On Wed, 5 Feb 2003, Maarten de Boer wrote:
> Hello,
>
> I am trying to write a pcm_jack plugin, which would allow existing alsa
> applications to use jack as input/output, without changing any code. The
> plugin will use some internal circular buffer for the data transfer
> between the alsa read/write calls and the async jack process callback.
>
> I have been studying the code of the various plugins, had some private
> communication on the subject with Paul Davis and Abramo, and written
> some code, but I am rather stuck... Your help would be greatly
> appreciated.
>
> As far as I understand, the easiest solution would be to have a pcm_plug
> plugin inbetween the alsa application and the pcm_jack plugin, to take
> care of sample rate and data format conversion. Then the pcm_jack plugin
> simply restrict it configuration space to the sample rate and data format
> (float) supplied by the jack server. Is this the right approach? Then,
Yep.
> what would be the right place to do this? In the hw_refine or in the
> hw_param function? Or somewhere else even? And how - what _snd_pcm helper
> functions should I best use?
Well, I think that it needs a bit deeper introduction to the alsa-lib
plugin system. I'm actually working on dmix plugin and facing with same
problems.
hw_refine must refine the given configuration to closest valid value (see
refine callback in dmix plugin)
hw_params receives only a valid configuration (validated via refine); it
might fail, but only when some necessary resources are allocated between
refine and hw_params calls with some other application or other event
> Jack will allow to open as many ports as necesary, so I will open one
> port for each channel. Is the prepare function the correct place to do
> so?
It depends, if you want to offer static or dynamic configuration. You may
start with static (you'll tell the user the static channel count and
preallocate these ports in jack).
> Most pcm plugins use a slave, but as the pcm_jack plugin will be an
> end-point, it will not have any slave. Do I see this correctly?
Yep. There are only few "terminate" plugins in ALSA like null, hw and shm.
Jaroslav
-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs
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