[alsa-devel] Buffer underrun in a not empty playback buffer
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.
Is this an allowed ALSA-driver behaviour or does the ALSA-driver developer has to hide all DMA internals to the application developer so that an underrun just occurs, when the playback buffer is empty? If yes, how can I get to know about the required minimum buffer filling level before an underrun occurs?
Regards, Christian
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
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
Christian Gruber wrote:
Am 25.07.2012 09:01, schrieb Clemens Ladisch:
(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.
The TLV320AIC3x are I²S codecs. DMA and buffering are done by the controller, which is managed by the platform driver.
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?
This is indeed not correct (if there isn't much time between reading the buffer fill level and the underrun).
It's likely that the platform driver does not report the buffer position correctly.
Regards, Clemens
participants (2)
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Christian Gruber -
Clemens Ladisch