ALSA: provide a simple mechanism to start playback at a given time

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/linux/timekeeping.h |  3 ++
  include/uapi/sound/asound.h |  6 +++
  kernel/time/timekeeping.c   | 16 ++++++++
  sound/core/pcm_native.c     | 99 +++++++++++++++++++++++++++++++++++++++++++++
  4 files changed, 124 insertions(+)

include/linux/timekeeping.h +3 -0

index 1caa6b0..74eb6b2 100644
@@ -32,6 +32,9 @@ extern void ktime_get_ts64(struct timespec64 *ts);
  extern int __getnstimeofday64(struct timespec64 *tv);
  extern void getnstimeofday64(struct timespec64 *tv);
 
+/*  Get the offset between monotonic and monotonic raw clocks */
+extern ktime_t ktime_get_raw_offset(void);
+
  #if BITS_PER_LONG == 64
  static inline int __getnstimeofday(struct timespec *ts)
  {

include/uapi/sound/asound.h +0 -0

index 6ee5867..c7aa88c 100644

include/uapi/sound/asound.h +1 -0

  enum {
  	SNDRV_PCM_TSTAMP_TYPE_GETTIMEOFDAY = 0,	/* gettimeofday equivalent */
  	SNDRV_PCM_TSTAMP_TYPE_MONOTONIC,	/* posix_clock_monotonic equivalent */

@@ -547,6 +552,7 @@ 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)
 
  /*****************************************************************************
   *                                                                           *

kernel/time/timekeeping.c +16 -0

index ec1791f..3d5d0bc 100644
@@ -789,6 +789,22 @@ error: /* even if we error out, we forwarded the time, so call update */
  }
  EXPORT_SYMBOL(timekeeping_inject_offset);
 
+/**
+ * ktime_get_raw_offset - get the offset in ktime format between
+ * the monotonic_raw clock and the monotonic clock
+ */
+ktime_t ktime_get_raw_offset(void)
+{
+        struct timespec rawtime = {0,0};
+        struct timespec now = {0,0};
+        struct timespec delta;
+
+        ktime_get_ts(&now);
+        getrawmonotonic(&rawtime);
+        delta = timespec_sub(now, rawtime);
+        return timespec_to_ktime(delta);
+}
+EXPORT_SYMBOL(ktime_get_raw_offset);
 
  /**
   * timekeeping_get_tai_offset - Returns current TAI offset from UTC

sound/core/pcm_native.c +87 -0

index 85fe1a2..1121127 100644
@@ -27,6 +27,7 @@
  #include <linux/pm_qos.h>
  #include <linux/aio.h>
  #include <linux/dma-mapping.h>
+#include <linux/time.h>
  #include <sound/core.h>
  #include <sound/control.h>
  #include <sound/info.h>

@@ -38,6 +39,9 @@
  #if defined(CONFIG_MIPS) && defined(CONFIG_DMA_NONCOHERENT)
  #include <dma-coherence.h>
  #endif
+#if defined(CONFIG_HIGH_RES_TIMERS)
+#include <linux/hrtimer.h>
+#endif
 
  /*
   *  Compatibility

@@ -1016,6 +1020,99 @@ static struct action_ops snd_pcm_action_start = {
  	.post_action = snd_pcm_post_start
  };
 
+#ifdef CONFIG_HIGH_RES_TIMERS
+/*
+ * hrtimer interface
+ */
+
+struct hrtimer_pcm {
+	struct hrtimer timer;
+	struct snd_pcm_substream *substream;
+};
+
+/*
+ * called from a hard irq context - no need for locks.
+ * only problem is that the caller might have gone away and closed the substream
+ * before the timer expires.
+ */
+enum hrtimer_restart snd_pcm_do_start_time(struct hrtimer *timer)
+{
+        struct hrtimer_pcm *pcm_timer;
+	struct snd_pcm_substream *substream;
+        int ret;
+
+        pcm_timer = container_of(timer, struct hrtimer_pcm, timer);
+        substream = pcm_timer->substream;
+
+        if (substream->runtime) {
+                ret = snd_pcm_do_start(substream, SNDRV_PCM_STATE_RUNNING);
+                if (ret == 0) {
+                        snd_pcm_post_start(substream, SNDRV_PCM_STATE_RUNNING);
+                }
+        }
+	kfree(pcm_timer);
+
+        return HRTIMER_NORESTART;
+}
+
+int snd_pcm_start_at(struct snd_pcm_substream *substream,
+        struct snd_clock_time __user *_start_time)
+{
+        struct hrtimer_pcm *pcm_timer;
+        struct snd_clock_time start_time;
+        int clock;
+        ktime_t start_kt;
+        ktime_t raw_offset;
+        int ret;
+
+	if (copy_from_user(&start_time, _start_time, sizeof(start_time)))
+		return -EFAULT;
+
+        if (!timespec_valid(&start_time.time))
+		return -EINVAL;
+
+        start_kt = timespec_to_ktime(start_time.time);
+        clock = start_time.clock;
+        if (start_time.clock == CLOCK_MONOTONIC_RAW) {
+                raw_offset = ktime_get_raw_offset();
+
+                /* convert to clock monotonic */
+                start_kt = ktime_add(raw_offset, start_kt);
+                clock = CLOCK_MONOTONIC;
+        }
+
+        /*  if not playback substream or less than 100 ms give up */
+	if (substream->stream != SNDRV_PCM_STREAM_PLAYBACK ||
+           (ktime_compare(start_kt, ktime_add_ns(ktime_get(), 100000000)) <= 0)) {
+		return snd_pcm_action_lock_irq(&snd_pcm_action_start,
+                        substream, SNDRV_PCM_STATE_RUNNING);
+        }
+
+        ret = snd_pcm_pre_start(substream, SNDRV_PCM_STATE_PREPARED);
+        if (ret != 0) {
+                return ret;
+        }
+
+        pcm_timer = kmalloc(sizeof(*pcm_timer), GFP_KERNEL);
+        hrtimer_init(&pcm_timer->timer, clock, HRTIMER_MODE_ABS);
+
+        pcm_timer->timer.function = snd_pcm_do_start_time;
+        pcm_timer->substream = substream;
+
+	hrtimer_start(&pcm_timer->timer, start_kt, HRTIMER_MODE_ABS);
+
+	return 0;
+}

