Re: [PATCH 0/2] drm/amdgpu/display: Make multi-plane configurations more flexible

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On Fri, 12 Apr 2024 10:28:52 -0400
Leo Li <> wrote:

> On 2024-04-12 04:03, Pekka Paalanen wrote:
> > On Thu, 11 Apr 2024 16:33:57 -0400
> > Leo Li <> wrote:
> >   


> >> That begs the question of what can be nailed down and what can left to
> >> independent implementation. I guess things like which plane should be enabled
> >> first (PRIMARY), and how zpos should be interpreted (overlay, underlay, mixed)
> >> can be defined. How to handle atomic test failures could be as well.  
> > 
> > What room is there for the interpretation of zpos values?
> > 
> > I thought they are unambiguous already: only the relative numerical
> > order matters, and that uniquely defines the KMS plane ordering.  
> The zpos value of the PRIMARY plane relative to OVERLAYS, for example, as a way
> for vendors to communicate overlay, underlay, or mixed-arrangement support. I
> don't think allowing OVERLAYs to be placed under the PRIMARY is currently
> documented as a way to support underlay.

I always thought it's obvious that the zpos numbers dictate the plane
order without any other rules. After all, we have the universal planes
concept, where the plane type is only informational to aid heuristics
rather than defining anything.

Only if the zpos property does not exist, the plane types would come
into play.

Of course, if there actually exists userspace that fails if zpos allows
an overlay type plane to be placed below primary, or fails if primary
zpos is not zero, then DRM needs a new client cap.

> libliftoff for example, assumes that the PRIMARY has the lowest zpos. So
> underlay arrangements will use an OVERLAY for the scanout plane, and the PRIMARY
> for the underlay view.

That's totally ok. It works, right? Plane type does not matter if the
KMS driver accepts the configuration.

What is a "scanout plane"? Aren't all KMS planes by definition scanout

IOW, if the KMS client understands zpos and can do a proper KMS
configuration search, and all planes have zpos property, then there is
no need to look at the plane type at all. That is the goal of the
universal planes feature.


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