Best practice device tree design for display subsystems/DRM

[Sebastian Hesselbarth]

On 07/03/13 13:43, Inki Dae wrote:
>> I do not understand why you keep referring to the SoC dtsi. Im my
>> example, I said that it is made up and joined from both SoC dtsi and
>> board dts.
>>
>> So, of course, lcd controller nodes and dcon are part of dove.dtsi
>> because they are physically available on every Dove SoC.
>>
>> Also, the connection from lcd0 to the external HDMI encoder node
>> is in the board dts because you can happily build a Dove SoC board
>> with a different HDMI encoder or with no encoder at all.
>>
>> The video-card super node is not in any way specific to DRM and
>
> In case of fbdev, framebuffer driver would use lcd0 or lcd1 driver, or lcd0
> and lcd1 drivers which are placed in drivers/video/backlight/.
>
> And let's assume the following:
>
> On board A
>       Display controller ------------- lcd 0
> On board B
>       Display controller ------------- lcd 1
> On board C
>       Display controller ------------- lcd 0 and lcd 1
>
> Without the super node, user could configure Linux kernel through menuconfig
> like below;
> 	board A:  enabling lcd 0, and disabling lcd 1,
> 	board B: disabling lcd 0, and enabling lcd 1,
> 	board C: enabling lcd 0 and lcd 1.
>
> All we have to do is to configure menuconfig to enable only drivers for
> certain board. Why does fbdev need the super node? Please give me comments
> if there is my missing point.

I assume when you say "configure menuconfig" you mean
"create a CONFIG_DISPLAY_CONTROLLER_AS_USED_ON_BOARD_A,
CONFIG_DISPLAY_CONTROLLER_AS_USED_ON_BOARD_B, ..." ?

This finally will require you to have
(a) #ifdef mess for every single board _and_ driver above
(b) new CONFIG_.._BOARD_D plus new #ifdefs in fbdev driver for every
     new board
(c) A new set of the above CONFIG_/#ifdef for DRM driver

This can also be done with device_tree and supernode approach,
so for your example above:

BoardA.dts:
video { card0 { video-devices = <&lcd0>; }; };

BoardB.dts:
video { card0 { video-devices = <&lcd1>; }; };

BoardC.dts:
video { card0 { video-devices = <&lcd0 &lcd1>; }; };

and in the driver your are prepared for looping over the video-devices
property and parsing the compatible string of the nodes passed.

Sebastian