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

[Daniel Vetter]

On Tue, Sep 10, 2019 at 12:38 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:
>
> 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

I think we should do an s/rule/recommendation to drivers to avoid
surprises in hw programming/ or something like that. Plus we allow
that for legacy ioctl, otherways it's impossible to change zpos with
legacy ioctls (not that I'm sure that's a reasonable use-case, but
whatever). With that also clarified I think Simon's patch is good to
go, same zpos value is really not something we want to encourage. Plus
"implicit by plane id" is already something we use in other places
(like figuring out which primary plan belongs to which crtc), and it's
just terrible uapi.
-Daniel

>     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



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch