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

[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@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


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

[Pekka Paalanen]

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On Tue, 10 Sep 2019 10:09:55 +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.
> 
> Signed-off-by: Simon Ser <contact@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().

Hi,

this seems to contradict what the docs say in another place:

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.

    See drm_plane_create_zpos_property() and
    drm_plane_create_zpos_immutable_property() for more details.

from https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference


Thanks,
pq

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

[Simon Ser]

On Tuesday, September 10, 2019 1:38 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:

> On Tue, 10 Sep 2019 10:09:55 +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.
> >
> > Signed-off-by: Simon Ser contact@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().
>
> Hi,
>
> this seems to contradict what the docs say in another place:

Except this comment is about drm_plane_state.zpos, an internal DRM
property. This is not about the zpos property itself.

The comment was introduced in v2 of [1], although the motivation for
the change isn't documented.

[1]: https://patchwork.freedesktop.org/series/13528/#rev2

> 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.
>
> See drm_plane_create_zpos_property() and
> drm_plane_create_zpos_immutable_property() for more details.
>
> from https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference
>
> Thanks,
> pq


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

[Pekka Paalanen]

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On Tue, 10 Sep 2019 11:20:16 +0000
Simon Ser <contact@emersion.fr> wrote:

> On Tuesday, September 10, 2019 1:38 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> 
> > On Tue, 10 Sep 2019 10:09:55 +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.
> > >
> > > Signed-off-by: Simon Ser contact@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().  
> >
> > Hi,
> >
> > this seems to contradict what the docs say in another place:  
> 
> Except this comment is about drm_plane_state.zpos, an internal DRM
> property. This is not about the zpos property itself.

Hi,

then I'm very confused. What's the difference?

The text you are changing was specifically added to explain uAPI
behaviour, that is, what the userspace sees and does with the zpos
property in uAPI.

Having two conflicting pieces of documentation is confusing, especially
when it is not absolutely clear which one is for driver implementors
and which one for uAPI consumers, or that one must ignore the other doc
depending on who you are.

> The comment was introduced in v2 of [1], although the motivation for
> the change isn't documented.
> 
> [1]: https://patchwork.freedesktop.org/series/13528/#rev2

That does not look like the comment you are changing.

Wait, which one is "this comment"?

> > 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.
> >
> > See drm_plane_create_zpos_property() and
> > drm_plane_create_zpos_immutable_property() for more details.
> >
> > from https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference

Me and others have taken the above to document uAPI. How could that not
be the case?

Is it wrong for userspace to assume that plane object ID order reflects
the plane stacking order? Weston does not do this yet, but since we just
recently found the above comment, we planned to make use of it.

Actually, if the ID ordering is true, then the DRM core could always
expose the zpos property as immutable to userspace, just manufacture it
from object IDs if nothing is set by a driver explicitly.

Or is the comment about object IDs wrong and should be changed to say
the opposite?


Thanks,
pq

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

[Simon Ser]

> On Tue, 10 Sep 2019 11:20:16 +0000
> Simon Ser <contact@emersion.fr> wrote:
>
> > On Tuesday, September 10, 2019 1:38 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> >
> > > On Tue, 10 Sep 2019 10:09:55 +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.
> > > >
> > > > Signed-off-by: Simon Ser contact@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().
> > >
> > > Hi,
> > >
> > > this seems to contradict what the docs say in another place:
> >
> > Except this comment is about drm_plane_state.zpos, an internal DRM
> > property. This is not about the zpos property itself.
>
> Hi,
>
> then I'm very confused. What's the difference?
>
> The text you are changing was specifically added to explain uAPI
> behaviour, that is, what the userspace sees and does with the zpos
> property in uAPI.
>
> Having two conflicting pieces of documentation is confusing, especially
> when it is not absolutely clear which one is for driver implementors
> and which one for uAPI consumers, or that one must ignore the other doc
> depending on who you are.

Yes, I agree that this is confusing.

To be completely honest, I don't really care what the outcome of this
discussion is, as long as there are no conflicting documentation comments.

So, my interpretation of the docs is that there are:

1. Some documentation for KMS properties, that is, what is exposed to
   user-space via DRM ioctls. This is the KMS uAPI.
2. Some documentation for kernel drivers, that is, internal DRM state that can
   be set by kernel developers. This includes helper functions and common
   structs.

Obviously as user-space developers we only care about (1).

Now, back to zpos' case: there are two doc comments about zpos.

The first one is [1], the one this patch changes:

> zpos:
> 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. 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().

This is in the "Plane Composition Properties". I believe this paragraph
documents the standard plane properties (standard as in not driver-specific).
So I'd say this documents the KMS uAPI.

The second one is [2], the one you've quoted:

> 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.
>
> See drm_plane_create_zpos_property() and
> drm_plane_create_zpos_immutable_property() for more details.

This is in the "Plane Functions Reference" section, more precisely in the
"struct drm_plane_state" docs. I believe this is really just about the common
DRM struct that can be used by all drivers. This struct isn't exposed to
user-space. It's just an implementation detail of DRM.

