[PATCH v2] 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 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.

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)

Signed-off-by: Simon Ser <contact at emersion.fr>
Cc: Pekka Paalanen <ppaalanen at gmail.com>
Cc: Marius Vlad <marius.vlad at collabora.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 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..51bd5454e50a 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 overlay 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..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.
 	 *
 	 * See drm_plane_create_zpos_property() and
 	 * drm_plane_create_zpos_immutable_property() for more details.
--
2.23.0