drm/blend: Define the direction of Z position values

We forgot that.

Proof is the one igt testcase we have:

https://gitlab.freedesktop.org/drm/igt-gpu-tools/blob/master/tests/kms_atomic.c#L280

While at it also document that we have immutable zpos properties in
some cases.

Reported-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906144459.16025-1-daniel.vetter@ffwll.ch

diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
index 37ac168..d02709d 100644 (file)
--- a/drivers/gpu/drm/drm_blend.c
+++ b/drivers/gpu/drm/drm_blend.c
@@ -130,7 +130,12 @@
  *     Z position is set up with drm_plane_create_zpos_immutable_property() and
  *     drm_plane_create_zpos_property(). It controls the visibility of overlapping
  *     planes. Without this property the primary plane is always below the cursor
- *     plane, and ordering between all other planes is undefined.
+ *     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().
  *
  * pixel blend mode:
  *     Pixel blend mode is set up with drm_plane_create_blend_mode_property().