[alsa-devel] [PATCH v3 kernel 1/3] snd_pcm_start_at and friends.

[Pierre-Louis Bossart]

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)
>>> +
>>>
>>>   /*****************************************************************************
>>>    *                                                                           *
>>>
>>
>