(We can see that even without this patch, these two comments already
kind of conflict. The first one says that without zpos ordering is
undefined. The second one says that two identical zpos values means the
plane ID should be used. However drm_plane_state is zero-filled, so a
driver not supporting zpos would have all drm_plane_state.zpos fields
set to zero? Since they are all equal, is the object ID ordering rule
relevant?)

[1]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-composition-properties
[2]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference

> > The comment was introduced in v2 of [1], although the motivation for
> > the change isn't documented.
> >
> > [1]: https://patchwork.freedesktop.org/series/13528/#rev2
>
> That does not look like the comment you are changing.
>
> Wait, which one is "this comment"?
>
> > > 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.
> > >
> > > See drm_plane_create_zpos_property() and
> > > drm_plane_create_zpos_immutable_property() for more details.
> > >
> > > from https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference
>
> Me and others have taken the above to document uAPI. How could that not
> be the case?

(See above)

> Is it wrong for userspace to assume that plane object ID order reflects
> the plane stacking order? Weston does not do this yet, but since we just
> recently found the above comment, we planned to make use of it.
>
> Actually, if the ID ordering is true, then the DRM core could always
> expose the zpos property as immutable to userspace, just manufacture it
> from object IDs if nothing is set by a driver explicitly.
>
> Or is the comment about object IDs wrong and should be changed to say
> the opposite?

I believe so. If zpos could always be obtained from plane object IDs, then what
would be the point of exposing the immutable zpos property at all?

I've heard that some drivers break this object ID assumption, which is the
reason why this zpos property exists. Can someone more familiar with the kernel
provide some feedback?
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Re: [PATCH] drm: two planes with the same zpos have undefined ordering

[Daniel Vetter]

On Tue, Sep 10, 2019 at 12:38 PM Pekka Paalanen <ppaalanen@gmail.com> wrote:
>
> On Tue, 10 Sep 2019 10:09:55 +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.
> >
> > Signed-off-by: Simon Ser <contact@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().
>
> Hi,
>
> this seems to contradict what the docs say in another place:
>
> 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

I think we should do an s/rule/recommendation to drivers to avoid
surprises in hw programming/ or something like that. Plus we allow
that for legacy ioctl, otherways it's impossible to change zpos with
legacy ioctls (not that I'm sure that's a reasonable use-case, but
whatever). With that also clarified I think Simon's patch is good to
go, same zpos value is really not something we want to encourage. Plus
"implicit by plane id" is already something we use in other places
(like figuring out which primary plan belongs to which crtc), and it's
just terrible uapi.
-Daniel

>     compare the plane object IDs; the plane with a higher ID must be
>     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.
>
> from https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference
>
>
> Thanks,
> pq



-- 
Daniel Vetter
Software Engineer, Intel Corporation
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[PATCH v2] 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.

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)

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..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().
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.
--
2.23.0


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Re: [PATCH v2] drm: two planes with the same zpos have undefined ordering

[Marius Vlad]

[-- Attachment #1.1.1: Type: text/plain, Size: 3620 bytes --]



On 9/17/19 8:24 PM, Simon Ser 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.
> 
> 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)
> 
> 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..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().

stacking of overlay (and or) scanout planes?


>   *
>   * 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.
> --
> 2.23.0
> 
> 

-- 
Marius Vlad


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Re: [PATCH v2] drm: two planes with the same zpos have undefined ordering

[Daniel Vetter]

On Wed, Sep 18, 2019 at 1:14 PM Marius Vlad <marius.vlad@collabora.com> wrote:
>
>
>
> On 9/17/19 8:24 PM, Simon Ser 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.
> >
> > 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)
> >
> > 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..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().
>
> stacking of overlay (and or) scanout planes?

Yeah might be better to drop the "overlay" here, this is for planes in
general. Overlay vs. other planes is really only relevant for the
legacy ioctls, to make sure they know which plane to pick. Otherwise
there's nothing special with them.
-Daniel

>
>
> >   *
> >   * 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.
> > --
> > 2.23.0
> >
> >
>
> --
> Marius Vlad
>


-- 
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Software Engineer, Intel Corporation
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[PATCH v3] 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.

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().
  *
  * 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.
--
2.23.0


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

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 7508 bytes --]

On Mon, 16 Sep 2019 09:19:09 +0000
Simon Ser <contact@emersion.fr> wrote:

> > On Tue, 10 Sep 2019 11:20:16 +0000
> > Simon Ser <contact@emersion.fr> wrote:
> >  
> > > On Tuesday, September 10, 2019 1:38 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> > >  
> > > > On Tue, 10 Sep 2019 10:09:55 +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.
> > > > >
> > > > > Signed-off-by: Simon Ser contact@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().  
> > > >
> > > > Hi,
> > > >
> > > > this seems to contradict what the docs say in another place:  
> > >
> > > Except this comment is about drm_plane_state.zpos, an internal DRM
> > > property. This is not about the zpos property itself.  
> >
> > Hi,
> >
> > then I'm very confused. What's the difference?
> >
> > The text you are changing was specifically added to explain uAPI
> > behaviour, that is, what the userspace sees and does with the zpos
> > property in uAPI.
> >
> > Having two conflicting pieces of documentation is confusing, especially
> > when it is not absolutely clear which one is for driver implementors
> > and which one for uAPI consumers, or that one must ignore the other doc
> > depending on who you are.  
> 
> Yes, I agree that this is confusing.
> 
> To be completely honest, I don't really care what the outcome of this
> discussion is, as long as there are no conflicting documentation comments.

