[alsa-devel] Can DMA address, periods, buffer size, etc change after hw_params is called?

Hans-Christian Egtvedt hans-christian.egtvedt at atmel.com
Thu Jan 29 11:10:34 CET 2009

On Thu, 29 Jan 2009 10:59:51 +0100
Clemens Ladisch <clemens at ladisch.de> wrote:

> Hans-Christian Egtvedt wrote:
> > Okay, so to be sure I setup and release my cyclic DMA buffer safely
> > I could use the following model:
> > 
> > in prepare callback setup the DMA buffer, once when called. The
> > hardware parameters should not change at this point.
> > 
> > If I get a hw_params callback after a prepare, then release the DMA
> > buffer, and set it up again in the prepare callback?
> > 
> > Final cleanup of the DMA buffer can/should be done in hw_free?
> This would work, but doing it in the hw_params callback would be a
> little more simple:
> * In hw_params, free the old buffer (if it exists), then setup the
>   buffer;

I think I was not 100% clear, in addition to call
snd_pcm_lib_malloc_pages() in hw_params, I also need to hand over the
DMA buffer information to a prepare function in the DMA driver I use
with this sound hardware.

So I follow the scheme for free/setup of buffers for my DMA controller
setup as well.

hw_params, release DMA controller if exists, then prepare DMA

> * in hw_free, free the buffer.

Which I do by calling snd_pcm_lib_free_pages().

and then in hw_free, release DMA controller if exists.

> As a rule of thumb, the callbacks open/close, hw_params/hw_free and
> start/stop trigger are mostly symmetrical, and any resources should be
> allocated and freed in the functions that belong together.  (The start
> trigger is a special case because it uses two callbacks, prepare and
> trigger with TRIGGER_START.)
> The hw_params and prepare callbacks can be called multiple times.

Noted, which means I should reprepare my DMA controller as well every
time I get a hw_params callback and do snd_pcm_lib_malloc_pages().

> > Is there a flow chart which shows how the different callbacks can be
> > called?
> Not yet.

Would probably help me a lot to see the actual flow of the callbacks.

Best regards,
Hans-Christian Egtvedt

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