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

Clemens Ladisch clemens at ladisch.de
Thu Jan 29 10:59:51 CET 2009

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
* in hw_free, free the buffer.

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.

> Is there a flow chart which shows how the different callbacks can be
> called?

Not yet.

Best regards,

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