Hi,

that is exactly what I want too.

> So, my interpretation of the docs is that there are:
> 
> 1. Some documentation for KMS properties, that is, what is exposed to
>    user-space via DRM ioctls. This is the KMS uAPI.
> 2. Some documentation for kernel drivers, that is, internal DRM state that can
>    be set by kernel developers. This includes helper functions and common
>    structs.
> 
> Obviously as user-space developers we only care about (1).

Theoretically yes, but the problem is that one cannot make that
distinction. I'm pretty sure both categories of comments are not
complete, and answers to some questions in one must be dug up from the
other, if documented at all.

So you cannot use wording that looks conflicting between the two. If
the wording is correct nevertheless, it needs more explaining on how it
doesn't actually conflict, so that people randomly reading just one
side or the other don't get the wrong idea.

> Now, back to zpos' case: there are two doc comments about zpos.
> 
> The first one is [1], the one this patch changes:
> 
> > zpos:
> > 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. 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().  
> 
> This is in the "Plane Composition Properties". I believe this paragraph
> documents the standard plane properties (standard as in not driver-specific).
> So I'd say this documents the KMS uAPI.
> 
> The second one is [2], the one you've quoted:
> 
> > 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.
> >
> > See drm_plane_create_zpos_property() and
> > drm_plane_create_zpos_immutable_property() for more details.  
> 
> This is in the "Plane Functions Reference" section, more precisely in the
> "struct drm_plane_state" docs. I believe this is really just about the common
> DRM struct that can be used by all drivers. This struct isn't exposed to
> user-space. It's just an implementation detail of DRM.
> 
> (We can see that even without this patch, these two comments already
> kind of conflict. The first one says that without zpos ordering is
> undefined. The second one says that two identical zpos values means the
> plane ID should be used. However drm_plane_state is zero-filled, so a
> driver not supporting zpos would have all drm_plane_state.zpos fields
> set to zero? Since they are all equal, is the object ID ordering rule
> relevant?)

Right, and we are suffering from that confusion already. Should
userspace use ID order if zpos property is not there or not? I have no
idea.

> [1]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-composition-properties
> [2]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference


Thanks,
pq

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Re: [PATCH v3] drm: two planes with the same zpos have undefined ordering

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 4716 bytes --]

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
> 
> 


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

[Daniel Vetter]

On Thu, Sep 19, 2019 at 9:18 AM Pekka Paalanen <ppaalanen@gmail.com> wrote:
>
> On Mon, 16 Sep 2019 09:19:09 +0000
> Simon Ser <contact@emersion.fr> wrote:
>
> > > On Tue, 10 Sep 2019 11:20:16 +0000
> > > Simon Ser <contact@emersion.fr> wrote:
> > >
> > > > On Tuesday, September 10, 2019 1:38 PM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> > > >
> > > > > On Tue, 10 Sep 2019 10:09:55 +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.
> > > > > >
> > > > > > Signed-off-by: Simon Ser contact@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().
> > > > >
> > > > > Hi,
> > > > >
> > > > > this seems to contradict what the docs say in another place:
> > > >
> > > > Except this comment is about drm_plane_state.zpos, an internal DRM
> > > > property. This is not about the zpos property itself.
> > >
> > > Hi,
> > >
> > > then I'm very confused. What's the difference?
> > >
> > > The text you are changing was specifically added to explain uAPI
> > > behaviour, that is, what the userspace sees and does with the zpos
> > > property in uAPI.
> > >
> > > Having two conflicting pieces of documentation is confusing, especially
> > > when it is not absolutely clear which one is for driver implementors
> > > and which one for uAPI consumers, or that one must ignore the other doc
> > > depending on who you are.
> >
> > Yes, I agree that this is confusing.
> >
> > To be completely honest, I don't really care what the outcome of this
> > discussion is, as long as there are no conflicting documentation comments.
>
> Hi,
>
> that is exactly what I want too.
>
> > So, my interpretation of the docs is that there are:
> >
> > 1. Some documentation for KMS properties, that is, what is exposed to
> >    user-space via DRM ioctls. This is the KMS uAPI.
> > 2. Some documentation for kernel drivers, that is, internal DRM state that can
> >    be set by kernel developers. This includes helper functions and common
> >    structs.
> >
> > Obviously as user-space developers we only care about (1).
>
> Theoretically yes, but the problem is that one cannot make that
> distinction. I'm pretty sure both categories of comments are not
> complete, and answers to some questions in one must be dug up from the
> other, if documented at all.
>
> So you cannot use wording that looks conflicting between the two. If
> the wording is correct nevertheless, it needs more explaining on how it
> doesn't actually conflict, so that people randomly reading just one
> side or the other don't get the wrong idea.
>
> > Now, back to zpos' case: there are two doc comments about zpos.
> >
> > The first one is [1], the one this patch changes:
> >
> > > zpos:
> > > 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. 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().
> >
> > This is in the "Plane Composition Properties". I believe this paragraph
> > documents the standard plane properties (standard as in not driver-specific).
> > So I'd say this documents the KMS uAPI.
> >
> > The second one is [2], the one you've quoted:
> >
> > > 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.
> > >
> > > See drm_plane_create_zpos_property() and
> > > drm_plane_create_zpos_immutable_property() for more details.
> >
> > This is in the "Plane Functions Reference" section, more precisely in the
> > "struct drm_plane_state" docs. I believe this is really just about the common
> > DRM struct that can be used by all drivers. This struct isn't exposed to
> > user-space. It's just an implementation detail of DRM.
> >
> > (We can see that even without this patch, these two comments already
> > kind of conflict. The first one says that without zpos ordering is
> > undefined. The second one says that two identical zpos values means the
> > plane ID should be used. However drm_plane_state is zero-filled, so a
> > driver not supporting zpos would have all drm_plane_state.zpos fields
> > set to zero? Since they are all equal, is the object ID ordering rule
> > relevant?)
>
> Right, and we are suffering from that confusion already. Should
> userspace use ID order if zpos property is not there or not? I have no
> idea.

