ALSA: core: add hooks for audio timestamps

ALSA did not provide any direct means to infer the audio time for A/V
sync and system/audio time correlations (eg. PulseAudio).
Applications had to track the number of samples read/written and
add/subtract the number of samples queued in the ring buffer.  This
accounting led to small errors, typically several samples, due to the
two-step process.  Computing the audio time in the kernel is more
direct, as all the information is available in the same routines.

Also add new .audio_wallclock routine to enable fine-grain synchronization
between monotonic system time and audio hardware time.
Using the wallclock, if supported in hardware, allows for a
much better sub-microsecond precision and a common drift tracking for
all devices sharing the same wall clock (master clock).

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_compat.c

@@ -190,7 +190,8 @@ struct snd_pcm_status32 {
 	u32 avail_max;
 	u32 overrange;
 	s32 suspended_state;
-	unsigned char reserved[60];
+	struct compat_timespec audio_tstamp;
+	unsigned char reserved[60-sizeof(struct compat_timespec)];
 } __attribute__((packed));
 
 
@@ -205,17 +206,16 @@ static int snd_pcm_status_user_compat(struct snd_pcm_substream *substream,
 		return err;
 
 	if (put_user(status.state, &src->state) ||
-	    put_user(status.trigger_tstamp.tv_sec, &src->trigger_tstamp.tv_sec) ||
-	    put_user(status.trigger_tstamp.tv_nsec, &src->trigger_tstamp.tv_nsec) ||
-	    put_user(status.tstamp.tv_sec, &src->tstamp.tv_sec) ||
-	    put_user(status.tstamp.tv_nsec, &src->tstamp.tv_nsec) ||
+	    compat_put_timespec(&status.trigger_tstamp, &src->trigger_tstamp) ||
+	    compat_put_timespec(&status.tstamp, &src->tstamp) ||
 	    put_user(status.appl_ptr, &src->appl_ptr) ||
 	    put_user(status.hw_ptr, &src->hw_ptr) ||
 	    put_user(status.delay, &src->delay) ||
 	    put_user(status.avail, &src->avail) ||
 	    put_user(status.avail_max, &src->avail_max) ||
 	    put_user(status.overrange, &src->overrange) ||
-	    put_user(status.suspended_state, &src->suspended_state))
+	    put_user(status.suspended_state, &src->suspended_state) ||
+	    compat_put_timespec(&status.audio_tstamp, &src->audio_tstamp))
 		return -EFAULT;
 
 	return err;
@@ -364,6 +364,7 @@ struct snd_pcm_mmap_status32 {
 	u32 hw_ptr;
 	struct compat_timespec tstamp;
 	s32 suspended_state;
+	struct compat_timespec audio_tstamp;
 } __attribute__((packed));
 
 struct snd_pcm_mmap_control32 {
@@ -426,12 +427,14 @@ static int snd_pcm_ioctl_sync_ptr_compat(struct snd_pcm_substream *substream,
 	sstatus.hw_ptr = status->hw_ptr % boundary;
 	sstatus.tstamp = status->tstamp;
 	sstatus.suspended_state = status->suspended_state;
+	sstatus.audio_tstamp = status->audio_tstamp;
 	snd_pcm_stream_unlock_irq(substream);
 	if (put_user(sstatus.state, &src->s.status.state) ||
 	    put_user(sstatus.hw_ptr, &src->s.status.hw_ptr) ||
-	    put_user(sstatus.tstamp.tv_sec, &src->s.status.tstamp.tv_sec) ||
-	    put_user(sstatus.tstamp.tv_nsec, &src->s.status.tstamp.tv_nsec) ||
+	    compat_put_timespec(&sstatus.tstamp, &src->s.status.tstamp) ||
 	    put_user(sstatus.suspended_state, &src->s.status.suspended_state) ||
+	    compat_put_timespec(&sstatus.audio_tstamp,
+		    &src->s.status.audio_tstamp) ||
 	    put_user(scontrol.appl_ptr, &src->c.control.appl_ptr) ||
 	    put_user(scontrol.avail_min, &src->c.control.avail_min))
 		return -EFAULT;

sound/core/pcm_lib.c

@@ -316,6 +316,7 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
 	unsigned long jdelta;
 	unsigned long curr_jiffies;
 	struct timespec curr_tstamp;
+	struct timespec audio_tstamp;
 	int crossed_boundary = 0;
 
 	old_hw_ptr = runtime->status->hw_ptr;
@@ -328,9 +329,14 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
 	 */
 	pos = substream->ops->pointer(substream);
 	curr_jiffies = jiffies;
-	if (runtime->tstamp_mode == SNDRV_PCM_TSTAMP_ENABLE)
+	if (runtime->tstamp_mode == SNDRV_PCM_TSTAMP_ENABLE) {
 		snd_pcm_gettime(runtime, (struct timespec *)&curr_tstamp);
 
+		if ((runtime->hw.info & SNDRV_PCM_INFO_HAS_WALL_CLOCK) &&
+			(substream->ops->wall_clock))
+			substream->ops->wall_clock(substream, &audio_tstamp);
+	}
+
 	if (pos == SNDRV_PCM_POS_XRUN) {
 		xrun(substream);
 		return -EPIPE;
@@ -520,9 +526,31 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
 		snd_BUG_ON(crossed_boundary != 1);
 		runtime->hw_ptr_wrap += runtime->boundary;
 	}
-	if (runtime->tstamp_mode == SNDRV_PCM_TSTAMP_ENABLE)
+	if (runtime->tstamp_mode == SNDRV_PCM_TSTAMP_ENABLE) {
 		runtime->status->tstamp = curr_tstamp;
 
+		if (!(runtime->hw.info & SNDRV_PCM_INFO_HAS_WALL_CLOCK)) {
+			/*
+			 * no wall clock available, provide audio timestamp
+			 * derived from pointer position+delay
+			 */
+			u64 audio_frames, audio_nsecs;
+
+			if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
+				audio_frames = runtime->hw_ptr_wrap
+					+ runtime->status->hw_ptr
+					- runtime->delay;
+			else
+				audio_frames = runtime->hw_ptr_wrap
+					+ runtime->status->hw_ptr
+					+ runtime->delay;
+			audio_nsecs = div_u64(audio_frames * 1000000000LL,
+					runtime->rate);
+			audio_tstamp = ns_to_timespec(audio_nsecs);
+		}
+		runtime->status->audio_tstamp = audio_tstamp;
+	}
+
 	return snd_pcm_update_state(substream, runtime);
 }
 

sound/core/pcm_native.c

@@ -594,6 +594,8 @@ int snd_pcm_status(struct snd_pcm_substream *substream,
 		snd_pcm_update_hw_ptr(substream);
 		if (runtime->tstamp_mode == SNDRV_PCM_TSTAMP_ENABLE) {
 			status->tstamp = runtime->status->tstamp;
+			status->audio_tstamp =
+				runtime->status->audio_tstamp;
 			goto _tstamp_end;
 		}
 	}