drm: Introduce plane SIZE_HINTS property

Add a new immutable plane property by which a plane can advertise
a handful of recommended plane sizes. This would be mostly exposed
by cursor planes as a slightly more capable replacement for
the DRM_CAP_CURSOR_WIDTH/HEIGHT caps, which can only declare
a one size fits all limit for the whole device.

Currently eg. amdgpu/i915/nouveau just advertize the max cursor
size via the cursor size caps. But always using the max sized
cursor can waste a surprising amount of power, so a better
strategy is desirable.

Most other drivers don't specify any cursor size at all, in
which case the ioctl code just claims that 64x64 is a great
choice. Whether that is actually true is debatable.

A poll of various compositor developers informs us that
blindly probing with setcursor/atomic ioctl to determine
suitable cursor sizes is not acceptable, thus the
introduction of the new property to supplant the cursor
size caps. The compositor will now be free to select a
more optimal cursor size from the short list of options.

Note that the reported sizes (either via the property or the
caps) make no claims about things such as plane scaling. So
these things should only really be consulted for simple
"cursor like" use cases.

Userspace consumer in the form of mutter seems ready:
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3165

v2: Try to add some docs
v3: Specify that value 0 is reserved for future use (basic idea from Jonas)
    Drop the note about typical hardware (Pekka)
v4: Update the docs to indicate the list is "in order of preference"
    Add a a link to the mutter MR
v5: Limit to cursors only for now (Simon)

Cc: Jonas Ådahl <jadahl@redhat.com>
Cc: Sameer Lattannavar <sameer.lattannavar@intel.com>
Reviewed-by: Sebastian Wick <sebastian.wick@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240318204408.9687-2-ville.syrjala@linux.intel.com

1 files changed, 5 insertions, 0 deletions

diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 973119a9176b..9d8acf7a10eb 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -942,6 +942,11 @@ struct drm_mode_config {
 	 */
 	struct drm_property *modifiers_property;
 
+	/**
+	 * @size_hints_propertty: Plane SIZE_HINTS property.
+	 */
+	struct drm_property *size_hints_property;
+
 	/* cursor size */
 	uint32_t cursor_width, cursor_height;