[PATCH v4] drm: two planes with the same zpos have undefined ordering

Pekka Paalanen ppaalanen at gmail.com
Tue Sep 24 07:26:39 UTC 2019

On Mon, 23 Sep 2019 12:39:20 +0000
Simon Ser <contact at emersion.fr> wrote:

> 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.

Hi Simon,

the commit message still talks about the drivers only, and I think that
is what lead me astray in the first place until I realized the
duplicate zpos value issue concerns only stupid userspace, not that
drivers were allowed to use duplicate zpos values which userspace then
needs to untangle with ID ordering. Drivers never have undefined plane
ordering themselves - if they do, that's a broken driver that doesn't
know what hardware it is driving. If the driver doesn't know, then
userspace definitely cannot know any better - if some userspace does,
that's a gfx driver stack misdesign. So there is no reason for a driver
to use ambiguous zpos values.

Anyway, it's fine. The actual doc below is what matters here.

> 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.
> 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)
> 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     | 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

Reviewed-by: Pekka Paalanen <pekka.paalanen at collabora.com>

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