linux-kernel - Re: [PATCH 2/3] drm: drm_blend.c: Improve drm_plane_create_rotation

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Message-ID: <20240417143435.6e1e414a.pekka.paalanen@collabora.com>
Date: Wed, 17 Apr 2024 14:34:35 +0300
From: Pekka Paalanen <pekka.paalanen@...labora.com>
To: Louis Chauvet <louis.chauvet@...tlin.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 seanpaul@...gle.com, marcheu@...gle.com, nicolejadeyee@...gle.com,
 thomas.petazzoni@...tlin.com
Subject: Re: [PATCH 2/3] drm: drm_blend.c: Improve
 drm_plane_create_rotation_property kernel doc

On Wed, 17 Apr 2024 00:30:58 +0200
Louis Chauvet <louis.chauvet@...tlin.com> wrote:

> Le 15/04/24 - 14:36, Pekka Paalanen a écrit :
> > On Tue, 09 Apr 2024 12:04:06 +0200
> > Louis Chauvet <louis.chauvet@...tlin.com> wrote:
> >   
> > > The expected behavior of the rotation property was not very clear. Add
> > > more examples to explain what is the expected result.
> > > 
> > > Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>
> > > ---
> > >  drivers/gpu/drm/drm_blend.c | 52 +++++++++++++++++++++++++++++++++------------
> > >  1 file changed, 38 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
> > > index 8d4b317eb9d7..6fbb8730d8b0 100644
> > > --- a/drivers/gpu/drm/drm_blend.c
> > > +++ b/drivers/gpu/drm/drm_blend.c
> > > @@ -104,6 +104,9 @@
> > >   *	Without this property the rectangle is only scaled, but not rotated or
> > >   *	reflected.
> > >   *
> > > + *	See drm_plane_create_rotation_property() for details about the expected rotation and
> > > + *	reflection behavior.  
> > 
> > I think internal function docs should be referring to UAPI docs, and
> > not vice versa. Internal functions can change, but UAPI cannot.
> >   
> > > + *
> > >   *	Possbile values:
> > >   *
> > >   *	"rotate-<degrees>":
> > > @@ -114,18 +117,6 @@
> > >   *		Signals that the contents of a drm plane is reflected along the
> > >   *		<axis> axis, in the same way as mirroring.
> > >   *
> > > - *	reflect-x::
> > > - *
> > > - *			|o |    | o|
> > > - *			|  | -> |  |
> > > - *			| v|    |v |
> > > - *
> > > - *	reflect-y::
> > > - *
> > > - *			|o |    | ^|
> > > - *			|  | -> |  |
> > > - *			| v|    |o |
> > > - *
> > >   * 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
> > > @@ -266,8 +257,41 @@ EXPORT_SYMBOL(drm_plane_create_alpha_property);
> > >   *
> > >   * Rotation is the specified amount in degrees in counter clockwise direction,
> > >   * the X and Y axis are within the source rectangle, i.e.  the X/Y axis before
> > > - * rotation. After reflection, the rotation is applied to the image sampled from
> > > - * the source rectangle, before scaling it to fit the destination rectangle.
> > > + * rotation.
> > > + *
> > > + * Here are some examples of rotation and reflections:
> > > + *
> > > + * |o  +|  REFLECT_X  |+  o|
> > > + * |    |  ========>  |    |
> > > + * |    |             |    |
> > > + *
> > > + * |o   |  REFLECT_Y  |+   |
> > > + * |    |  ========>  |    |
> > > + * |+   |             |o   |
> > > + *
> > > + * |o  +|  ROTATE_90  |+   |
> > > + * |    |  ========>  |    |
> > > + * |    |             |o   |
> > > + *
> > > + * |o   |  ROTATE_180 |   +|
> > > + * |    |  ========>  |    |
> > > + * |+   |             |   o|
> > > + *
> > > + * |o   |  ROTATE_270 |+  o|
> > > + * |    |  ========>  |    |
> > > + * |+   |             |    |
> > > + *
> > > + * Rotation and reflection can be combined to handle more situations In this condition, the
> > > + * reflection is applied first and the rotation in second.  
> > 
> > When going in which direction? Is the first image the FB source
> > rectangle contents, and the second image what the plane looks like in
> > CRTC frame of reference?  
> 
> The first is the FB source, the second is the expected result on the CRTC 
> output.
> 
> I will add a sentence before the schemas:
> 
>  * Here are some examples of rotation and reflections, on the left it is 
>  * the content of the source frame buffer, on the right is the expected 
>  * result on the CRTC output.
> 
> >   
> > > + *
> > > + * For example the expected result for DRM_MODE_ROTATE_90 | DRM_MODE_REFLECT_X is:
> > > + *
> > > + * |o  +|  REFLECT_X  |+  o|  ROTATE_90  |o   |
> > > + * |    |  ========>  |    |  ========>  |    |
> > > + * |    |             |    |             |+   |
> > > + *
> > > + * It is not possible to pass multiple rotation at the same time. (ie ROTATE_90 | ROTATE_180 is
> > > + * not the same as ROTATE_270 and is not accepted).
> > >   */
> > >  int drm_plane_create_rotation_property(struct drm_plane *plane,
> > >  				       unsigned int rotation,
> > >   
> > 
> > These are definitely improvements. I think they should just be in the
> > UAPI section rather than implementation details.  
> 
> So, somewhere in [1]? It feel strange because this is in the `GPU Driver 
> Developer’s Guide` section, not a `UAPI interfaces`.

The whole kernel documentation layout is a big mess. I *still*
spend ages trying to find the pages I know exist.

https://docs.kernel.org/gpu/drm-kms.html#plane-composition-properties
is where properties are documented for userspace developers to look at.

Let's see... I'm cheating and looking what hierarchy I need to follow
to find the place I am at:

The Linux Kernel
-> Subsystems
  -> Human Interfaces: GPU Driver Developer's Guide
    -> Kernel Mode Setting (KMS)
    <- oops, don't click that, take a step back
    -> Kernel Mode Setting (KMS): KMS properties
    <- oops, don't click that, take a step back
    -> Kernel Mode Setting (KMS): KMS properties: Plane Composition Properties

So yeah, UAPI docs are under graphics driver developer's guide, inside
human interface subsystems.


Thanks,
pq

> [1]: https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html
> 
> Thanks,
> Louis Chauvet
> 
> > Disclaimer again to everyone else: I cannot tell if this is the correct
> > documentation or its inverse.
> > 
> > 
> > Thanks,
> > pq  
> 
> 
> 


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