Christian Gruber wrote:
I had a problem with an ALSA-driver, which causes an underrun in the playback stream before the playback buffer was completely empty (snd_pcm_avail() < playback buffer size). The driver developer told me, that this is correct, since for correct DMA transfer a minimum buffer filling level is required.
There is indeed a minimum buffer filling level. If the device reads data out of the buffer x bytes at a time, then you'd better make sure that there are at least x bytes available.
Is this an allowed ALSA-driver behaviour
This has nothing to do with the driver itself; the ALSA framework checks for underruns.
Furthermore, the underrun does not happen before the buffer is empty; underruns are alway detected after the fact. It's just that your application will never be able to read snd_pcm_avail()==0 because that value would automatically trigger the underrun detection.
(Or does that driver, which you did not name, do additional checks?)
how can I get to know about the required minimum buffer filling level before an underrun occurs?
The minimum buffer filling level _before_ an underrun is one frame. However, to ensure that the device has enough data available, and to protect against random scheduling delays, your application should try to keep the buffer always as much filled as possible.
There is no function to read the device's read block size. However, for large block sizes, snd_pcm_hw_params_is_block_transfer() returns 1, and the block size cannot be larger than one period. (Quite a few embedded devices indeed transfer entire periods.)
Regards, Clemens