[PATCH] drm: two planes with the same zpos have undefined ordering

[Simon Ser]

> On Tue, 10 Sep 2019 11:20:16 +0000
> Simon Ser <contact at emersion.fr> wrote:
>
> > On Tuesday, September 10, 2019 1:38 PM, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >
> > > On Tue, 10 Sep 2019 10:09:55 +0000
> > > Simon Ser contact at emersion.fr wrote:
> > >
> > > > Currently the property docs don't specify whether it's okay for two planes to
> > > > have the same zpos value and what user-space should expect in this case.
> > > > The rule mentionned in the past was to disambiguate with object IDs. However
> > > > some drivers break this rule (that's why the ordering is documented as
> > > > unspecified in case the zpos property is missing). Additionally it doesn't
> > > > really make sense for a driver to user identical zpos values if it knows their
> > > > relative position: the driver can just pick different values instead.
> > > > So two solutions would make sense: either disallow completely identical zpos
> > > > values for two different planes, either make the ordering unspecified. To allow
> > > > drivers that don't know the relative ordering between two planes to still
> > > > expose the zpos property, choose the latter solution.
> > > >
> > > > Signed-off-by: Simon Ser contact at emersion.fr
> > > >
> > > > ---------------------------------------------
> > > >
> > > > Err, I'm sorry about the double-post. I sent this to intel-gfx by mistake.
> > > > drivers/gpu/drm/drm_blend.c | 8 ++++----
> > > > 1 file changed, 4 insertions(+), 4 deletions(-)
> > > > diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
> > > > index d02709dd2d4a..51bd5454e50a 100644
> > > > --- a/drivers/gpu/drm/drm_blend.c
> > > > +++ b/drivers/gpu/drm/drm_blend.c
> > > > @@ -132,10 +132,10 @@
> > > >
> > > > -   planes. Without this property the primary plane is always below the cursor
> > > > -   plane, and ordering between all other planes is undefined. The positive
> > > > -   Z axis points towards the user, i.e. planes with lower Z position values
> > > >
> > > > -   -   are underneath planes with higher Z position values. Note that the Z
> > > > -   -   position value can also be immutable, to inform userspace about the
> > > > -   -   hard-coded stacking of overlay planes, see
> > > > -   -   drm_plane_create_zpos_immutable_property().
> > > >
> > > > -   -   are underneath planes with higher Z position values. Two planes with the
> > > > -   -   same Z position value have undefined ordering. Note that the Z position
> > > > -   -   value can also be immutable, to inform userspace about the hard-coded
> > > > -   -   stacking of overlay planes, see drm_plane_create_zpos_immutable_property().
> > > >     -
> > > >     -   pixel blend mode:
> > > >     -   Pixel blend mode is set up with drm_plane_create_blend_mode_property().
> > >
> > > Hi,
> > >
> > > this seems to contradict what the docs say in another place:
> >
> > Except this comment is about drm_plane_state.zpos, an internal DRM
> > property. This is not about the zpos property itself.
>
> Hi,
>
> then I'm very confused. What's the difference?
>
> The text you are changing was specifically added to explain uAPI
> behaviour, that is, what the userspace sees and does with the zpos
> property in uAPI.
>
> Having two conflicting pieces of documentation is confusing, especially
> when it is not absolutely clear which one is for driver implementors
> and which one for uAPI consumers, or that one must ignore the other doc
> depending on who you are.

Yes, I agree that this is confusing.

To be completely honest, I don't really care what the outcome of this
discussion is, as long as there are no conflicting documentation comments.

So, my interpretation of the docs is that there are:

1. Some documentation for KMS properties, that is, what is exposed to
   user-space via DRM ioctls. This is the KMS uAPI.
2. Some documentation for kernel drivers, that is, internal DRM state that can
   be set by kernel developers. This includes helper functions and common
   structs.

Obviously as user-space developers we only care about (1).

Now, back to zpos' case: there are two doc comments about zpos.

The first one is [1], the one this patch changes:

> zpos:
> Z position is set up with drm_plane_create_zpos_immutable_property() and
> drm_plane_create_zpos_property(). It controls the visibility of overlapping
> planes. Without this property the primary plane is always below the cursor
> plane, and ordering between all other planes is undefined. The positive Z
> axis points towards the user, i.e. planes with lower Z position values are
> underneath planes with higher Z position values. Note that the Z position
> value can also be immutable, to inform userspace about the hard-coded
> stacking of overlay planes, see drm_plane_create_zpos_immutable_property().

This is in the "Plane Composition Properties". I believe this paragraph
documents the standard plane properties (standard as in not driver-specific).
So I'd say this documents the KMS uAPI.

The second one is [2], the one you've quoted:

> zpos
>
> Priority of the given plane on crtc (optional).
>
> Note that multiple active planes on the same crtc can have an identical zpos
> value. The rule to solving the conflict is to compare the plane object IDs;
> the plane with a higher ID must be stacked on top of a plane with a lower ID.
>
> See drm_plane_create_zpos_property() and
> drm_plane_create_zpos_immutable_property() for more details.

This is in the "Plane Functions Reference" section, more precisely in the
"struct drm_plane_state" docs. I believe this is really just about the common
DRM struct that can be used by all drivers. This struct isn't exposed to
user-space. It's just an implementation detail of DRM.

(We can see that even without this patch, these two comments already
kind of conflict. The first one says that without zpos ordering is
undefined. The second one says that two identical zpos values means the
plane ID should be used. However drm_plane_state is zero-filled, so a
driver not supporting zpos would have all drm_plane_state.zpos fields
set to zero? Since they are all equal, is the object ID ordering rule
relevant?)

[1]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-composition-properties
[2]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference

> > The comment was introduced in v2 of [1], although the motivation for
> > the change isn't documented.
> >
> > [1]: https://patchwork.freedesktop.org/series/13528/#rev2
>
> That does not look like the comment you are changing.
>
> Wait, which one is "this comment"?
>
> > > zpos
> > >
> > > Priority of the given plane on crtc (optional).
> > >
> > > Note that multiple active planes on the same crtc can have an
> > > identical zpos value. The rule to solving the conflict is to
> > > compare the plane object IDs; the plane with a higher ID must be
> > > stacked on top of a plane with a lower ID.
> > >
> > > See drm_plane_create_zpos_property() and
> > > drm_plane_create_zpos_immutable_property() for more details.
> > >
> > > from https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference
>
> Me and others have taken the above to document uAPI. How could that not
> be the case?

(See above)

> Is it wrong for userspace to assume that plane object ID order reflects
> the plane stacking order? Weston does not do this yet, but since we just
> recently found the above comment, we planned to make use of it.
>
> Actually, if the ID ordering is true, then the DRM core could always
> expose the zpos property as immutable to userspace, just manufacture it
> from object IDs if nothing is set by a driver explicitly.
>
> Or is the comment about object IDs wrong and should be changed to say
> the opposite?

I believe so. If zpos could always be obtained from plane object IDs, then what
would be the point of exposing the immutable zpos property at all?

I've heard that some drivers break this object ID assumption, which is the
reason why this zpos property exists. Can someone more familiar with the kernel
provide some feedback?