drm/doc: document the type plane property

Add a new entry for "type" in the section for standard plane properties.

v3: improve paragraph about mixing legacy IOCTLs with explicit usage,
note that a driver may support cursors without cursor planes (Daniel)

v4: fixing rebase gone wrong

v5:
- Fix typo (Daniel)
- Mention CAP_ATOMIC instead of CAP_UNIVERSAL_PLANES when referring to
  atomic test-only commits (Daniel)
- Add newlines at end of sections (Daniel)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115110626.12233-2-contact@emersion.fr

drivers/gpu/drm/drm_plane.c

@@ -50,10 +50,8 @@
  * &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
@@ -73,6 +71,58 @@
  *
  * 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_ATOMIC 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+ *     the plane type is just a hint and is mostly superseded by atomic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.
+ *
+ *     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.
+ *
+ *         Some 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.
+ *
+ *     "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.
+ *
+ *         Some 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.
+ *
  * IN_FORMATS:
  *     Blob property which contains the set of buffer format and modifier
  *     pairs supported by this plane. The blob is a struct