[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 09:43:32 CET 2009

On Thu, 29 Jan 2009 09:11:40 +0100
Clemens Ladisch <clemens at ladisch.de> wrote:

> Hans-Christian Egtvedt wrote:
> > I wonder if I must take care handling changes in DMA address,
> > number of periods and period size in the prepare callback?
> > 
> > Or are these locked after the first time prepare is called?
> The hardware parameters like buffer/period size and sample format are
> set by calling the hw_params callback; they are valid until a call to
> hw_free (or another call to hw_params), which doesn't happen while the
> stream is running.

Is there a chance that the following might happen:
 - stop trigger,
 - hw_free
 - hw_params
 - prepare
 - start trigger

I.e. without calling close and open?

> The DMA address is selected by the driver in the hw_params callback,
> but most drivers let the framework handle this by calling
> snd_pcm_lib_malloc_pages().

Yes, this is what I do today. I am considering if I should release the
DMA stuff in hw_free and then in the prepare callback check if it needs
to be prepared again (since it was released in hw_free).

> The prepare and start/stop trigger callbacks are called only between
> hw_params/hw_free, but they could be called multiple times, so you
> should handle this stuff in the prepare callback only if your device
> requires that this is done every time before a stream is started.

Yes, I setup the cyclic DMA buffer the first time I get a prepare
callback, and free the cyclic DMA buffer in the hw_free callback.

This works fine as it is now, but I just want to make sure I handle all
cases which might appear on the way.

Best regards,
Hans-Christian Egtvedt

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