Best practice device tree design for display subsystems/DRM

[Russell King]

On Wed, Jul 03, 2013 at 11:02:42AM +0200, Sascha Hauer wrote:
> On Wed, Jul 03, 2013 at 05:57:18PM +0900, Inki Dae wrote:
> > > video {
> > > 	/* Single video card w/ multiple lcd controllers */
> > > 	card0 {
> > > 		compatible = "marvell,armada-510-display";
> > > 		reg = <0 0x3f000000 0x1000000>; /* video-mem hole */
> > > 		/* later: linux,video-memory-size = <0x1000000>; */
> > > 		marvell,video-devices = <&lcd0 &lcd1 &dcon>;
> > > 	};
> > > 
> > > 	/* OR: Multiple video card w/ single lcd controllers */
> > > 	card0 {
> > > 		compatible = "marvell,armada-510-display";
> > > 		...
> > > 		marvell,video-devices = <&lcd0>;
> > > 	};
> > > 
> > > 	card1 {
> > > 		compatible = "marvell,armada-510-display";
> > > 		...
> > > 		marvell,video-devices = <&lcd1>;
> > > 	};
> > > };
> > 
> > Sorry but I'd like to say that this cannot be used commonly. Shouldn't you
> > really consider Linux framebuffer or other subsystems? The above dtsi file
> > is specific to DRM subsystem. And I think the dtsi file has no any
> > dependency on certain subsystem so board dtsi file should be considered for
> > all device drivers based on other subsystems: i.e., Linux framebuffer, DRM,
> > and so no. So I *strongly* object to it. All we have to do is to keep the
> > dtsi file as is, and to find other better way that can be used commonly in
> > DRM.
> 
> +1 for not encoding the projected usecase of the graphics subsystem into
> the devicetree. Whether the two LCD controllers shall be used together
> or separately should not affect the devicetree. devicetree is about
> hardware description, not configuration.

And if we listen to that argument, then this problem is basically
impossible to solve sanely.

Are we really saying that we have no acceptable way to represent
componentized devices in DT?  If that's true, then DT fails to represent
quite a lot of ARM hardware, and frankly we shouldn't be using it.  I
can't believe that's true though.

The problem is that even with an ASoC like approach, that doesn't work
here because there's no way to know how many "components" to expect.
That's what the "supernode" is doing - telling us what components group
together to form a device.

Moreover, if you pay attention to my proposal, what you will realise is
that it isn't DRM specific - it's totally subsystem agnostic.  All it's
doing is collecting a set of other devices together and only then
publishing a device representing the full set of sub-devices.

Now think about that: what is DRM specific about that solution?  What
is the DRM specific about "collecting a set of devices together and
publishing a new device" ?

How is this not "describing the hardware"?  If I attach a HDMI transceiver
to the DCON which is then connected to LCD0, is it not "describing the
hardware" to put into DT that LCD0, DCON, and the HDMI transceiver are
all connected together and therefore are required?  One of the points of
DT after all is that it can and should be used to represent the
relationship between devices.

No - using the tree approach doesn't work, because LCD0, LCD1 and DCON
are all on the same physical bus, but are themselves connected together.
If you like, there are multiple heirarchies here - there's the bus
heirarchy, and then there's the device heirarchy.  Both of these
heirarchies need to be represented in DT, otherwise you're not describing
the hardware properly.

-- 
Russell King