ALSA: pcm: allow for trigger_tstamp snapshot in .trigger

Don't use generic snapshot of trigger_tstamp if low-level driver or
hardware can get a more precise value for better audio/system time
synchronization.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm.h

@@ -285,6 +285,7 @@ struct snd_pcm_runtime {
 	/* -- Status -- */
 	struct snd_pcm_substream *trigger_master;
 	struct timespec trigger_tstamp;	/* trigger timestamp */
+	bool trigger_tstamp_latched;     /* trigger timestamp latched in low-level driver/hardware */
 	int overrange;
 	snd_pcm_uframes_t avail_max;
 	snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */

sound/core/pcm_native.c

@@ -810,6 +810,7 @@ static void snd_pcm_trigger_tstamp(struct snd_pcm_substream *substream)
 	if (runtime->trigger_master == NULL)
 		return;
 	if (runtime->trigger_master == substream) {
+		if (!runtime->trigger_tstamp_latched)
 			snd_pcm_gettime(runtime, &runtime->trigger_tstamp);
 	} else {
 		snd_pcm_trigger_tstamp(runtime->trigger_master);
@@ -979,6 +980,7 @@ static int snd_pcm_pre_start(struct snd_pcm_substream *substream, int state)
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 	    !snd_pcm_playback_data(substream))
 		return -EPIPE;
+	runtime->trigger_tstamp_latched = false;
 	runtime->trigger_master = substream;
 	return 0;
 }