[PATCH v4 2/5] drm/doc: document the type plane property

[Daniel Vetter]

On Tue, Dec 22, 2020 at 02:35:21PM +0100, Simon Ser wrote:
> Add a new entry for "type" in the section for standard plane properties.
> 
> v3: improve paragraph about mixing legacy IOCTLs with explicit usage,
> not that a driver may support cursors without cursor planes (Daniel)

s/not/note/

> 
> v4: fixing rebase gone wrong
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> ---
>  drivers/gpu/drm/drm_plane.c | 58 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 54 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index b15b65e48555..9dce5be5780c 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -49,10 +49,8 @@
>   * &struct drm_plane (possibly as part of a larger structure) and registers it
>   * with a call to drm_universal_plane_init().
>   *
> - * The type of a plane is exposed in the immutable "type" enumeration property,
> - * which has one of the following values: "Overlay", "Primary", "Cursor" (see
> - * enum drm_plane_type). A plane can be compatible with multiple CRTCs, see
> - * &drm_plane.possible_crtcs.
> + * Each plane has a type, see enum drm_plane_type. A plane can be compatible
> + * with multiple CRTCs, see &drm_plane.possible_crtcs.
>   *
>   * Each CRTC must have a unique primary plane userspace can attach to enable
>   * the CRTC. In other words, userspace must be able to attach a different
> @@ -72,6 +70,58 @@
>   *
>   * DRM planes have a few standardized properties:
>   *
> + * type:
> + *     Immutable property describing the type of the plane.
> + *
> + *     For user-space which has enabled the &DRM_CLIENT_CAP_UNIVERSAL_PLANES

s/UNIVERSAL_PLANES/ATOMIC/ here?

With just universal planes you don't have atomic test-only. But I guess it
also works as-is, I'm just not entirely clear what you want to state here.

> + *     capability, the plane type is just a hint and is mostly superseded by
> + *     atomic test-only commits. The type hint can still be used to come up
> + *     more easily with a plane configuration accepted by the driver. Note that
> + *     &DRM_CLIENT_CAP_UNIVERSAL_PLANES is implicitly enabled by
> + *     &DRM_CLIENT_CAP_ATOMIC.
> + *
> + *     The value of this property can be one of the following:
> + *
> + *     "Primary":
> + *         To light up a CRTC, attaching a primary plane is the most likely to
> + *         work if it covers the whole CRTC and doesn't have scaling or
> + *         cropping set up.
> + *
> + *         Drivers may support more features for the primary plane, user-space
> + *         can find out with test-only atomic commits.
> + *
> + *         Primary planes are implicitly used by the kernel in the legacy
> + *         IOCTLs &DRM_IOCTL_MODE_SETCRTC and &DRM_IOCTL_MODE_PAGE_FLIP.
> + *         Therefore user-space must not mix explicit usage of any primary
> + *         plane (e.g. through an atomic commit) with these legacy IOCTLs.

Empty line here for reading comfort in plain text? Same below.

Since you mention formats below, I also wonder whether we should state
here that xrgb8888 is generally supported, worst case through software
emulation. That's defacto the uapi we have to adhere to.

> + *     "Cursor":
> + *         To enable this plane, using a framebuffer configured without scaling
> + *         or cropping and with the following properties is the most likely to
> + *         work:
> + *
> + *         - If the driver provides the capabilities &DRM_CAP_CURSOR_WIDTH and
> + *           &DRM_CAP_CURSOR_HEIGHT, create the framebuffer with this size.
> + *           Otherwise, create a framebuffer with the size 64x64.
> + *         - If the driver doesn't support modifiers, create a framebuffer with
> + *           a linear layout. Otherwise, use the IN_FORMATS plane property.
> + *
> + *         Drivers may support more features for the cursor plane, user-space
> + *         can find out with test-only atomic commits.
> + *
> + *         Cursor planes are implicitly used by the kernel in the legacy
> + *         IOCTLs &DRM_IOCTL_MODE_CURSOR and &DRM_IOCTL_MODE_CURSOR2.
> + *         Therefore user-space must not mix explicit usage of any cursor
> + *         plane (e.g. through an atomic commit) with these legacy IOCTLs.
> + *
> + *         Some drivers may support cursors even if no cursor plane is exposed.
> + *         In this case, the legacy cursor IOCTLs can be used to configure the
> + *         cursor.
> + *     "Overlay":
> + *         Neither primary nor cursor.
> + *
> + *         Overlay planes are the only planes exposed when the
> + *         &DRM_CLIENT_CAP_UNIVERSAL_PLANES capability is disabled.
> + *

Just some thoughts about, feel free to merge as-is.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

>   * IN_FORMATS:
>   *     Blob property which contains the set of buffer format and modifier
>   *     pairs supported by this plane. The blob is a struct
> -- 
> 2.29.2
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch