[alsa-devel] DP1.2 MST audio support discussion

[David Henningsson]

On 2015-11-02 08:30, Yang, Libin wrote:
>
>>> 4. power management
>>> If we close the PCM when disconnecting monitor, the driver will
>> return
>>> -ENODEV, and we assume user space will close the PCM at once .
>> ... if we implement it.  Actually this behavior (returning -ENODEV) is
>> not mandatory and needs investigation beforehand, whether this
>> breaks
>> any applications, especially PA.
> Yes. I heard that PA has dummy PCM and when there is no HW to
> play the audio, PA can smoothly move the PCM from HW to dummy
> PCM. If it's true, stopping PCM will not break PA, I think.

Well, in most cases (not all) there will be an analog output available, 
so we're more likely to move there than to a dummy PCM. The problem is 
that PA does not currently figure out that a -ENODEV means that one 
should switch from a digital to an analog profile. So, at least at this 
point, don't expect a "smooth move".

I'm not sure if this is a good idea or not, but just brainstorming here:

If PA (or other application) tries to play back to a PCM, and that PCM 
is currently unassigned. What would happen if we actually assigned a 
converter node at that point, but no pin node connected to that 
converter node? Would that give a backwards compatible behaviour? I e, 
audio userspace could successfully play back audio through the converter 
node, but then the audio goes nowhere because there is no pin node assigned?

(Or perhaps we PA developers should actually start supporting this 
properly like we said that we should do, for like three years or 
something...)

// David