Nope. I think the only options for this case are:
- file bug against upstream driver so they add zpos
- you magically know how planes work on that hw
- you don't overlap planes at all
- cursor is above primary, that much we can guarantee

Yes it's kinda uapi fail we didn't add zpos from the start :-/
-Daniel

>
> > [1]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-composition-properties
> > [2]: https://01.org/linuxgraphics/gfx-docs/drm/gpu/drm-kms.html#plane-functions-reference
>
>
> Thanks,
> pq



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

[Pekka Paalanen]

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On Thu, 19 Sep 2019 10:18:04 +0200
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> On Thu, Sep 19, 2019 at 9:18 AM Pekka Paalanen <ppaalanen@gmail.com> wrote:
> >

...

> > Right, and we are suffering from that confusion already. Should
> > userspace use ID order if zpos property is not there or not? I have no
> > idea.  
> 
> Nope. I think the only options for this case are:
> - file bug against upstream driver so they add zpos
> - you magically know how planes work on that hw
> - you don't overlap planes at all
> - cursor is above primary, that much we can guarantee
> 
> Yes it's kinda uapi fail we didn't add zpos from the start :-/

Good. Weston does the last two. The confusion did not last long
enough to let us add code using the object ID to infer stacking order.

Although, Weston does have the assumption that overlays are in unknown
order between primary and cursor, which now seems false.


Thanks,
pq

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

[Daniel Vetter]

On Thu, Sep 19, 2019 at 11:02 AM Pekka Paalanen <ppaalanen@gmail.com> wrote:
>
> On Thu, 19 Sep 2019 10:18:04 +0200
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> > On Thu, Sep 19, 2019 at 9:18 AM Pekka Paalanen <ppaalanen@gmail.com> wrote:
> > >
>
> ...
>
> > > Right, and we are suffering from that confusion already. Should
> > > userspace use ID order if zpos property is not there or not? I have no
> > > idea.
> >
> > Nope. I think the only options for this case are:
> > - file bug against upstream driver so they add zpos
> > - you magically know how planes work on that hw
> > - you don't overlap planes at all
> > - cursor is above primary, that much we can guarantee
> >
> > Yes it's kinda uapi fail we didn't add zpos from the start :-/
>
> Good. Weston does the last two. The confusion did not last long
> enough to let us add code using the object ID to infer stacking order.
>
> Although, Weston does have the assumption that overlays are in unknown
> order between primary and cursor, which now seems false.

I think cursor is always on top, but some of the overlays can be underlays.
-Daniel
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[PATCH v4] 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.

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@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     | 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


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

[Pekka Paalanen]

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On Mon, 23 Sep 2019 12:39:20 +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.

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@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     | 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@collabora.com>


Thanks,
pq

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

[Simon Ser]

On Tuesday, September 24, 2019 10:26 AM, Pekka Paalanen <ppaalanen@gmail.com> wrote:

> On Mon, 23 Sep 2019 12:39:20 +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.
>
> 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.

This patch in fact explains why duplicate plane IDs are allowed. The
two alternatives are mentioned: either disallow duplicate plane zpos
values, or either make ordering undefined.

I chose to allow duplicate values because some HW might not know the
zpos ordering between e.g. some overlay planes, but might know primary
is under overlays.

If that doesn't make sense at all, I'd be happy to change the spec to
say that duplicate zpos values are a kernel bug. We'll need to make
sure this doesn't happen, e.g. with a check in the function creating
the property.

