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 unspoken, legacy rule used in the past was to make user-space figure out the zpos from object IDs. However some drivers break this rule, that's why the ordering is documented as unspecified in case the zpos property is missing. User-space should rely on the zpos property only.
There are some cases in which user-space might read identical zpos values for different planes.
For instance, in case the property is mutable, user-space might set two planes' zpos to the same value. This is necessary to support user-space using the legacy DRM API where atomic commits are not possible: user-space needs to update the planes' zpos one by one.
Because of this, user-space should handle multiple planes with the same zpos.
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. In other words, when user-space sets the same zpos on two planes, drivers should rely on the plane object ID.
v2: clarify drm_plane_state.zpos docs (Daniel)
v3: zpos is for all planes (Marius, Daniel)
v4: completely reword the drm_plane_state.zpos docs to make it clear the recommendation to use plane IDs is for drivers in case user-space uses duplicate zpos values (Pekka)
v5: reword commit message (Pekka, James)
v6: remove mention of Arm GPUs having planes which can't overlap, because this isn't uAPI yet (Daniel)
Signed-off-by: Simon Ser contact@emersion.fr Reviewed-by: Pekka Paalanen ppaalanen@gmail.com Cc: Marius Vlad marius.vlad@collabora.com Cc: Daniel Vetter daniel.vetter@ffwll.ch Cc: James Qian Wang james.qian.wang@arm.com --- drivers/gpu/drm/drm_blend.c | 8 ++++---- include/drm/drm_plane.h | 9 +++++---- 2 files changed, 9 insertions(+), 8 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(). * * 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..328773690851 100644 --- a/include/drm/drm_plane.h +++ b/include/drm/drm_plane.h @@ -140,10 +140,11 @@ struct drm_plane_state { * @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. + * User-space may set mutable zpos properties so that multiple active + * planes on the same CRTC have identical zpos values. This is a + * user-space bug, but drivers can solve the conflict by comparing the + * plane object IDs; the plane with a higher ID is 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. -- 2.23.0