[Patch 2/4] dt-bindings: display/ti: Add plane binding to dispc node

[Tomi Valkeinen]

On 19/04/18 09:34, Daniel Vetter wrote:

>> But the kernel cannot know what the user wants to do, so it cannot
>> configure the planes. If we have an HDMI output which supports 2k+ and a
>> -2k LCD, and 4 hw planes, we can set up the planes at least in the
>> following ways:
>>
>> - One virtual plane on HDMI, two normal planes on LCD. Here the normal
>> planes can also be used on the HDMI, as long as the input width is -2k.
>>
>> - One virtual plane on HDMI, one virtual plane on LCD, but sometimes
>> both planes on the same display (either HDMI or LCD).
>>
>> - No virtual planes (and fbdev support disabled). This needs the
>> userspace to take care not to configure 2k+ planes. But considering that
>> the modes supported are still quit close to 2k in width, I believe
>> upscaling a 2k plane cover the whole display would provide quite ok image.
>>
>> Each of those requires a different virtual plane setup.
> 
> Why do you want to hardcode this in DT? The recommended approach is to
> have a bunch of virtual planes, and runtime assign whatever hw resources
> you need to get there. If you run out of hw planes you just fail with
> -EINVAL in atomic_check.

That is possible, but how many userspace apps will work correctly if the
planes work or don't work in a random manner (from userspace's point of
view)? I like the idea more that the DRM driver exposes a lesser number
of planes which always work, instead of exposing larger number of planes
which sometimes do not work.

And with userspace apps, I don't mainly mean Weston and X, but instead
the numerous custom applications the customers write themselves. Perhaps
I'm worrying too much, but I can imagine a flood of support requests
about why plane setup is not working when one does this or that simple
setup.

Also one complication with runtime assignment is that the hw planes are
not identical: some support YUV modes, some don't, some support scaling,
some don't. That's probably not a show stopper, but it does limit the
options as e.g. we can't have all virtual planes advertising YUV support
when we have a hw plane that doesn't support YUV.

 Tomi

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