Note that user-space needs to handle undefined order between planes
anyway in case zpos isn't advertised.
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Re: [PATCH v4] drm: two planes with the same zpos have undefined ordering

[Pekka Paalanen]

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On Tue, 24 Sep 2019 07:34:30 +0000
Simon Ser <contact@emersion.fr> wrote:

> On Tuesday, September 24, 2019 10:26 AM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> 
> > On Mon, 23 Sep 2019 12:39:20 +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.  
> >
> > 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.  
> 
> This patch in fact explains why duplicate plane IDs are allowed. The
> two alternatives are mentioned: either disallow duplicate plane zpos
> values, or either make ordering undefined.
> 
> I chose to allow duplicate values because some HW might not know the
> zpos ordering between e.g. some overlay planes, but might know primary
> is under overlays.

Ok, seems like we are still crossing some streams here. There are two
different and independent cases:
- zpos set by drivers
- zpos set by userspace

Zpos set by drivers into nonsensical (duplicate) values is a driver
bug to me. Drivers do have the opportunity to not advertise zpos at all
if they don't know, even if that makes no sense to me.

I assumed that zpos set by userspace must be allowed to have duplicate
zpos values, because such broken userspace already exists. But do they
exist? If probably not, then atomic check failing on duplicate zpos
would be nice. But there must be some reason why the doc already
mentions that ID ordering rule for making sense of ambiguous userspace.

> If that doesn't make sense at all, I'd be happy to change the spec to
> say that duplicate zpos values are a kernel bug. We'll need to make
> sure this doesn't happen, e.g. with a check in the function creating
> the property.

I thought that was already the case.

> Note that user-space needs to handle undefined order between planes
> anyway in case zpos isn't advertised.

Of course.


Thanks,
pq

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

[Simon Ser]

On Tuesday, September 24, 2019 11:48 AM, Pekka Paalanen <ppaalanen@gmail.com> wrote:

> On Tue, 24 Sep 2019 07:34:30 +0000
> Simon Ser contact@emersion.fr wrote:
>
> > On Tuesday, September 24, 2019 10:26 AM, Pekka Paalanen ppaalanen@gmail.com wrote:
> >
> > > On Mon, 23 Sep 2019 12:39:20 +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.
> > >
> > > 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.
> >
> > This patch in fact explains why duplicate plane IDs are allowed. The
> > two alternatives are mentioned: either disallow duplicate plane zpos
> > values, or either make ordering undefined.
> > I chose to allow duplicate values because some HW might not know the
> > zpos ordering between e.g. some overlay planes, but might know primary
> > is under overlays.
>
> Ok, seems like we are still crossing some streams here. There are two
> different and independent cases:
>
> -   zpos set by drivers
> -   zpos set by userspace

I think we sorted out the "zpos set by userspace" properly. Let's just
focus on "zpos set by drivers".

> Zpos set by drivers into nonsensical (duplicate) values is a driver
> bug to me. Drivers do have the opportunity to not advertise zpos at all
> if they don't know, even if that makes no sense to me.

I don't know about that. Let's say a driver knows it has underlay
planes, but doesn't know their relative ordering for each revision of
the HW. Maybe it could just set the same zpos for all underlays to
signal to user-space that these planes are under the primary plane?

I'm no driver developer so I don't know whether this could happen. The
goal of this patch is to allow drivers to provide partial zpos info to
user-space (better than no info at all, especially when overlay planes
have undefined ordering wrt. the primary plane when zpos is not
advertised).

> I assumed that zpos set by userspace must be allowed to have duplicate
> zpos values, because such broken userspace already exists. But do they
> exist? If probably not, then atomic check failing on duplicate zpos
> would be nice. But there must be some reason why the doc already
> mentions that ID ordering rule for making sense of ambiguous userspace.

I don't know, I haven't found anything explaining where these dup zpos
values set by user-space are coming from.

> > If that doesn't make sense at all, I'd be happy to change the spec to
> > say that duplicate zpos values are a kernel bug. We'll need to make
> > sure this doesn't happen, e.g. with a check in the function creating
> > the property.
>
> I thought that was already the case.

No, this patch specifically says duplicate zpos (coming from the driver
or set by user-space) will have undefined ordering.

> > Note that user-space needs to handle undefined order between planes
> > anyway in case zpos isn't advertised.
>
> Of course.
>
> Thanks,
> pq


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

[james qian wang (Arm Technology China)]

On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.

Some Arm's usage cases about two planes with same zpos.

- "Smart layer"
which can accepts 4 different fbs with different src/display rect,
but this smart layer has one restriction:

4 display rects must have no overlaps in V direction
(A optimization for android window like Toolbar/Navigation bar)

So when map this Smart-layer to drm world, it might be 4 different
drm-planes, but with same zorder to indicate that these 4 planes are
smart-laye planes.

- "VR-Layer"
One VR-layer comprises two different viewports which can be configured
with totoally different fbs, src/display rect.
we use two differernt drm-planes to drive on HW "VR-Layer", and these
two drm-planes must be configured with same zpos.

