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

[Simon Ser]

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.

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

> 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
>
> Thanks,
> pq