[alsa-devel] [PATCH v3 kernel 1/3] snd_pcm_start_at and friends.
Pierre-Louis Bossart
pierre-louis.bossart at linux.intel.com
Tue Feb 10 21:37:00 CET 2015
On 2/10/15 2:21 PM, Nick Stoughton wrote:
> The most important clock is MONOTONIC_RAW, from which the audio clocks
> are ultimately derived. Any system clock that is adjusted by NTP has the
> problem that Pierre-Louis describes. My implementation allowed for other
> clocks, but my actual use-case only uses MONOTONIC_RAW, and we are
> achieving very high accuracy using it.
>
> The primary driver (at least for me) for snd_pcm_start_at() is to be
> able to have two separate devices (which are in the background
> exchanging information about their respective CLOCK_MONOTONIC_RAW
> counters) start playing the same audio at the same time. Therefore, we
> need a clock that runs at a constant frequency and from which the I2S
> bit clock is derived.
Sounds like a very specific case. You *may* derive your MONOTONIC_RAW
counters and bitclocks from the same osc in your hardware, but that's
not typically how things work. There are quite a few systems where the
bitclock or wallclock isn't aligned with the system time reported with
MONOTONIC_RAW, and you really need to track sample/bitclock/wallclock
counters to track the drift which by nature differs between devices.
>
> *______________________________*
> *Nick Stoughton*
> *Aether Things Inc *
> **San Francisco**
> +1 (510) 388 1413
>
> On Fri, Feb 6, 2015 at 1:02 PM, Pierre-Louis Bossart
> <pierre-louis.bossart at linux.intel.com
> <mailto:pierre-louis.bossart at linux.intel.com>> wrote:
>
> On 02/06/2015 11:08 AM, Tim Cussins wrote:
> > Hi Pierre,
> >
> > You got here quick!
>
> That's what 6am flights do to you...
>
> >
> > On 06/02/15 16:32, Pierre-Louis Bossart wrote:
> >> On 02/06/2015 10:16 AM, Tim Cussins wrote:
> >>> We introduce the kernel-side of the START_AT ioctl.
> >>>
> >>> struct runtime is updated to hold information about the currently
> >>> active start_at timer, if any. This facilitates cancellation via
> >>> snd_pcm_start_at_abort(), and querying via snd_pcm_status().
> >>>
> >>> struct snd_start_at holds a startat operation and its arguments.
> >>>
> >>> Signed-off-by: Tim Cussins <timcussins at eml.cc>
> >>>
> >>> diff --git a/include/uapi/sound/asound.h
> b/include/uapi/sound/asound.h
> >>> index 0e88e7a..2943e1a 100644
> >>> --- a/include/uapi/sound/asound.h
> >>> +++ b/include/uapi/sound/asound.h
> >>> @@ -421,7 +421,10 @@ struct snd_pcm_status {
> >>> snd_pcm_state_t suspended_state; /* suspended stream state */
> >>> __u32 reserved_alignment; /* must be filled with zero */
> >>> struct timespec audio_tstamp; /* from sample counter
> or wall clock */
> >>> - unsigned char reserved[56-sizeof(struct timespec)]; /*
> must be filled with zero */
> >>> + int startat_pending; /* 1 if a start_at timer is
> pending, 0 otherwise */
> >>> + int startat_clock_type; /* start_at clock type, if
> pending */
> >>> + struct timespec startat_start_time; /* start_at start
> time, if pending */
> >>> + unsigned char reserved[48-(2*sizeof(struct timespec))]; /*
> must be filled with zero */
> >>> };
> >>>
> >>> struct snd_pcm_mmap_status {
> >>> @@ -473,6 +476,34 @@ enum {
> >>> SNDRV_PCM_TSTAMP_TYPE_LAST =
> SNDRV_PCM_TSTAMP_TYPE_MONOTONIC_RAW,
> >>> };
> >>>
> >>> +enum {
> >>> + SNDRV_PCM_STARTAT_OP_SET = 0,
> >>> + SNDRV_PCM_STARTAT_OP_CANCEL,
> >>> + SNDRV_PCM_STARTAT_OP_STATUS,
> >>> + SNDRV_PCM_STARTAT_OP_LAST = SNDRV_PCM_STARTAT_OP_STATUS,
> >>> +};
> >>> +
> >>> +enum {
> >>> + SNDRV_PCM_STARTAT_CLOCK_TYPE_GETTIMEOFDAY = 0,
> >>> + SNDRV_PCM_STARTAT_CLOCK_TYPE_MONOTONIC,
> >>> + SNDRV_PCM_STARTAT_CLOCK_TYPE_LINK,
> >>> + SNDRV_PCM_STARTAT_CLOCK_TYPE_LAST =
> SNDRV_PCM_STARTAT_CLOCK_TYPE_LINK,
> >>> +};
> >>
> >> Looks like you went back to the original design with all clocks
> mixed. I think it's a better idea to split system timers and audio
> clocks.