Hi again Nick!

Quoted 34 lines · 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/linux/timekeeping.h |  3 ++
  include/uapi/sound/asound.h |  6 +++
  kernel/time/timekeeping.c   | 16 ++++++++
  sound/core/pcm_native.c     | 99 +++++++++++++++++++++++++++++++++++++++++++++
  4 files changed, 124 insertions(+)

include/linux/timekeeping.h +3 -0

index 1caa6b0..74eb6b2 100644
@@ -32,6 +32,9 @@ extern void ktime_get_ts64(struct timespec64 *ts);
  extern int __getnstimeofday64(struct timespec64 *tv);
  extern void getnstimeofday64(struct timespec64 *tv);
 
+/*  Get the offset between monotonic and monotonic raw clocks */
+extern ktime_t ktime_get_raw_offset(void);
+
  #if BITS_PER_LONG == 64
  static inline int __getnstimeofday(struct timespec *ts)
  {

include/uapi/sound/asound.h +0 -0

index 6ee5867..c7aa88c 100644
@@ -462,6 +462,11 @@ struct snd_xfern {
  	snd_pcm_uframes_t frames;
  };
 

include/uapi/sound/asound.h struct snd_xfern { · line 469

Quoted 5 lines

include/uapi/sound/asound.h +5 -0

+struct snd_clock_time {
+        clockid_t       clock;
+        struct timespec time;
+};
+

Takashi added snd_pcm_tstamp_type_t in July - If the final start_at patch includes some "struct snd_clock_time", then it'd probably use Takashi's type in favour of a raw clockid_t.

I like the idea of a single struct that includes the context for the timespec btw. Cool.

Quoted 19 lines

include/uapi/sound/asound.h +1 -0

  enum {
  	SNDRV_PCM_TSTAMP_TYPE_GETTIMEOFDAY = 0,	/* gettimeofday equivalent */
  	SNDRV_PCM_TSTAMP_TYPE_MONOTONIC,	/* posix_clock_monotonic equivalent */

@@ -547,6 +552,7 @@ 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)
 
  /*****************************************************************************
   *                                                                           *

kernel/time/timekeeping.c +0 -0

index ec1791f..3d5d0bc 100644
@@ -789,6 +789,22 @@ error: /* even if we error out, we forwarded the time, so call update */
  }
  EXPORT_SYMBOL(timekeeping_inject_offset);
 

Quoted 6 lines

kernel/time/timekeeping.c +6 -0

+/**
+ * ktime_get_raw_offset - get the offset in ktime format between
+ * the monotonic_raw clock and the monotonic clock
+ */
+ktime_t ktime_get_raw_offset(void)
+{

kernel/time/timekeeping.c error: /* even if we error out, we forwarded the time, so call update */ · line 807

Quoted 10 lines

kernel/time/timekeeping.c +10 -0

+        struct timespec rawtime = {0,0};
+        struct timespec now = {0,0};
+        struct timespec delta;
+
+        ktime_get_ts(&now);
+        getrawmonotonic(&rawtime);
+        delta = timespec_sub(now, rawtime);
+        return timespec_to_ktime(delta);
+}
+EXPORT_SYMBOL(ktime_get_raw_offset);

Quoted 81 lines

kernel/time/timekeeping.c +0 -0

   * timekeeping_get_tai_offset - Returns current TAI offset from UTC

sound/core/pcm_native.c +58 -0

index 85fe1a2..1121127 100644
@@ -27,6 +27,7 @@
  #include <linux/pm_qos.h>
  #include <linux/aio.h>
  #include <linux/dma-mapping.h>
+#include <linux/time.h>
  #include <sound/core.h>
  #include <sound/control.h>
  #include <sound/info.h>

