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

[David Henningsson]

On 2015-11-04 15:17, Yang, Libin wrote:
> + Jani, who is from Intel gfx team.
>
>> -----Original Message-----
>> From: David Henningsson [mailto:david.henningsson at canonical.com]
>> Sent: Monday, November 02, 2015 3:55 PM
>> To: Yang, Libin; Takashi Iwai
>> Cc: Lin, Mengdong; Raymond Yau; airlied at linux.ie; tanuk at iki.fi; ALSA
>> Development Mailing List; Girdwood, Liam R; Lu, Han
>> Subject: Re: [alsa-devel] DP1.2 MST audio support discussion
>>
>>
>>
>> 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?
>
> GFX team hope us that if there is no monitor connected, audio
> should release gfx so gfx driver can enter power save mode.
> Based on the requirement, audio driver may need stop playback
> as soon as the monitor is disconnected.

I'm not talking about continuous playback here, so power consumption 
should not be a big concern. E g, if it takes a second or so before PA 
reroutes audio somewhere else, it's not a big deal from a power save 
perspective.

So I'm asking my question again. What would happen (on Intel hardware), 
if you stream audio to a converter node on the audio codec, and there is 
no pin node connected to that converter node?

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic