On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.
Some Arm's usage cases about two planes with same zpos.
- "Smart layer" which can accepts 4 different fbs with different src/display rect, but this smart layer has one restriction:
4 display rects must have no overlaps in V direction (A optimization for android window like Toolbar/Navigation bar)
So when map this Smart-layer to drm world, it might be 4 different drm-planes, but with same zorder to indicate that these 4 planes are smart-laye planes.
- "VR-Layer" One VR-layer comprises two different viewports which can be configured with totoally different fbs, src/display rect. we use two differernt drm-planes to drive on HW "VR-Layer", and these two drm-planes must be configured with same zpos.
Thanks James
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)
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)
Signed-off-by: Simon Ser contact@emersion.fr Cc: Pekka Paalanen ppaalanen@gmail.com Cc: Marius Vlad marius.vlad@collabora.com Cc: Daniel Vetter daniel.vetter@ffwll.ch
drivers/gpu/drm/drm_blend.c | 8 ++++---- include/drm/drm_plane.h | 9 +++++---- 2 files changed, 9 insertions(+), 8 deletions(-)
-- 2.23.0
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.