Thanks
James

> 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@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     | 9 +++++----
>  2 files changed, 9 insertions(+), 8 deletions(-)
> 
> --
> 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..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.
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Re: [v4] drm: two planes with the same zpos have undefined ordering

[Simon Ser]

Hi,

> On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.
>
> Some Arm's usage cases about two planes with same zpos.
>
> - "Smart layer"
> which can accepts 4 different fbs with different src/display rect,
> but this smart layer has one restriction:
>
> 4 display rects must have no overlaps in V direction
> (A optimization for android window like Toolbar/Navigation bar)
>
> So when map this Smart-layer to drm world, it might be 4 different
> drm-planes, but with same zorder to indicate that these 4 planes are
> smart-laye planes.
>
> - "VR-Layer"
> One VR-layer comprises two different viewports which can be configured
> with totoally different fbs, src/display rect.
> we use two differernt drm-planes to drive on HW "VR-Layer", and these
> two drm-planes must be configured with same zpos.

Thanks a lot for your feedback James, that's exactly what I was looking for.
Both of these use-cases make sense to me.

I think user-space should be prepared to handle identical immutable zpos values.
Pekka and Daniel, thoughts?

In any case, this doesn't change the patch itself. Probably something worth
mentionning in the commit message.
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Re: [v4] drm: two planes with the same zpos have undefined ordering

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 2353 bytes --]

On Sun, 29 Sep 2019 20:30:44 +0000
Simon Ser <contact@emersion.fr> wrote:

> Hi,
> 
> > On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.  
> >
> > Some Arm's usage cases about two planes with same zpos.
> >
> > - "Smart layer"
> > which can accepts 4 different fbs with different src/display rect,
> > but this smart layer has one restriction:
> >
> > 4 display rects must have no overlaps in V direction
> > (A optimization for android window like Toolbar/Navigation bar)
> >
> > So when map this Smart-layer to drm world, it might be 4 different
> > drm-planes, but with same zorder to indicate that these 4 planes are
> > smart-laye planes.
> >
> > - "VR-Layer"
> > One VR-layer comprises two different viewports which can be configured
> > with totoally different fbs, src/display rect.
> > we use two differernt drm-planes to drive on HW "VR-Layer", and these
> > two drm-planes must be configured with same zpos.  
> 
> Thanks a lot for your feedback James, that's exactly what I was looking for.
> Both of these use-cases make sense to me.
> 
> I think user-space should be prepared to handle identical immutable zpos values.
> Pekka and Daniel, thoughts?

Hi,

given those explained use cases, sure, I agree.

> In any case, this doesn't change the patch itself. Probably something worth
> mentionning in the commit message.

Yes, recording these use cases would be very nice.


Thanks,
pq

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

[Daniel Vetter]

On Mon, Sep 30, 2019 at 10:07:07AM +0300, Pekka Paalanen wrote:
> On Sun, 29 Sep 2019 20:30:44 +0000
> Simon Ser <contact@emersion.fr> wrote:
> 
> > Hi,
> > 
> > > On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.  
> > >
> > > Some Arm's usage cases about two planes with same zpos.
> > >
> > > - "Smart layer"
> > > which can accepts 4 different fbs with different src/display rect,
> > > but this smart layer has one restriction:
> > >
> > > 4 display rects must have no overlaps in V direction
> > > (A optimization for android window like Toolbar/Navigation bar)
> > >
> > > So when map this Smart-layer to drm world, it might be 4 different
> > > drm-planes, but with same zorder to indicate that these 4 planes are
> > > smart-laye planes.
> > >
> > > - "VR-Layer"
> > > One VR-layer comprises two different viewports which can be configured
> > > with totoally different fbs, src/display rect.
> > > we use two differernt drm-planes to drive on HW "VR-Layer", and these
> > > two drm-planes must be configured with same zpos.  
> > 
> > Thanks a lot for your feedback James, that's exactly what I was looking for.
> > Both of these use-cases make sense to me.
> > 
> > I think user-space should be prepared to handle identical immutable zpos values.
> > Pekka and Daniel, thoughts?
> 
> Hi,
> 
> given those explained use cases, sure, I agree.
> 
> > In any case, this doesn't change the patch itself. Probably something worth
> > mentionning in the commit message.
> 
> Yes, recording these use cases would be very nice.

There's a lot more hw that does the same tricks (qualcom is one for sure).
Imo before we encode this we should make sure that:
a) everyone is happy with this new uapi
b) drivers are consistent
c) maybe even testcases in igt
d) definitely an open source user
e) no breaking of existing userspace

I.e. definitely a separate patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [v4] drm: two planes with the same zpos have undefined ordering

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 3215 bytes --]

