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

Clemens Ladisch clemens at ladisch.de
Mon Mar 16 09:54:28 CET 2009

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
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.


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