Best practice device tree design for display subsystems/DRM

[Sascha Hauer]

On Thu, Jul 04, 2013 at 11:44:41AM +0200, Sebastian Hesselbarth wrote:
> On 07/04/13 11:30, Sascha Hauer wrote:
> >On Thu, Jul 04, 2013 at 10:11:31AM +0100, Russell King wrote:
> >>On Thu, Jul 04, 2013 at 10:58:17AM +0200, Sascha Hauer wrote:
> >>>On Thu, Jul 04, 2013 at 09:40:52AM +0100, Russell King wrote:
> >>>>Wrong.  Please read the example with the diagrams I gave.  Consider
> >>>>what happens if you have two display devices connected to a single
> >>>>output, one which fixes the allowable mode and one which _can_
> >>>>reformat the selected mode.
> >>>
> >>>What you describe here is a forced clone mode. This could be described
> >>>in the devicetree so that a driver wouldn't start before all connected
> >>>displays (links) are present, but this should be limited to the affected
> >>>path, not to the whole componentized device.
> >>
> >>Okay, to throw a recent argument back at you: so what in this scenario
> >>if you have a driver for the fixed-mode device but not the other device?
> >>
> >>It's exactly the same problem which you were describing to Sebastian
> >>just a moment ago with drivers missing from the supernode approach -
> >>you can't start if one of those "forced clone" drivers is missing.
> >
> >Indeed, then you will see nothing on your display, but I rather make
> >this setup a special case than the rather usual case that we do not
> >have compiled in all drivers for all devices referenced in the
> >supernode.
> 
> The super-node links SoC internal devices that do not necessarily match
> with the subsystem driver. You have one single DRM driver exploiting
> several device nodes for a single video card.
> 
> But you need one device node to hook the driver to.

Currently on i.MX we use a platform_device for this purpose.

Sascha

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