[PATCH] 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.

Signed-off-by: Simon Ser <contact at emersion.fr>
---

Err, I'm sorry about the double-post. I sent this to intel-gfx by mistake.

 drivers/gpu/drm/drm_blend.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 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().
--
2.23.0