@@ -38,6 +39,9 @@
  #if defined(CONFIG_MIPS) && defined(CONFIG_DMA_NONCOHERENT)
  #include <dma-coherence.h>
  #endif
+#if defined(CONFIG_HIGH_RES_TIMERS)
+#include <linux/hrtimer.h>
+#endif
 
  /*
   *  Compatibility

@@ -1016,6 +1020,99 @@ static struct action_ops snd_pcm_action_start = {
  	.post_action = snd_pcm_post_start
  };
 
+#ifdef CONFIG_HIGH_RES_TIMERS
+/*
+ * hrtimer interface
+ */
+
+struct hrtimer_pcm {
+	struct hrtimer timer;
+	struct snd_pcm_substream *substream;
+};
+
+/*
+ * called from a hard irq context - no need for locks.
+ * only problem is that the caller might have gone away and closed the substream
+ * before the timer expires.
+ */
+enum hrtimer_restart snd_pcm_do_start_time(struct hrtimer *timer)
+{
+        struct hrtimer_pcm *pcm_timer;
+	struct snd_pcm_substream *substream;
+        int ret;
+
+        pcm_timer = container_of(timer, struct hrtimer_pcm, timer);
+        substream = pcm_timer->substream;
+
+        if (substream->runtime) {
+                ret = snd_pcm_do_start(substream, SNDRV_PCM_STATE_RUNNING);
+                if (ret == 0) {
+                        snd_pcm_post_start(substream, SNDRV_PCM_STATE_RUNNING);
+                }
+        }
+	kfree(pcm_timer);
+
+        return HRTIMER_NORESTART;
+}
+
+int snd_pcm_start_at(struct snd_pcm_substream *substream,
+        struct snd_clock_time __user *_start_time)
+{
+        struct hrtimer_pcm *pcm_timer;
+        struct snd_clock_time start_time;
+        int clock;
+        ktime_t start_kt;
+        ktime_t raw_offset;
+        int ret;
+
+	if (copy_from_user(&start_time, _start_time, sizeof(start_time)))
+		return -EFAULT;
+
+        if (!timespec_valid(&start_time.time))
+		return -EINVAL;
+
+        start_kt = timespec_to_ktime(start_time.time);
+        clock = start_time.clock;
+        if (start_time.clock == CLOCK_MONOTONIC_RAW) {

sound/core/pcm_native.c static struct action_ops snd_pcm_action_start = { · line 1084

Quoted 8 lines

sound/core/pcm_native.c +8 -0

+                raw_offset = ktime_get_raw_offset();
+
+                /* convert to clock monotonic */
+                start_kt = ktime_add(raw_offset, start_kt);
+                clock = CLOCK_MONOTONIC;
+        }
+
+        /*  if not playback substream or less than 100 ms give up */

Quoted 21 lines

sound/core/pcm_native.c +21 -0

+	if (substream->stream != SNDRV_PCM_STREAM_PLAYBACK ||
+           (ktime_compare(start_kt, ktime_add_ns(ktime_get(), 100000000)) <= 0)) {
+		return snd_pcm_action_lock_irq(&snd_pcm_action_start,
+                        substream, SNDRV_PCM_STATE_RUNNING);
+        }
+
+        ret = snd_pcm_pre_start(substream, SNDRV_PCM_STATE_PREPARED);
+        if (ret != 0) {
+                return ret;
+        }
+
+        pcm_timer = kmalloc(sizeof(*pcm_timer), GFP_KERNEL);
+        hrtimer_init(&pcm_timer->timer, clock, HRTIMER_MODE_ABS);
+
+        pcm_timer->timer.function = snd_pcm_do_start_time;
+        pcm_timer->substream = substream;
+
+	hrtimer_start(&pcm_timer->timer, start_kt, HRTIMER_MODE_ABS);
+
+	return 0;
+}

sound/core/pcm_native.c static struct action_ops snd_pcm_action_start = { · line 1115

Quoted 10 lines

sound/core/pcm_native.c +10 -0

+#else
+/* without high res time, interface is identical to snd_pcm_start() */
+int snd_pcm_start_at(struct snd_pcm_substream *substream,
+        struct snd_clock_time __user *_start_time)
+{
+        return snd_pcm_action_lock_irq(&snd_pcm_action_start,
+                substream, SNDRV_PCM_STATE_RUNNING);
+}
+#endif
+

I'd prefer to return an error code when snd_pcm_start_at cannot be supported, rather than lying to userspace that it will be satisfied :)

There might be a number of reasons that snd_pcm_start_at cannot be
satisfied: unsupported clock-type, timespec in the past, incompatible kernel config are just three.

While one might argue that start_at is a "best effort" mechanism, on principle the function should make every effort to return an error when it would be meaningful (read: handleable) by userspace. Not sure how you feel about that ... :)