[alsa-devel] alsalib and snd_pcm_hw_params_set_rate_minmax

[Takashi Iwai]

On Thu, 15 Mar 2018 14:39:33 +0100,
Oleksandr Andrushchenko wrote:
> 
> On 03/15/2018 03:23 PM, Takashi Iwai wrote:
> > 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.
> >
> Clear, thank you
> >>> 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.
> Yes, I now start thinking of the same, e.g. if we pass
> all 5 parameters (mask for formats and intervals for rate, channels,
> buffer and period), then on backend side I can do something like:
> 
> 1. snd_pcm_hw_params_any
> 2. snd_pcm_hw_params_set_format_mask
> 3. snd_pcm_hw_params_set_rate_minmax
> 4. snd_pcm_hw_params_set_channels_minmax
> 5. snd_pcm_hw_params_set_buffer_size_minmax
> 6. snd_pcm_hw_params_set_period_size_minmax
> 
> So, when finished the above confirms that configuration is possible.
> The only concern here is that so many calls on backend side
> might introduce start-up latency on frontend side though
> 
> > IOW, the query stuff won't be modal, it just tries to reduce the given
> > configuration space to the acceptable ranges.
> Do you think the above solution with 5 parameters and the
> corresponding snd_pcm_hw_params_set_xxx calls will do?

I guess so, but let's model & test :)


thanks,

Takashi