[alsa-devel] How to tell user level hardware not support a certain format in a certain sample rate

Richard Zhao linuxzsc at gmail.com
Tue Mar 17 07:57:09 CET 2009

On Mon, Mar 16, 2009 at 4:54 PM, Clemens Ladisch <clemens at ladisch.de> wrote:
> Richard Zhao wrote:
>> Sure I can add rules. but the rules will be checked as hw_param. If
>> hw_param failed, does alsalib convert fmt or sample rate
>> automatically?
>> For example:
>> One codec, support:
>> 44.1k : s8 s16 s24 s32
>> 96k:  s24 s32
>> When user try to play s8/s16 96k audio file, hw_param will fail.
> This is not how the rules work.
> Initializing a device works like this:
> 1) The application opens the device, and the driver's open callback gets
>   called.
> 2) The application chooses hardware parameters; ALSA checks that these
>   values conform to the device's limitations.
> 3) After all hardware parameters are known, the driver's hw_params
>   callback gets called.
> When you driver adds constraint rules, it has do this in the open
> callback (so that the rules are available in step 2 above).  Your
> constraint rules will be called when the application tries to select
> parameters; this is before the hw_params callback gets called.
> In your example above, the open callback would install two rule
> functions: one to remove s8/s16 if 96k has been selected, and one to
> remove 96k if s8/s16 has been selected.
> When the hw_params callback is called, ALSA guarantees that the
> parameters are valid for the device.
> Clemens

Hi Clemens,

Thank you!  I see. After set every hw parm, snd_pcm_hw_refine will
called, then the corresponding constraints(rules) will be checked.

Thanks again!

More information about the Alsa-devel mailing list