On Tue, 8 Oct 2019 11:59:04 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Mon, Sep 30, 2019 at 10:07:07AM +0300, Pekka Paalanen wrote:
> > On Sun, 29 Sep 2019 20:30:44 +0000
> > Simon Ser <contact@emersion.fr> wrote:
> >   
> > > Hi,
> > >   
> > > > On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.    
> > > >
> > > > Some Arm's usage cases about two planes with same zpos.
> > > >
> > > > - "Smart layer"
> > > > which can accepts 4 different fbs with different src/display rect,
> > > > but this smart layer has one restriction:
> > > >
> > > > 4 display rects must have no overlaps in V direction
> > > > (A optimization for android window like Toolbar/Navigation bar)
> > > >
> > > > So when map this Smart-layer to drm world, it might be 4 different
> > > > drm-planes, but with same zorder to indicate that these 4 planes are
> > > > smart-laye planes.
> > > >
> > > > - "VR-Layer"
> > > > One VR-layer comprises two different viewports which can be configured
> > > > with totoally different fbs, src/display rect.
> > > > we use two differernt drm-planes to drive on HW "VR-Layer", and these
> > > > two drm-planes must be configured with same zpos.    
> > > 
> > > Thanks a lot for your feedback James, that's exactly what I was looking for.
> > > Both of these use-cases make sense to me.
> > > 
> > > I think user-space should be prepared to handle identical immutable zpos values.
> > > Pekka and Daniel, thoughts?  
> > 
> > Hi,
> > 
> > given those explained use cases, sure, I agree.
> >   
> > > In any case, this doesn't change the patch itself. Probably something worth
> > > mentionning in the commit message.  
> > 
> > Yes, recording these use cases would be very nice.  
> 
> There's a lot more hw that does the same tricks (qualcom is one for sure).
> Imo before we encode this we should make sure that:
> a) everyone is happy with this new uapi

Sorry, what new UAPI?

We're just trying to make the documentation match what currently
happens, right?


Thanks,
pq


> b) drivers are consistent
> c) maybe even testcases in igt
> d) definitely an open source user
> e) no breaking of existing userspace
> 
> I.e. definitely a separate patch.
> -Daniel


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

[Daniel Vetter]

On Tue, Oct 8, 2019 at 1:39 PM Pekka Paalanen <ppaalanen@gmail.com> wrote:
>
> On Tue, 8 Oct 2019 11:59:04 +0200
> Daniel Vetter <daniel@ffwll.ch> wrote:
>
> > On Mon, Sep 30, 2019 at 10:07:07AM +0300, Pekka Paalanen wrote:
> > > On Sun, 29 Sep 2019 20:30:44 +0000
> > > Simon Ser <contact@emersion.fr> wrote:
> > >
> > > > Hi,
> > > >
> > > > > On Mon, Sep 23, 2019 at 12:39:20PM +0000, Simon Ser 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.
> > > > >
> > > > > Some Arm's usage cases about two planes with same zpos.
> > > > >
> > > > > - "Smart layer"
> > > > > which can accepts 4 different fbs with different src/display rect,
> > > > > but this smart layer has one restriction:
> > > > >
> > > > > 4 display rects must have no overlaps in V direction
> > > > > (A optimization for android window like Toolbar/Navigation bar)
> > > > >
> > > > > So when map this Smart-layer to drm world, it might be 4 different
> > > > > drm-planes, but with same zorder to indicate that these 4 planes are
> > > > > smart-laye planes.
> > > > >
> > > > > - "VR-Layer"
> > > > > One VR-layer comprises two different viewports which can be configured
> > > > > with totoally different fbs, src/display rect.
> > > > > we use two differernt drm-planes to drive on HW "VR-Layer", and these
> > > > > two drm-planes must be configured with same zpos.
> > > >
> > > > Thanks a lot for your feedback James, that's exactly what I was looking for.
> > > > Both of these use-cases make sense to me.
> > > >
> > > > I think user-space should be prepared to handle identical immutable zpos values.
> > > > Pekka and Daniel, thoughts?
> > >
> > > Hi,
> > >
> > > given those explained use cases, sure, I agree.
> > >
> > > > In any case, this doesn't change the patch itself. Probably something worth
> > > > mentionning in the commit message.
> > >
> > > Yes, recording these use cases would be very nice.
> >
> > There's a lot more hw that does the same tricks (qualcom is one for sure).
> > Imo before we encode this we should make sure that:
> > a) everyone is happy with this new uapi
>
> Sorry, what new UAPI?
>
> We're just trying to make the documentation match what currently
> happens, right?

It's so much new uapi that I've sent out a few reverts for 5.4 (it
landed in that merge window) to undo the stuff the arm driver team has
done (it didn't come with userspace, proper discussion on dri-devel,
docs or testcases in igt). I also just spotted that a leftover is that
arm/komeda still computes its own version of normalized_zpos, which
probably should be ditched too (it's not actually different from the
one in helpers without the reverted uapi).

