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
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