

sound/usb/endpoint.c

@@ -794,6 +794,10 @@ void snd_usb_endpoint_set_callback(struct snd_usb_endpoint *ep,
 {
 	ep->prepare_data_urb = prepare;
 	ep->retire_data_urb = retire;
+	if (data_subs)
+		ep->lowlatency_playback = data_subs->lowlatency_playback;
+	else
+		ep->lowlatency_playback = false;
 	WRITE_ONCE(ep->data_subs, data_subs);
 }
 

sound/usb/pcm.c

@@ -581,6 +581,22 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream)
 	return 0;
 }
 
+/* check whether early start is needed for playback stream */
+static int lowlatency_playback_available(struct snd_usb_substream *subs)
+{
+	struct snd_usb_audio *chip = subs->stream->chip;
+
+	if (subs->direction == SNDRV_PCM_STREAM_CAPTURE)
+		return false;
+	/* disabled via module option? */
+	if (!chip->lowlatency)
+		return false;
+	/* too short periods? */
+	if (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes)
+		return false;
+	return true;
+}
+
 /*
  * prepare callback
  *
@@ -614,13 +630,8 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
 	subs->period_elapsed_pending = 0;
 	runtime->delay = 0;
 
-	/* check whether early start is needed for playback stream */
-	subs->early_playback_start =
-		subs->direction == SNDRV_PCM_STREAM_PLAYBACK &&
-		(!chip->lowlatency ||
-		 (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes));
-
-	if (subs->early_playback_start)
+	subs->lowlatency_playback = lowlatency_playback_available(subs);
+	if (!subs->lowlatency_playback)
 		ret = start_endpoints(subs);
 
  unlock:
@@ -1412,7 +1423,7 @@ static void prepare_playback_urb(struct snd_usb_substream *subs,
 		subs->trigger_tstamp_pending_update = false;
 	}
 
-	if (period_elapsed && !subs->running && !subs->early_playback_start) {
+	if (period_elapsed && !subs->running && subs->lowlatency_playback) {
 		subs->period_elapsed_pending = 1;
 		period_elapsed = 0;
 	}
@@ -1466,7 +1477,7 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea
 					      prepare_playback_urb,
 					      retire_playback_urb,
 					      subs);
-		if (!subs->early_playback_start &&
+		if (subs->lowlatency_playback &&
 		    cmd == SNDRV_PCM_TRIGGER_START) {
 			err = start_endpoints(subs);
 			if (err < 0) {