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, 5 insertions, 2 deletions

diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
index 92b104e496b5..85b2e4dde883 100644
--- a/include/uapi/sound/asound.h
+++ b/include/uapi/sound/asound.h
@@ -136,7 +136,7 @@ struct snd_hwdep_dsp_image {
  *                                                                           *
  *****************************************************************************/
 
-#define SNDRV_PCM_VERSION               SNDRV_PROTOCOL_VERSION(2, 0, 10)
+#define SNDRV_PCM_VERSION               SNDRV_PROTOCOL_VERSION(2, 0, 11)
 
 typedef unsigned long snd_pcm_uframes_t;
 typedef signed long snd_pcm_sframes_t;
@@ -258,6 +258,7 @@ typedef int __bitwise snd_pcm_subformat_t;
 #define SNDRV_PCM_INFO_JOINT_DUPLEX     0x00200000      /* playback and capture stream are somewhat correlated */
 #define SNDRV_PCM_INFO_SYNC_START       0x00400000      /* pcm support some kind of sync go */
 #define SNDRV_PCM_INFO_NO_PERIOD_WAKEUP 0x00800000      /* period wakeup can be disabled */
+#define SNDRV_PCM_INFO_HAS_WALL_CLOCK   0x01000000      /* has audio wall clock for audio/system time sync */
 #define SNDRV_PCM_INFO_FIFO_IN_FRAMES   0x80000000      /* internal kernel flag - FIFO size is in frames */
 
 typedef int __bitwise snd_pcm_state_t;
@@ -406,7 +407,8 @@ struct snd_pcm_status {
         snd_pcm_uframes_t avail_max;    /* max frames available on hw since last status */
         snd_pcm_uframes_t overrange;    /* count of ADC (capture) overrange detections from last status */
         snd_pcm_state_t suspended_state; /* suspended stream state */
-        unsigned char reserved[60];     /* must be filled with zero */
+        struct timespec audio_tstamp;   /* from sample counter or wall clock */
+        unsigned char reserved[60-sizeof(struct timespec)]; /* must be filled with zero */
 };
 
 struct snd_pcm_mmap_status {
@@ -415,6 +417,7 @@ struct snd_pcm_mmap_status {
         snd_pcm_uframes_t hw_ptr;       /* RO: hw ptr (0...boundary-1) */
         struct timespec tstamp;         /* Timestamp */
         snd_pcm_state_t suspended_state; /* RO: suspended stream state */
+        struct timespec audio_tstamp;   /* from sample counter or wall clock */
 };
 
 struct snd_pcm_mmap_control {