[alsa-devel] Buffer underrun in a not empty playback buffer

Christian Gruber christian.gruber at voiceinterconnect.de
Wed Jul 25 12:13:11 CEST 2012

Am 25.07.2012 09:01, schrieb Clemens Ladisch:
> 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.
Ok, this is understandable and clear to me.
>> 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.
That's clear, too.
> (Or does that driver, which you did not name, do additional checks?)
The mentioned driver is an ALSA-driver, which was originally developed by Texas 
Instruments for their Audio-Codec family TLV320AIC3x, but was adapted to a specific 
customer board.
>> 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.)
You are right, I did not refer to the frame size and the period size. But the behaviour of 
the mentioned driver is, that an underrun occurs, even when there are lots of periods (> 6 
periods) inside the playback buffer. And this is not correct, isn't it?
> Regards,
> Clemens

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