[alsa-devel] [PATCH 2/2] Improve hw_params documentation

[Raymond Yau]

2011/11/17 Andrew Eikum <aeikum at codeweavers.com>:
> On Wed, Nov 16, 2011 at 10:20:07PM +0100, Clemens Ladisch wrote:
>> Andrew Eikum wrote:
>> > +++ b/include/pcm.h
>> > @@ -44,8 +44,20 @@ extern "C" {
>> >
>> >  /** PCM generic info container */
>> >  typedef struct _snd_pcm_info snd_pcm_info_t;
>> > -/** PCM hardware configuration space container */
>> > +
>> > +/** PCM hardware configuration space container
>> > + *
>> > + *  snd_pcm_hw_params_t is an opaque structure which contains a set of possible
>> > + *  PCM hardware configurations. For example, a given instance might include a
>> > + *  range of buffer sizes, a range of period sizes, and a set of several sample
>> > + *  formats. Some subset of all possible combinations these sets may be valid,
>> > + *  but not necessarily any combination will be valid.
>> > + *
>> > + *  No validation is done by the various snd_pcm_hw_params_set* functions.
>>
>> These functions do validate the value that the application is trying to set
>> and adjust all other dependent limits.
>>
>
> I didn't find that to be the case in my testing, at least between
> periods, period_size, and buffer_size. I've attached a test program
> here. It sets periods to 10, period_size to 1024, and buffer_size_max
> to be the same as the return from get_buffer_size_min (in my case,
> 80). The first call to fail is snd_pcm_hw_params(). If what you said
> is true, I would expect the set_buffer_size_max() call to fail.
>
> $ gcc -I /usr/include/alsa -lasound -o alsa_period_count alsa_period_count.c
> $ ./alsa_period_count
> min_buffer_frames: 80
> set max buffer size: 80
> snd_pcm_hw_params: -12
> $
>
> Andrew
>

The logic of your program is wrong since your program did not set
channel, rate and format

There are sound cards which support S16 and U8, mono and stereo.

As  "default" device using "plug:dmix" , "pulse" or "hw"

The buffer and period calculation is only valid after you have choose
the format and channels

http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html

Hardware related parameters
The ALSA PCM devices use the parameter refining system for hardware
parameters - snd_pcm_hw_params_t. It means, that application choose
the full-range of configurations at first and then application sets
single parameters until all parameters are elementary (definite).