[alsa-devel] Rationale behind snd_pcm_htimestamp()

[Takashi Iwai]

At Wed, 18 Feb 2009 22:51:57 +0100,
Lennart Poettering wrote:
> 
> On Mon, 16.02.09 17:08, Takashi Iwai (tiwai at suse.de) wrote:
> 
> > > An alternative way seemed to be to go via the snd_pcm_status
> > > structure. However querying that seems not to sync the hw index
> > > data. Hmm... so let's call snd_pcm_hwsync() right before querying the
> > > struct -- as it turns out however, that function is now deprecated. So
> > > which function should I call? The doxygen comments of _hwsync() kind
> > > of suggest in a way to call snd_pcm_avail_update(), whose doxygen
> > > comments however declare that it does in fact *not* sync the hw state
> > > either, but suggest to use snd_pcm_avail() for that.
> > > 
> > > Ok, so I am now tempted to first call snd_pcm_avail() and then
> > > snd_pcm_status() and have all three values atomically. But unfortunately
> > > that's not the case. Again the avail value of the two calls differs sometimes.
> > > 
> > > How can I query the three values atomically? Can i do that at all?
> > 
> > Hmm, there is no way to get them in atomic way anyway, I guess.
> > Even in *_avail_delay(),  basically each of them is called
> > sequentially in alsa-lib code.
> > 
> > But, I wonder whether calling snd_pcm_delay() then
> > snd_pcm_htimestamp() doesn't work?  snd_pcm_avail_delay() is nothing
> > but a sequential call of snd_pcm_delay() and avail_update().  And,
> > htimestamp is avail_update() and timestamp check.
> > 
> > Well, *_htimestamp() has a loop to sync with avail, so the similar
> > logic could be used to sync all of them...
> 
> Hmm, so I understood you correctly then the API would need changing to
> get the timestamp, the delay and the avail value that belong together?

Yes, if you stick with snd_pcm_htimetamp(), snd_pcm_avil() and co.

But basically snd_pcm_status() does sync and fetch all status data.
Shouldn't calling this suffice for your purpose?


Takashi