[alsa-devel] [RFC] ALSA: provide a simple mechanism to start playback at a given time
Tim Cussins
timcussins at eml.cc
Thu Oct 16 12:53:07 CEST 2014
Hi again Nick!
On 14/10/14 09:29, Nick Stoughton wrote:
> 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(+)
>
> diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
> index 1caa6b0..74eb6b2 100644
> --- a/include/linux/timekeeping.h
> +++ b/include/linux/timekeeping.h
> @@ -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)
> {
> diff --git a/include/uapi/sound/asound.h b/include/uapi/sound/asound.h
> index 6ee5867..c7aa88c 100644
> --- a/include/uapi/sound/asound.h
> +++ b/include/uapi/sound/asound.h
> @@ -462,6 +462,11 @@ struct snd_xfern {
> snd_pcm_uframes_t frames;
> };
>
> +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.
> 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)
>
> /*****************************************************************************
> * *
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index ec1791f..3d5d0bc 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -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
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 85fe1a2..1121127 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -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;
> +}
> +#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 ... :)
> /**
> * snd_pcm_start - start all linked streams
> * @substream: the PCM substream instance
> @@ -2698,6 +2795,8 @@ static int snd_pcm_common_ioctl1(struct file *file,
> return snd_pcm_reset(substream);
> case SNDRV_PCM_IOCTL_START:
> return snd_pcm_action_lock_irq(&snd_pcm_action_start, substream, SNDRV_PCM_STATE_RUNNING);
> + case SNDRV_PCM_IOCTL_START_AT:
> + return snd_pcm_start_at(substream, arg);
> case SNDRV_PCM_IOCTL_LINK:
> return snd_pcm_link(substream, (int)(unsigned long) arg);
> case SNDRV_PCM_IOCTL_UNLINK:
>
I'm really not qualified to pass comment on the kernel half of start_at:
I'll have to some back to that, or defer to others...
Cheers,
Tim
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