[Intel-gfx] [RFC] set up an sync channel between audio and display driver (i.e. ALSA and DRM)

[Babu, Ramesh]

> On Tue, May 20, 2014 at 05:29:07PM +0300, Imre Deak wrote:
> > On Tue, 2014-05-20 at 05:52 +0300, Lin, Mengdong wrote:
> > > This RFC is based on previous discussion to set up a generic
> > > communication channel between display and audio driver and an
> > > internal design of Intel MCG/VPG HDMI audio driver. It's still an
> > > initial draft and your advice would be appreciated to improve the
> > > design.
> > >
> > > The basic idea is to create a new avsink module and let both drm and
> > > alsa depend on it.
> > > This new module provides a framework and APIs for synchronization
> > > between the display and audio driver.
> > >
> > > 1. Display/Audio Client
> > >
> > > The avsink core provides APIs to create, register and lookup a
> > > display/audio client.
> > > A specific display driver (eg. i915) or audio driver (eg. HD-Audio
> > > driver) can create a client, add some resources objects (shared
> > > power wells, display outputs, and audio inputs, register ops) to the
> > > client, and then register this client to avisink core. The peer
> > > driver can look up a registered client by a name or type, or both.
> > > If a client gives a valid peer client name on registration, avsink
> > > core will bind the two clients as peer for each other. And we expect
> > > a display client and an audio client to be peers for each other in a
> > > system.
> >
> > One problem we have at the moment is the order of calling the system
> > suspend/resume handlers of the display driver wrt. that of the audio
> > driver. Since the power well control is part of the display HW block,
> > we need to run the display driver's resume handler first, initialize
> > the HW, and only then let the audio driver's resume handler run. For
> > similar reasons we have to call the audio suspend handler first and
> > only then the display driver resume handler. Currently we solve this
> > using the display driver's late/early suspend/resume hooks, but we'd
> > need a more robust solution.
> >
> > This seems to be a similar issue to the load time ordering problem
> > that you describe later. Having a real device for avsync that would be
> > a child of the display device would solve the ordering issue in both
> > cases. I admit I haven't looked into it if this is feasible, but I
> > would like to see some solution to this as part of the plan.
> 
> If we are able create and mandate that HDMI display controller is parent and
> audio is child device, then this wouldn't be an issue and PM frameowrk will
> ensure parent is suspended last.
> 
If there is a scenario where HDMI audio has to active but display has to go to low power, then
parent-child device is not optimal.  There needs to be a mechanism to turn on/off individual hw blocks within
the controller.