[PATCH] RFC: drm/drm_plane: Expose the plane capability and interoperability

[Shankar, Uma]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Xaver
> Hugl
> Sent: Thursday, July 11, 2024 9:06 PM
> To: intel-gfx at lists.freedesktop.org
> Subject: Re: [PATCH] RFC: drm/drm_plane: Expose the plane capability and
> interoperability
> 
> Hi,
> 
> On Dienstag, 9. Juli 2024 09:46:56 MESZ Arun R Murthy wrote:
> > Each plane has its own capability or interoperability based on the
> > harware restrictions. If this is exposed to the user, then user can
> > read it once on boot and store this. Later can be used as a lookup
> > table to check a corresponding capability is supported by plane then
> > only go ahead with the framebuffer creation/calling atomic_ioctl.
> >
> > For Ex: There are few restiction as to async flip doesnt support all
> > the formats/modifiers. Similar restrictions on scaling. With the
> > availabililty of this info to user, failures in atomic_check can be
> > avoided as these are more the hardware capabilities.
> >
> > There are two options on how this can be acheived.
> > Option 1:
> >
> > Add a new element to struct drm_mode_get_plane that holds the addr to
> > the array of a new struct. This new struct consists of the formats
> > supported and the corresponding plane capabilities.
> >
> > Option 2:
> >
> > These can be exposed to user as a plane property so that the user can
> > get to know the limitation ahead and avoid failures in atomic_check.
> >
> > Usually atomic_get_property is controlled over the state struct for
> > the parameters/elements that can change. But here these capabilities
> > or the interoperabilities are rather hardware restrictions and wont
> > change over flips. Hence having as a plane_property may not make much sense.
> > On the other hand, Option 1 include changes in the uapi struct
> > drm_mode_get_plane. Shouldnt have impact on backward compatibility,
> > but if userspace has some implementation so as to check the size of
> > the struct, then it might a challenge.
> 
> Adding fields to the struct should be okay, but adding a per-plane immutable
> property is also fine and IMO the cleaner option. We already have the same thing
> with "IN_FORMATS" and "type" that don't ever change either.
> 
> Either way, having a capability flag per format+modifier pair sounds good to me
> and should be both easy to make use of in userspace.

Adding new fields to a structure part of an existing UAPI may not be a great idea.
This can have some issues of backward compatibility and multiple structure definitions
with various kernel and user space header versions. So this doesn't look very promising.
Others can chime in and share their perspective.
 
We can expose an immutable plane capability property (with interoperability info) which userspace
can read and based on the capabilities exposed plane the composition. So, option 2 looks more
viable to me.

Regards,
Uma Shankar

> > Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
> > ---
> >  drivers/gpu/drm/drm_atomic_uapi.c |  3 +++
> >  include/drm/drm_plane.h           |  8 ++++++++
> >  include/uapi/drm/drm_mode.h       | 20 ++++++++++++++++++++
> >  3 files changed, 31 insertions(+)
> >
> > =============Option 2========================
> >
> > diff --git a/drivers/gpu/drm/drm_atomic_uapi.c
> > b/drivers/gpu/drm/drm_atomic_uapi.c index 22bbb2d83e30..b46177d5fc8c
> > 100644
> > --- a/drivers/gpu/drm/drm_atomic_uapi.c
> > +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> > @@ -631,6 +631,9 @@ drm_atomic_plane_get_property(struct drm_plane
> *plane,
> >  		*val = state->hotspot_x;
> >  	} else if (property == plane->hotspot_y_property) {
> >  		*val = state->hotspot_y;
> > +	} else if (property == config->prop_plane_caps) {
> > +		*val = (state->plane_caps) ?
> > +			state->plane_caps->base.id : 0;
> >  	} else {
> >  		drm_dbg_atomic(dev,
> >  			       "[PLANE:%d:%s] unknown property
> [PROP:%d:%s]\n",
> > diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h index
> > dd718c62ac31..dfe931677d0a 100644
> > --- a/include/drm/drm_plane.h
> > +++ b/include/drm/drm_plane.h
> > @@ -260,6 +260,14 @@ struct drm_plane_state {
> >  	 * flow.
> >  	 */
> >  	bool color_mgmt_changed : 1;
> > +
> > +	/**
> > +	 * @plane_caps:
> > +	 *
> > +	 * Blob representing plane capcabilites and interoperability.
> > +	 * This element is a pointer to the array of struct
> drm_format_blob.
> > +	 */
> > +	struct drm_property_blob *plane_caps;
> >  };
> >
> >  static inline struct drm_rect
> >
> > =============Option 1========================
> >
> > diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> > index d390011b89b4..0b5c1b65ef63 100644
> > --- a/include/uapi/drm/drm_mode.h
> > +++ b/include/uapi/drm/drm_mode.h
> > @@ -312,6 +312,20 @@ struct drm_mode_set_plane {
> >  	__u32 src_w;
> >  };
> >
> > +#define DRM_FORMAT_PLANE_CAP_LINEAR_TILE	BIT(0)
> > +#define DRM_FORMAT_PLANE_CAP_X_TILE		BIT(1)
> > +#define DRM_FORMAT_PLANE_CAP_Y_TILE		BIT(2)
> > +#define DRM_FORMAT_PLANE_CAP_Yf_TILE		BIT(3)
> > +#define DRM_FORMAT_PLANE_CAP_ASYNC_FLIP		BIT(4)
> > +#define DRM_FORMAT_PLANE_CAP_FBC		BIT(5)
> > +#define DRM_FORMAT_PLANE_CAP_RC			BIT(6)
> > +
> > +struct drm_format_blob {
> > +	__u64 modifier;
> > +	__u32 plane_caps;
> > +
> > +};
> > +
> >  /**
> >   * struct drm_mode_get_plane - Get plane metadata.
> >   *
> > @@ -355,6 +369,12 @@ struct drm_mode_get_plane {
> >  	 * supported by the plane. These formats do not require modifiers.
> >  	 */
> >  	__u64 format_type_ptr;
> > +	/**
> > +	 * @ format_blob_ptr: Pointer to the array of struct
> drm_format_blob.
> > +	 * Specify the plane capabilites/restrictions w.r.t tiling/sync-
> async
> > +	 * flips etc
> > +	 */
> > +	__u64 format_blob_ptr;
> >  };
> >
> >  struct drm_mode_get_plane_res {
> 
> 
>