ALSA: pcm: Comment why read blocks when PCM is not running

This avoids bringing back the problem introduced by
62ba568f ("ALSA: pcm: Return 0 when size <
start_threshold in capture") and fixed in 00a399ca
("ALSA: pcm: Revert capture stream behavior change in
blocking mode"), which prevented the user from starting
capture from another thread.
Signed-off-by: Ricardo Biehl Pasquali <pasqualirb@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_lib.c

@@ -2176,6 +2176,10 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 	if (runtime->status->state == SNDRV_PCM_STATE_RUNNING)
 		snd_pcm_update_hw_ptr(substream);
 
+	/*
+	 * If size < start_threshold, wait indefinitely. Another
+	 * thread may start capture
+	 */
 	if (!is_playback &&
 	    runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
 	    size >= runtime->start_threshold) {