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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. The positive Z axis points towards the user, i.e. planes with lower Z position values 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(). If any plane has a zpos property (either mutable or immutable), then all planes shall have a zpos property.