On Wed, 18 Sep 2019 16:33:47 +0000
Simon Ser <contact@xxxxxxxxxxx> 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.
>
> While at it, remove the assumption that zpos is only for overlay planes.
>
> Additionally, update the drm_plane_state.zpos docs to clarify that zpos
> disambiguation via plane object IDs is a recommendation for drivers, not
> something user-space can rely on.
>
> v2: clarify drm_plane_state.zpos docs (Daniel)
>
> v3: zpos is for all planes (Marius, Daniel)
>
> Signed-off-by: Simon Ser <contact@xxxxxxxxxxx>
> Cc: Pekka Paalanen <ppaalanen@xxxxxxxxx>
> Cc: Marius Vlad <marius.vlad@xxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> drivers/gpu/drm/drm_blend.c | 8 ++++----
> include/drm/drm_plane.h | 6 +++---
> 2 files changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
> index d02709dd2d4a..121481f6aa71 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 planes, see drm_plane_create_zpos_immutable_property().
Hi,
the above looks good to me.
> *
> * pixel blend mode:
> * Pixel blend mode is set up with drm_plane_create_blend_mode_property().
> diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
> index cd5903ad33f7..7ac68057b19d 100644
> --- a/include/drm/drm_plane.h
> +++ b/include/drm/drm_plane.h
> @@ -141,9 +141,9 @@ struct drm_plane_state {
> * 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.
> + * identical zpos value. To solve the conflict, the recommendation to
> + * drivers to avoid surprises is to compare the plane object IDs; the
> + * plane with a higher ID is stacked on top of a plane with a lower ID.
While better, I think this would need a less subtle clarification:
+ "Userspace should never rely on stacking order derived from IDs."
Oh, I think I realised it only now. This comment is for the drivers to
handle a case where userspace is being stupid and assigns mutable zpos
properties with duplicate values, right?
It does *not* allow drivers themselves to assign duplicate zpos values.
So I've been looking at this from the wrong angle.
Maybe it should just say that then?
Instead of:
"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 paragraph could start with:
"Userspace may set mutable zpos properties so that multiple
active planes on the same crtc have identical zpos value. This
is a userspace bug, but drivers can solve the conflict
deterministically by comparing the plane object IDs;"
Thanks,
pq
> *
> * See drm_plane_create_zpos_property() and
> * drm_plane_create_zpos_immutable_property() for more details.
> --
> 2.23.0
>
>
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