| Message ID | WHzXxgNIkxb4Mn1hdeAAwutlt0P-G2onwFr4FpA083VEVh1nIcstHOBRataUD7skpgtWd7BMDPF4v0GHJmQcLrnicVF1hpaUz_5WeSBUTWk=@emersion.fr (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3] drm: two planes with the same zpos have undefined ordering | expand |
On Wed, 18 Sep 2019 16:33:47 +0000 Simon Ser <contact@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. > > 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) > > Signed-off-by: Simon Ser <contact@emersion.fr> > Cc: Pekka Paalanen <ppaalanen@gmail.com> > Cc: Marius Vlad <marius.vlad@collabora.com> > Cc: Daniel Vetter <daniel.vetter@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..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(). Hi, the above looks good to me. > * > * 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. While better, I think this would need a less subtle clarification: + "Userspace should never rely on stacking order derived from IDs." Oh, I think I realised it only now. This comment is for the drivers to handle a case where userspace is being stupid and assigns mutable zpos properties with duplicate values, right? It does *not* allow drivers themselves to assign duplicate zpos values. So I've been looking at this from the wrong angle. Maybe it should just say that then? Instead of: "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 paragraph could start with: "Userspace may set mutable zpos properties so that multiple active planes on the same crtc have identical zpos value. This is a userspace bug, but drivers can solve the conflict deterministically by comparing the plane object IDs;" Thanks, pq > * > * See drm_plane_create_zpos_property() and > * drm_plane_create_zpos_immutable_property() for more details. > -- > 2.23.0 > >
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..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.
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. 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) Signed-off-by: Simon Ser <contact@emersion.fr> Cc: Pekka Paalanen <ppaalanen@gmail.com> Cc: Marius Vlad <marius.vlad@collabora.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> --- drivers/gpu/drm/drm_blend.c | 8 ++++---- include/drm/drm_plane.h | 6 +++--- 2 files changed, 7 insertions(+), 7 deletions(-) -- 2.23.0