[alsa-devel] [RFC][PATCH 0/8] ALSA: dice: constrain PCM substreams to current sampling transfer frequency

[Takashi Iwai]

On Sat, 21 Nov 2015 01:29:24 +0100,
Takashi Sakamoto wrote:
> > In your original statement:
> >> As a result, userspace applications can request PCM substreams at current
> >> sampling transfer frequency. Therefore, when users want to start PCM
> >> substreams at different rate, they should set the rate in advance by the
> >> other ways (i.e. ffado-dbus-server/ffado-mixer).
> > 
> > So, an application cannot change the PCM rate other than the value
> > currently set by another tool.  Is it correct?
> 
> Correct.

The single rate restriction is fairly common among many drivers.
As this appears like a hardware limitation on DICE, it's fine, per se.
But, requiring a special tool to set the sample rate is different; it
sounds strange to me.

Why it must be *only* by another tool, not by PCM interface itself?
Suppose you playing a single application.  Kernel driver also knows
that it's currently only a single process accessing the hardware.
What prevents it changing the sample rate? 

And, even if we implement in that way -- allowing only the locked
sample rate -- by some reason (e.g. due to the code complexity), why
can't it be controlled via a more common interface like a normal mixer
element or such?  Some drivers do so, simply by providing an enum
control for the master sample rate.

So again: restricting the PCM per one rate itself is understandable.
The main question is, however, how to manage the current sample rate.
If the first-user-allowed rule is applied, there won't be a big
regression, for example.


thanks,

Takashi