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

[Jani Nikula]

On Wed, 04 Nov 2015, "Yang, Libin" <libin.yang at intel.com> 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.

Rather, as soon as the userspace responding to hotplug uevents has
reacted and disabled the display. See [1] for a better explanation. In
other words, it would be desirable that you call the put power hook in
response to intel_audio_codec_disable().

BR,
Jani.


[1] https://bugs.freedesktop.org/show_bug.cgi?id=90447#c19

>
>> 
>> (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
>

-- 
Jani Nikula, Intel Open Source Technology Center