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 05:07:51PM +0200, Daniel Vetter wrote:
> On 20/05/2014 16:57, Thierry Reding wrote:
> >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.
> That's why we need a new avsink device which is a proper child of the gfx
> device, and the audio driver needs to use the componentized device framework
> so that the suspend/resume ordering works correctly. Or at least that's been
> my idea, might be we have some small gaps here and there.

The component/master helpers don't allow you to do that. Essentially
what it does is provide a way to glue together multiple devices (the
components) to produce a meta-device (the master). What you get is a
pair of .bind()/.unbind() functions that are called on each of the
components when the master binds or unbinds the meta-device. I don't
see how that could be made to work for suspend/resume.

Thierry

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