Best practice device tree design for display subsystems/DRM

[Sebastian Hesselbarth]

On 07/02/2013 09:19 PM, Russell King wrote:
> On Tue, Jul 02, 2013 at 08:42:55PM +0200, Jean-Francois Moine wrote:
>> It seems that you did not look at the NVIDIA Tegra driver (I got its
>> general concept for my own driver, but I used a simple atomic counter):
>>
>> - at probe time, the main driver (drivers/gpu/host1x/drm/drm.c) scans
>>    the DT and finds its external modules. These ones are put in a
>>    "clients" list.
>>
>> - when loaded, the other modules register themselves into the main
>>    driver. This last one checks if each module is in the "client" list.
>>    If so, the module is moved from the "client" to an "active" list".
>>
>> - when the "client" list is empty, this means all modules are started,
>>    and, so, the main driver starts the drm stuff.
>>
>> The active list is kept for module unloading.
>
> Please tell me how this works with the two LCD controllers if you wish
> to drive the two LCD controllers as entirely separate devices.  Given
> that the above requires the use of global data in the driver, how do
> you distinguish between the two?
>
>> Putting "phandle"s in the 'display' seems more flexible (I did not do so
>> because I knew the hardware - 2 LCDs and the dcon/ire).
>
> Except you haven't looked at the bigger picture - the Armada 510 is
> unusual in that it has two LCD controllers and the DCON.  All of the
> other SoCs using this IP block that I've been able to research have
> only one LCD controller and no DCON.  I don't think they even have an
> IRE (image rotation engine) either.
>
> Neither have you considered the case where you may wish to keep the
> two LCD controllers entirely separate (eg, you want X to drive one
> but something else on the other.)  X drives the DRM device as a whole,
> including all CRTCs which make up that device - with them combined
> into one DRM device, you can't ask X to leave one controller alone
> because you're doing something else with it.  (This is just the simple
> extension of the common case of a single LCD controller, so it's
> really nothing special.)
>
> So, the unusual case _is_ the Armada 510 with its two LCD controllers
> and DCON which we _could_ work out some way of wrapping up into one
> DRM device, or we could just ignore the special case, ignore the DCON
> and just keep the two LCD CRTCs as two separate and independent DRM
> devices.
>
> I'm actually starting to come towards the conclusion that we should go
> for the easiest solution, which is the one I just mentioned, and forget
> trying to combine these devices into one super DRM driver.

I am against a super node which contains lcd and dcon/ire nodes. You can
enable those devices on a per board basis. We add them to dove.dtsi but
disable them by default (status = "disabled").

The DRM driver itself should get a video-card node outside of 
soc/internal-regs where you can put e.g. video memory hole (or video
mem size if it will be taken from RAM later)

About the unusual case, I guess we should try to get both lcd
controllers into one DRM driver. Then support mirror or screen
extension X already provides. For those applications where you want
X on one lcd and some other totally different video stream - wait
for someone to come up with a request or proposal.

Sebastian