[alsa-devel] alsalib and snd_pcm_hw_params_set_rate_minmax

[Takashi Iwai]

On Thu, 15 Mar 2018 13:20:14 +0100,
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 :(

Yes, that's the design.  The only way to expand is to reset the whole,
space and reduce again to the given size.


> > 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.

Right.  At the point where snd_pcm_hw_params_any() is called, the
whole configuration gets reset.  That's the reason I thought we may
need to pass all 5 parameters in the query protocol.

IOW, the query stuff won't be modal, it just tries to reduce the given
configuration space to the acceptable ranges.


Takashi