[alsa-devel] HDMI codec, way forward?

[Russell King - ARM Linux]

On Wed, Oct 21, 2015 at 07:59:06PM +0200, Takashi Iwai wrote:
> On Wed, 21 Oct 2015 19:34:37 +0200,
> Russell King - ARM Linux wrote:
> > 
> > On Wed, Oct 21, 2015 at 09:49:28PM +0530, Vinod Koul wrote:
> > > On Wed, Oct 21, 2015 at 03:37:47PM +0100, Russell King - ARM Linux wrote:
> > > > In any case, this doesn't (and can't) solve the CEC problem, so it's not
> > > > a solution to the problem at hand.
> > > 
> > > Sorry am not sure I follow the reasons for that, wouldn't CEC be another
> > > slave in such an interface? I though component fwk did allow us to have
> > > multiple slaves..
> > 
> > Not with the way you're using the component helper here.
> > 
> > I guess that not all my message is being read, because people keep
> > replying half-way down my messages...
> > 
> > You can only register a struct device _once_ as a slave device.
> > 
> > With the way you're using it here for audio, you're registering the
> > i915 DRM device as a slave component device, and the audio side as
> > the master.  That means the audio master can bind to the DRM slave
> > component device.
> > 
> > You can't then have a CEC master bind to the i915 DRM slave device
> > (it's already bound to the audio master device), and you can't
> > register the i915 DRM device as a second slave component device.
> > It becomes indistinguishable from the first, and there's no way
> > to tell which of the two different 'ops' structures should be used
> > with which master.
> > 
> > I said this in my message 20151021140307.GE32532 at n2100.arm.linux.org.uk
> > which was two of my replies ago in this sub-thread.
> 
> Can't the limitation of single slave dev be extended simply?  e.g. add
> some matching semantics to component_master_add_child() like a shared
> key in both master and slave, and let assign only the matched slave.

I've already explained that in the email message I referred to by
message ID above (which was a reply to one of your previous messages)

This is why I find email such a poor communication medium - I'm quite
sure most people only read half an email message before hitting reply,
and then they stick their reply after where they stopped reading and
never bother reading the rest of the message.  Normally, at this point,
I'll start getting grumpy and sending frustrated emails... which would
eventually deride into flames, but let's _try_ to keep this civil.

Here's the relevant paragraph from the above referenced message, and
to make the appropriate bit stand out, I've underlined it with ^.

> 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.)

In a subsequent sentence, I also address the issue that would occur
if any of the already componentised DRM drivers were to attempt what
you're doing in i915 - although I say it's solvable, I've resisted
that because it makes the locking in there _much_ more hairy, and I'm
now not certain that my more complex locking implementation would
even cope with that scenario.

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