Best practice device tree design for display subsystems/DRM

[Inki Dae]

> -----Original Message-----
> From: Sebastian Hesselbarth [mailto:sebastian.hesselbarth at gmail.com]
> Sent: Wednesday, July 03, 2013 8:52 PM
> To: Inki Dae
> Cc: 'Russell King'; devicetree-discuss at lists.ozlabs.org; 'Jean-Francois
> Moine'; 'Sascha Hauer'; 'Daniel Drake'; dri-devel at lists.freedesktop.org
> Subject: Re: Best practice device tree design for display subsystems/DRM
> 
> 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.
> 

As I mentioned before, fbdev don't need the super node, card0. Please see
the below,

BoardA.dts:
video { dcon: display-controller at 830000 { video-devices = <&lcd0>; }; };

BoardB.dts:
video { dcon: display-controller at 830000 { video-devices = <&lcd1>; }; };

BoardC.dts:
video { dcon: display-controller at 830000 { video-devices = <&lcd0 &lcd1>; };
};

With the above dts file, does the fbdev have any problem? I just changed the
super node to real hardware node. That is why the super node is specific to
DRM.

Thanks,
Inki Dae

> Sebastian