[alsa-devel] HDMI codec, way forward?

[Russell King - ARM Linux]

On Tue, Oct 20, 2015 at 07:31:48PM +0530, Vinod Koul wrote:
> On Tue, Oct 20, 2015 at 09:08:00AM +0100, Russell King - ARM Linux wrote:
> > > > Currently i915/audio component works as you described.  The audio is
> > > > optional and HDMI graphics works without audio, while HDMI HD-audio
> > > > mandates i915 graphics.
> > > 
> > > Right, but we also add additional interface on top of this to allow
> > > things like ensuring display is on when audio wants to run and now
> > > notification for events.
> > > 
> > > I don't see a reason why this can be used as single interface to bind as
> > > well as talk to display from various components, unless I missed obvious
> > > which prevent us from doing this in non i915 cases...
> > 
> > Maybe I can comment more specifically if I saw the code.  Right now all
> > I'm aware of is this idea without any code, and I don't like it.
> 
> Ok, i will be post my patches tomorrow. FWIW uses interface in
> sound/hda/hdac_i915.c for display power up/down

This:

        component_match_add(dev, &match, hdac_component_master_match, bus);
        ret = component_master_add_with_match(dev, &hdac_component_master_ops,
                                              match);
        if (ret < 0)
                goto out_err;

        /*
         * Atm, we don't support deferring the component binding, so make sure
         * i915 is loaded and that the binding successfully completes.
         */
        request_module("i915");

        if (!acomp->ops) {
                ret = -ENODEV;
                goto out_master_del;
        }

is really not nice.

The whole point of the component helper is that you don't care when the
slaves come along.  The above code does care about that, because it
expects that the master and slaves will be bound either inside
component_master_add_with_match(), or via that request_module (which
may happen asynchronously) and ends up setting ->ops.

This won't work for systems in the presence of deferred probing, since
there's only a weak connection between loading a module and the driver
successfully probing its device.

If you arrange for the audio device to finish probing in the master's
bind callback, that would be _far_ better, and would be in line with
how the component helper is supposed to work.  It also means that if
your slave (the video side) goes away, you'll know about it as the
unbind callback will be called (at which point you should tear down
the audio device.)

However, there's a lurking problem: you can't register the same struct
device as a slave more than once into the component helpers - that's
because it's designed to look for devices.  There's intentionally no
linkage between the slave ops and master ops to allow for generic
slave drivers (like tda998x) to be used with different master drivers
(armada, tilcdc, etc).  In theory, you can register the master device
of one componentised system as a slave device of another, but that
requires a much more complicated locking setup in component.c (I have
patches for that, but I've resisted sending them because it makes the
locking very messy.)

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