[alsa-devel] does snd_pcm_info_get_sync work at all?

Clemens Ladisch clemens at ladisch.de
Fri May 18 23:21:15 CEST 2012

Pierre-Louis Bossart wrote:
> After looking a number of times at the code to start multiple devices at
> the same time, I wonder if the alsa-lib snd_pcm_info_get_sync() routine
> works at all.
> The documentation says "Because the drivers cannot guarantee the
> synchronization (sample resolution) on hardware lacking this feature,
> the #snd_pcm_info_get_sync() function returns synchronization ID,
> which is equal for hardware synchronized streams".

This is somewhat misleading.

> It seems some drivers set a value when opening the stream using the
> snd_pcm_set_sync() helper. What exactly is the purpose of this id

This ID has the same value for two streams if those streams _can_ be
started at exactly the same time.

> The value returned seems to be the card#, the same value no matter if
> streams are synchronized or not.

Typically, a card that supports simultaneous starting of multiple
streams does so for all its streams (with a register that has one bit
for each stream), so the card number is the correct ID to describe this


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