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 _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
