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>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 28fd9f95f9ba..45c1981c9ca2 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -71,6 +71,8 @@ struct snd_pcm_ops {
         int (*prepare)(struct snd_pcm_substream *substream);
         int (*trigger)(struct snd_pcm_substream *substream, int cmd);
         snd_pcm_uframes_t (*pointer)(struct snd_pcm_substream *substream);
+        int (*wall_clock)(struct snd_pcm_substream *substream,
+                          struct timespec *audio_ts);
         int (*copy)(struct snd_pcm_substream *substream, int channel,
                     snd_pcm_uframes_t pos,
                     void __user *buf, snd_pcm_uframes_t count);