[PATCH v5] drm: two planes with the same zpos have undefined ordering

[Simon Ser]

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:

- 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.
- Some drivers might expose sets of planes which can't overlap, in which
  case it makes sense for the driver to assign the same zpos to these
  planes (e.g. some Arm GPUs).

For these reasons, 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)

Signed-off-by: Simon Ser <contact at emersion.fr>
Reviewed-by: Pekka Paalanen <ppaalanen at gmail.com>
Cc: Marius Vlad <marius.vlad at collabora.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: James Qian Wang <james.qian.wang at 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