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.
- 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