> >> And you are missing MONOTONIC_RAW (no NTP corrections).
> >
> > It's not /quite/ the original design. The start_at API now has
> its own enum of clock types, which are separate from, but
> semantically the same as, timestamping clock types.
> >
> > For the implementation of snd_pcm_start_at, I genuinely prefer
> the unified approach. From the point of view of a /client/ of
> snd_pcm_start_at, system and audio clocks are merely different
> points in time with no other distinguishing characteristics.
> Directing users to 2 different tables of clocks could be considered
> obtuse, if not Kafkaesque :P
> >
> > Ok, so maybe that's putting it a bit too strong :D Let see if I
> can see it your way: I can see two reasons for having 2 categories
> of clocks.
> >
> > (1) Because the underlying implementation of snd_pcm_start_at (or
> some other API) is different.
> > (2) Because you want to gather pairs of timestamps
> (a_system_timestamp, an_audio_timestamp) in order to estimate clock
> drift.
> >
> > I think we'd agree that (1) is not a strong justification: We
> shouldn't allow implementation details to propagate into the API
> without good cause.
> >
> > The second point is interesting, but doesn't seem to preclude
> having an dedicated clock type enum for use with snd_pcm_start_at.
> >
> > I guess what I'm arguing here is that the timestamping evolutions
> and start_at aren't /required/ to use the same clock type enums, and
> it seems more sensible to me that start_at has it's own, unified,
> clock type enum.
> >
> > I hope I haven't mischaracterised your view on split enums. Hit
> me back with some corrections.
> >
> > More importantly, I hope I haven't stepped on your toes too much
> - I really value your efforts and feedback on this stuff!
>
> I understand the logic of making things unified to specify the
> start, but the audio or link clocks are really different in nature
> from the system ones. I guess my reaction really comes from the
> number of times i've had to explain that a system timer can't
> control audio playback unless the same oscillator is used. Pretty
> basic but not everyone gets it. It'd also be weird to define a clock
> type to specify the start time and use a second one to know the time
> once the stream has started.
>
> >
> >>
> >>> + woul
> >>> +struct snd_start_at {
> >>> + int op; /* startat operation to be performed */
> >>> + union { /* fields for setting a startat
> timer */
> >>> + struct {
> >>> + int clock_type; /* clock type e.g.
> SNDRV_PCM_STARTAT_CLOCK_TYPE_GETTIMEOFDAY */
> >>> + struct timespec start_time; /* start time */
> >>> + } set;
> >>> + struct {
> >>> + int clock_type;
> >>> + struct timespec current_time;
> >>> + } status;
> >>> + } args;
> >>> +};
> >>> +
> >>> /* channel positions */
> >>> enum {
> >>> SNDRV_CHMAP_UNKNOWN = 0,
> >>> @@ -551,6 +582,8 @@ 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 _IOW('A', 0x62, struct
> snd_start_at)
> >>> +
> >>>
> >>>
> /*****************************************************************************
> >>> *
> *
> >>>
> >>
> >
>
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