So yeah, separate patch :-)
-Daniel
--
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Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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Re: [v4] drm: two planes with the same zpos have undefined ordering

[Simon Ser]

On Tuesday, October 8, 2019 6:03 PM, Daniel Vetter <daniel@ffwll.ch> wrote:

> > > > > In any case, this doesn't change the patch itself. Probably something worth
> > > > > mentionning in the commit message.
> > > >
> > > > Yes, recording these use cases would be very nice.
> > >
> > > There's a lot more hw that does the same tricks (qualcom is one for sure).
> > > Imo before we encode this we should make sure that:
> > > a) everyone is happy with this new uapi
> >
> > Sorry, what new UAPI?
> > We're just trying to make the documentation match what currently
> > happens, right?
>
> It's so much new uapi that I've sent out a few reverts for 5.4 (it
> landed in that merge window) to undo the stuff the arm driver team has
> done (it didn't come with userspace, proper discussion on dri-devel,
> docs or testcases in igt). I also just spotted that a leftover is that
> arm/komeda still computes its own version of normalized_zpos, which
> probably should be ditched too (it's not actually different from the
> one in helpers without the reverted uapi).
>
> So yeah, separate patch :-)

I don't get it. Do you want to split the docs changes for user-space,
only keeping the doc changes for drivers in this patch?

User-space could already see duplicate zpos because of the non-atomic
API. I don't think this introduces a new uAPI.
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Re: [v4] drm: two planes with the same zpos have undefined ordering

[Daniel Vetter]

On Tue, Oct 8, 2019 at 5:11 PM Simon Ser <contact@emersion.fr> wrote:
>
> On Tuesday, October 8, 2019 6:03 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
>
> > > > > > In any case, this doesn't change the patch itself. Probably something worth
> > > > > > mentionning in the commit message.
> > > > >
> > > > > Yes, recording these use cases would be very nice.
> > > >
> > > > There's a lot more hw that does the same tricks (qualcom is one for sure).
> > > > Imo before we encode this we should make sure that:
> > > > a) everyone is happy with this new uapi
> > >
> > > Sorry, what new UAPI?
> > > We're just trying to make the documentation match what currently
> > > happens, right?
> >
> > It's so much new uapi that I've sent out a few reverts for 5.4 (it
> > landed in that merge window) to undo the stuff the arm driver team has
> > done (it didn't come with userspace, proper discussion on dri-devel,
> > docs or testcases in igt). I also just spotted that a leftover is that
> > arm/komeda still computes its own version of normalized_zpos, which
> > probably should be ditched too (it's not actually different from the
> > one in helpers without the reverted uapi).
> >
> > So yeah, separate patch :-)
>
> I don't get it. Do you want to split the docs changes for user-space,
> only keeping the doc changes for drivers in this patch?
>
> User-space could already see duplicate zpos because of the non-atomic
> API. I don't think this introduces a new uAPI.

I'm talking specifically about the "duplicated zpos values indicate
special cloned planes like in the arm example" clarification. Not
about splitting the zpos documentation any more, that's indeed just
documenting existing uapi. But assigning the special meaning to
duplicated zpos values (instead of just "can happen because non-atomic
legacy userspace"), that is new uapi. Especially if we allow
duplicated zpos for immutable properties (afaik that doesn't exist
yet).
-Daniel
-- 
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Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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Re: [v4] drm: two planes with the same zpos have undefined ordering

[Simon Ser]

On Tuesday, October 8, 2019 6:16 PM, Daniel Vetter <daniel@ffwll.ch> wrote:

> On Tue, Oct 8, 2019 at 5:11 PM Simon Ser contact@emersion.fr wrote:
>
> > On Tuesday, October 8, 2019 6:03 PM, Daniel Vetter daniel@ffwll.ch wrote:
> >
> > > > > > > In any case, this doesn't change the patch itself. Probably something worth
> > > > > > > mentionning in the commit message.
> > > > > >
> > > > > > Yes, recording these use cases would be very nice.
> > > > >
> > > > > There's a lot more hw that does the same tricks (qualcom is one for sure).
> > > > > Imo before we encode this we should make sure that:
> > > > > a) everyone is happy with this new uapi
> > > >
> > > > Sorry, what new UAPI?
> > > > We're just trying to make the documentation match what currently
> > > > happens, right?
> > >
> > > It's so much new uapi that I've sent out a few reverts for 5.4 (it
> > > landed in that merge window) to undo the stuff the arm driver team has
> > > done (it didn't come with userspace, proper discussion on dri-devel,
> > > docs or testcases in igt). I also just spotted that a leftover is that
> > > arm/komeda still computes its own version of normalized_zpos, which
> > > probably should be ditched too (it's not actually different from the
> > > one in helpers without the reverted uapi).
> > > So yeah, separate patch :-)
> >
> > I don't get it. Do you want to split the docs changes for user-space,
> > only keeping the doc changes for drivers in this patch?
> > User-space could already see duplicate zpos because of the non-atomic
> > API. I don't think this introduces a new uAPI.
>
> I'm talking specifically about the "duplicated zpos values indicate
> special cloned planes like in the arm example" clarification. Not
> about splitting the zpos documentation any more, that's indeed just
> documenting existing uapi. But assigning the special meaning to
> duplicated zpos values (instead of just "can happen because non-atomic
> legacy userspace"), that is new uapi. Especially if we allow
> duplicated zpos for immutable properties (afaik that doesn't exist
> yet).

Oh, I see. That makes sense, will send a new version.
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[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@emersion.fr>
---
 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


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