[alsa-devel] alsalib and snd_pcm_hw_params_set_rate_minmax

[Takashi Sakamoto]

On Mar 15 2018 21:20, Oleksandr Andrushchenko wrote:
> On 03/15/2018 01:59 PM, Takashi Sakamoto wrote:
>> Hi,
>>
>> On Mar 15 2018 19:45, Oleksandr Andrushchenko wrote:
>>> Is it possible for user-space to reduce configuration space
>>> with snd_pcm_hw_params_set_rate_minmax and then change it
>>> with another snd_pcm_hw_params_set_rate_minmax with values
>>> out of the reduced config?
>>>
>>> For example, the initial min/max is 44100/48000 and I set 44100
>>> first, e.g.
>>>
>>> snd_pcm_hw_params_set_rate_minmax(handle, hw_params, 44100, 0, 44100, 0)
>>>
>>> and then want
>>>
>>> snd_pcm_hw_params_set_rate_minmax(handle, hw_params, 48000, 0, 48000, 0)
>>>
>>> Obviously, the last call fails as we have already a reduced
>>> space of [44100; 44100].
>>>
>>> Is there a way I can still set the range to [48000; 48000]?
>>>
>>> Thank you,
>>> Oleksandr
>>>
>>> P.S. This is in context of work done for [1]
>>>
>>> [1] https://www.spinics.net/lists/alsa-devel/msg75382.html
>>
>> We can't. Once shrinking available interval of a parameter, we cannot
>> expand it again without initializing the parameter on memory object for
>> 'struct snd_pcm_hw_params_t', in which actual layout is never disclosed
>> to user applications.
> So, this effectively means that this is a one way road, if you need to 
> change
> some parameter you'll need to start all over, so the whole configuration
> space remains consistent :(

You miss whole a design of ALSA PCM interface and protocol.

The protocol consists of interactions between applications, ALSA PCM
core and drivers:
1.Userspace applications are expected to allocate 'struct
   snd_pcm_hw_params' in its VMA, then fill it with maximum range and
   set of parameters.
2.The applications are expected to execute ioctl(2) with
    SNDRV_PCM_IOCTL_HW_REFINE, to inquire available set of parameters.
3.In kernel land, ALSA PCM core handles the ioctl request, then shrink
   range and set of parameters on the applications' VMA according to
   runtime information registered by used drivers.
4.In this time, in userspace, memory object for the
   'struct snd_pcm_hw_params' represents available range and set
   parameters. The applications are expected to decide actual value of
   the parameters. If no interests, leave them as is.
5.The applications are expected to execute ioctl(2) with
   SNDRV_PCM_IOCTL_HW_PARAMS finally.
6.In kernel land, ALSA PCM core handles the request, then choose final
   range and set of parameters.

In this protocol, one way is better because expanding range and set of
parameters brings ambiguity of available range and set.

>> If you can initialize whole the parameters, snd_pcm_hw_params_any() is
>> available for your purpose, then set min/max rate again. 
> This is what I do now but...
>> But just for
>> one of the parameters, in my opinion, we need to open an internal API; 
>> snd_pcm_hw_param_any()[1].
> IMO, this will lead to the false assumption that configuration is possible.
> For example, I set 4 channels and 44100, but then, after 
> snd_pcm_hw_params_any,
> set 48000 and might assume that the configuration is still possible. But 
> this may not
> be true: it is true for the configuration returned by snd_pcm_hw_param_any
> as we don't know about 4 channels yet. But might not be allowed if we 
> want 4 channels
> and 48000 at the same time.

Exactly. In short, it's not better idea to expand range and set of
parameters during the above steps.

For your information, in runtime of PCM substream, several parameters
can have dependency. Actually, to handle the dependency,
SNDRV_PCM_IOCTL_HW_REFINE should be called several times to inquire
available range and set of parameters to drivers which satisfies
hardware specification.


Regards

Takashi Sakamoto