[alsa-devel] Latency and timestamps

[Takashi Iwai]

At Wed, 21 Nov 2007 15:26:05 +0200,
Heikki Lindholm wrote:
> 
> Takashi Iwai kirjoitti:
> > At Mon, 19 Nov 2007 22:46:31 -0200,
> > Claudio Matsuoka wrote:
> >> Hi,
> >>
> >> I'm adding latency control to an application and didn't find much
> >> documentation about the pcm status functions aside from a very brief
> >> description and the latency.c example. What exactly are the "trigger
> >> timestamp" and "now timestamp" returned by
> >> snd_pcm_status_get_trigger_tstamp() and snd_pcm_status_get_tstamp()?
> > 
> > The trigger_tstamp is the time-stamp at the last time the PCM status
> > change occured.  For example, when the PCM is really triggered to
> > start, or stopped, or XRUN, etc.  It won't be changed as long as the 
> > PCM status is kept.
> > 
> > OTOH, the tstamp is the current timestamp (now).  But, this value has
> > a slightly different meaning when tstamp_mode is set to
> > SND_PCM_TSTAMP_MMAP.  Then it keeps the timestamp of the last period
> > update time instead of the now.
> 
> I looked at the code and I'm not sure I understand the above "last 
> period update time" correctly. I'd like to think that MMAP timestamp is 
> updated by the driver interrupt handler when a new period arrives _and 
> only there_, but the code seems somewhat different:
> - the timestamp is generated in snd_pcm_update_hw_ptr_pos
> - snd_pcm_update_hw_ptr_pos is called by snd_pcm_update_hw_ptr_interrupt
> - the driver interrupt updates the timestamp in snd_pcm_period_elapsed
> BUT if the user calls snd_pcm_status
> - snd_pcm_status calls snd_pcm_update_hw_ptr
> - snd_pcm_update_hw_ptr calls snd_pcm_update_hw_ptr_pos
> and therefore it seems that the timestamp is updated when calling 
> snd_pcm_status also, effectively making the timestamp a "now" timestamp 
> anyway.

Oh, yeah, it's broken.  I think the original design was supposed to
update the mmap update.  Let's fix this.


thanks,

Takashi