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

[Yang, Libin]

Hi David,


> -----Original Message-----
> From: David Henningsson [mailto:david.henningsson at canonical.com]
> Sent: Thursday, November 05, 2015 5:07 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; Nikula, Jani
> Subject: Re: [alsa-devel] DP1.2 MST audio support discussion
> 
> 
> 
> 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?

I changed the audio driver and did the test today. The test is:
1. Pin 5 connect to converter 2 and playback, there is sound from pin 5.
2. Pin 5 connect to converter 2 (no other pin is connected to cvt2), no sound.
3. Pin 5 connect back to cvt 2, there is sound, playback is normal.

And in step 2, the playback is still ongoing but no sound is out from any pin.

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