[alsa-devel] PCM buffering ussues ( or probably the lack of understanding )
marcelsbox at quicknet.nl
Sat Apr 20 09:13:34 CEST 2013
I am working on a PCM driver, have some issues and thing which are
unclear to me, hopefully somebody here can clarify something for me.
My PCM driver is straigh forward:
- Copy callback copies the frames received in the callback into a small
ringbuffer and starts a DMA transfer to the hardware.
- DMA interrupt each times take the next chunk from the ringbuffer.
- In the DMA interrupt the number of frames is counted, if te number >=
period_size reported in runtime_struct pcm_period_elapsed() is called.
- the pointer which is read by the pcm_pointer() callback is updated in
each DMA interrupt.
When using mplayer this works without problems, however with other
applications like mpg123 its causes a lot of problems, for some reaosns
I do not understand yet.
It has something to do with buffer handling.
basically in the cm driver I can influence a few parameters:
buffer_size ( should be sufficient to hold periods_max with
period_bytes_max I assume )
As far as I understand periods_min/periods_max is the size (number of
buffer entries) of the internal ringbuffer Alsa uses.
- Why is there both a min and a max ? How is chosen what will be used ?
- Currently my period_min is set to 1 and my period_max set to 4, If I
increase the period_max value it does not start properly, it seems that
the samples are not deliverered in time.
Is there any explanation for this, does the Alsa layer not any
pre-buffering before the first copy() is called ?
- Is there any advice / guidline for period min/max values ? I checked
sourecs of severl drivers, some use 1 and some use values like 32 or
more for period_max.
The same applies to buffer_bytes min/max, currently my min is set to
128bytes , max to 2kB, larger values of max seem to cause more problems.
From the documentation I understand that it does not matter if
pcm_period_elapsed() is not always called, How dows the ALSA layer
then know how many samples are played.
the pointer value read the pcm_pointer() only counts from
[0..period_max] so what if 2 or more calls to period_elapsed() are
Suppose I want to have some buffering inside my pcm driver, how will I
be able to get some samples in advance before starting playback ? Can I
just call pcm_period_elapsed() a few times in a row to get the buffer
before starting the actual playcback, or does this cause side effects ?
I noticed that the number of frames passed to pcm_copy is often less
than the periods_size, is this determined by the application ?
Lots of questions, but I try to understand the odd behaviour with
certain applications ( and the best settings for a pcm driver which will
work with most applications )
Any help would be appreciated.
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