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@xxxxxxxxxxx>
Reviewed-by: Pekka Paalanen <ppaalanen@xxxxxxxxx>
Cc: Marius Vlad <marius.vlad@xxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: James Qian Wang <james.qian.wang@xxxxxxx>
---
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
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