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