Re: [alsa-devel] [RFC] set up an sync channel between audio and display driver (i.e. ALSA and DRM)

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, May 20, 2014 at 04:45:56PM +0200, Daniel Vetter wrote:
> On Tue, May 20, 2014 at 4:29 PM, Imre Deak <imre.deak@intel.com> 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.
> 
> Yeah, this is a big reason why I want real devices - we have piles of
> infrastructure to solve these ordering issues as soon as there's a
> struct device around. If we don't use that, we need to reinvent all
> those wheels ourselves.

To make the driver core's magic work I think you'd need to find a way to
reparent the audio device under the display device. Presumably they come
from two different parts of the device tree (two different PCI devices I
would guess for Intel, two different platform devices on SoCs). Changing
the parent after a device has been registered doesn't work as far as I
know. But even assuming that would work, I have trouble imagining what
the implications would be on the rest of the driver model.

I faced similar problems with the Tegra DRM driver, and the only way I
can see to make this kind of interaction between devices work is by
tacking on an extra layer outside the core driver model.

Thierry

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