[alsa-devel] [RFC] pcm: provide a simple mechanism to start playback at a given time

[Nick Stoughton]

Initial implementation / Request For Comment.

Given an absolute time based on a given clock (CLOCK_MONOTONIC,
CLOCK_MONOTONIC_RAW, CLOCK_REALTIME etc), setup a high resolution timer
to cause playback to be triggered at that time.
---
 include/global.h       | 10 +++++++++-
 include/pcm.h          |  2 ++
 include/sound/asound.h |  7 +++++++
 src/pcm/pcm.c          | 22 +++++++++++++++++++++-
 src/pcm/pcm_hw.c       | 15 +++++++++++++++
 src/pcm/pcm_local.h    |  1 +
 6 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/include/global.h b/include/global.h
index 16a26dc..f0fb661 100644
--- a/include/global.h
+++ b/include/global.h
@@ -144,6 +144,8 @@ struct timespec {
 	time_t		tv_sec;		/* seconds */
 	long		tv_nsec;	/* nanoseconds */
 };
+
+typedef int clockid_t;
 #endif
 #endif
 
@@ -151,7 +153,13 @@ struct timespec {
 typedef struct timeval snd_timestamp_t;
 /** Hi-res timestamp */
 typedef struct timespec snd_htimestamp_t;
-
+/** clock type */
+typedef clockid_t snd_clock_t;
+/** a combined clock and time */
+typedef struct {
+        snd_clock_t             clock;
+        snd_htimestamp_t        time;
+} snd_clock_time_t;
 /** \} */
 
 #ifdef __cplusplus
diff --git a/include/pcm.h b/include/pcm.h
index db88ad5..dfaf69f 100644
--- a/include/pcm.h
+++ b/include/pcm.h
@@ -474,6 +474,8 @@ int snd_pcm_prepare(snd_pcm_t *pcm);
 int snd_pcm_reset(snd_pcm_t *pcm);
 int snd_pcm_status(snd_pcm_t *pcm, snd_pcm_status_t *status);
 int snd_pcm_start(snd_pcm_t *pcm);
+int snd_pcm_start_at(snd_pcm_t *pcm, snd_clock_time_t *start_time);
+#define SND_PCM_START_AT(pcm, start_time) snd_pcm_start_at(pcm, start_time)
 int snd_pcm_drop(snd_pcm_t *pcm);
 int snd_pcm_drain(snd_pcm_t *pcm);
 int snd_pcm_pause(snd_pcm_t *pcm, int enable);
diff --git a/include/sound/asound.h b/include/sound/asound.h
index 6ee5867..92647ce 100644
--- a/include/sound/asound.h
+++ b/include/sound/asound.h
@@ -434,6 +434,11 @@ struct snd_pcm_mmap_control {
 	snd_pcm_uframes_t avail_min;	/* RW: min available frames for wakeup */
 };
 
+struct snd_clock_time {
+        clockid_t       clock;
+        struct timespec time;
+};
+
 #define SNDRV_PCM_SYNC_PTR_HWSYNC	(1<<0)	/* execute hwsync */
 #define SNDRV_PCM_SYNC_PTR_APPL		(1<<1)	/* get appl_ptr from driver (r/w op) */
 #define SNDRV_PCM_SYNC_PTR_AVAIL_MIN	(1<<2)	/* get avail_min from driver */
@@ -547,6 +552,8 @@ enum {
 #define SNDRV_PCM_IOCTL_READN_FRAMES	_IOR('A', 0x53, struct snd_xfern)
 #define SNDRV_PCM_IOCTL_LINK		_IOW('A', 0x60, int)
 #define SNDRV_PCM_IOCTL_UNLINK		_IO('A', 0x61)
+#define SNDRV_PCM_IOCTL_START_AT        _IOR('A', 0x62, struct snd_clock_time)
+
 
 /*****************************************************************************
  *                                                                           *
diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c
index 4a7be6c..7051a22 100644
--- a/src/pcm/pcm.c
+++ b/src/pcm/pcm.c
@@ -1084,6 +1084,25 @@ int snd_pcm_start(snd_pcm_t *pcm)
 }
 
 /**
+ * \brief Start a PCM at a specified point in the future
+ * \param pcm PCM handle
+ * \param start_time time to start - based on specified clock
+ * \return 0 on success otherwise a negative error code
+ */
+int snd_pcm_start_at(snd_pcm_t *pcm, snd_clock_time_t *start_at)
+{
+        assert(pcm);
+        assert(start_at);
+	if (CHECK_SANITY(! pcm->setup)) {
+		SNDMSG("PCM not set up");
+		return -EIO;
+	}
+        if (pcm->fast_ops->start_at)
+                return pcm->fast_ops->start_at(pcm->fast_op_arg, start_at);
+        return -EINVAL;
+}
+
+/**
  * \brief Stop a PCM dropping pending frames
  * \param pcm PCM handle
  * \return 0 on success otherwise a negative error code
@@ -2444,8 +2463,9 @@ int snd_pcm_wait_nocheck(snd_pcm_t *pcm, int timeout)
 		                continue;
 			return -errno;
                 }
-		if (! err_poll)
+		if (! err_poll) {
 			break;
+                }
 		err = snd_pcm_poll_descriptors_revents(pcm, pfd, npfds, &revents);
 		if (err < 0)
 			return err;
diff --git a/src/pcm/pcm_hw.c b/src/pcm/pcm_hw.c
index c34b766..9888253 100644
--- a/src/pcm/pcm_hw.c
+++ b/src/pcm/pcm_hw.c
@@ -620,6 +620,20 @@ static int snd_pcm_hw_start(snd_pcm_t *pcm)
 	return 0;
 }
 
+static int snd_pcm_hw_start_at(snd_pcm_t *pcm, snd_clock_time_t *start_time)
+{
+	snd_pcm_hw_t *hw = pcm->private_data;
+	int err;
+
+	sync_ptr(hw, 0);
+	if (ioctl(hw->fd, SNDRV_PCM_IOCTL_START_AT, start_time) < 0) {
+		err = -errno;
+		SYSMSG("SNDRV_PCM_IOCTL_START_AT failed (%i)", err);
+		return err;
+	}
+	return 0;
+}
+
 static int snd_pcm_hw_drop(snd_pcm_t *pcm)
 {
 	snd_pcm_hw_t *hw = pcm->private_data;
@@ -1336,6 +1350,7 @@ static const snd_pcm_fast_ops_t snd_pcm_hw_fast_ops = {
 	.prepare = snd_pcm_hw_prepare,
 	.reset = snd_pcm_hw_reset,
 	.start = snd_pcm_hw_start,
+	.start_at = snd_pcm_hw_start_at,
 	.drop = snd_pcm_hw_drop,
 	.drain = snd_pcm_hw_drain,
 	.pause = snd_pcm_hw_pause,
diff --git a/src/pcm/pcm_local.h b/src/pcm/pcm_local.h
index 394505f..c3e58af 100644
--- a/src/pcm/pcm_local.h
+++ b/src/pcm/pcm_local.h
@@ -154,6 +154,7 @@ typedef struct {
 	int (*prepare)(snd_pcm_t *pcm);
 	int (*reset)(snd_pcm_t *pcm);
 	int (*start)(snd_pcm_t *pcm);
+	int (*start_at)(snd_pcm_t *pcm, snd_clock_time_t *start_time);
 	int (*drop)(snd_pcm_t *pcm);
 	int (*drain)(snd_pcm_t *pcm);
 	int (*pause)(snd_pcm_t *pcm, int enable);
-- 
1.9.3