[alsa-devel] usb: offset in trigger_tstamp?

[Clemens Ladisch]

Pierre-Louis Bossart wrote:
> I was trying to find a bug in the timestamping logic I am working on
> and after some time I realized the USB audio class driver seems to
> have an issue.
> If I instrument the code in sound/usb/pcm.c, I see a offset of up to
> 6ms between the  snd_usb_substream_playback_trigger and
> prepare_playback_urb steps
>
> [45053.102625] trigger_start
> [45053.108320] prepare_playback_urb
>
> My understanding is that empty URBs are submitted in the prepare step
> to avoid underflows, which is fine. The problem with that is that the
> trigger_tstamp value is captured before audio transfers actually start
> and there is a constant offset when trying to track audio/system time
> drifts or do basic a/v sync.

Most other devices have a FIFO that gets filled quickly when the stream
is started.  This results in a small offset in the opposite direction.

AFAICS you cannot avoid having an offset when trying to relate the
trigger_tstamp value to the sample clock.


Regards,
Clemens