Best practice device tree design for display subsystems/DRM

[Sascha Hauer]

On Thu, Jul 04, 2013 at 11:40:17AM +0200, Sebastian Hesselbarth wrote:
> On 07/04/13 11:23, Sascha Hauer wrote:
> >
> >With this you can describe the whole graph of devices you have in the
> >devicetree. The examples in this file have a path from a camera sensor
> >via a MIPI converter to a capture interface.
> >
> >The difference to a supernode is that this approach describes the data
> >flow in the devicetree so that we can iterate over it to find links
> >between source and sink rather than relying on a list of subdevices to
> >be completed.
> 
> Agree. But that is not that different from linux,video-external-encoder
> property I made up, except that the name is different.
> 
> And, I still see no way with that source/sink linking _alone_ how to
> tell that either lcd0 and lcd1 act as a _single_ video card or lcd0 and
> lcd1 are used in a _two_ video card setup.
> 
> There is no single device node on Dove that would sufficiently act as
> the top node for a working video card on all boards. And there is no
> framebuffer node to link each of the lcd0/1 nodes to.
> 
> That is what the super-node is for, form a virtual device called
> video card to act as a container for all those SoC devices that are
> not sufficient for a working video setup on their own.
> 
> If lcd0 needs that hdmi-transmitter you link it to the lcd0 node -
> not the super-node. If lcd0 needs some pll clock you link it to the
> lcd0 node - again not the super-node.
> 
> The super-node(s) just connects all SoC devices that shall be part of
> your board-specific video card(s) - for Dove that is any combination of
> lcd0, lcd0, dcon and video memory allocation.

So with the supernode approach you would have one/two supernodes and
with a v4l2 approach you would have either one graph containing lcd0 and
lcd1 or to graphs (without a connection in between).

Sascha

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