Best practice device tree design for display subsystems/DRM

[Rob Clark]

On Wed, Jul 3, 2013 at 5:02 AM, Sascha Hauer <s.hauer at pengutronix.de> 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.

I do like the supernode idea, it would have avoided the ugly
global-list-of-sub-devices in tilcdc (and probably some other
drivers).

As for projection of use-case, and whether it is something drm
specific or not..  if there is a way to make it generic enough that it
could work for fbdev, well I suppose that is nice.  Not a hard
requirement in my mind, or at least it is a secondary priority
compared to having a better way to having drm devices composed of
sub-{devices,nodes,whatever}.

BR,
-R

> Sascha
>
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