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

[Daniel Vetter]

On Tue, May 20, 2014 at 4:29 PM, Imre Deak <imre.deak at 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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch