drm/doc: document drm_mode_get_plane

Add a small description and document struct fields of
drm_mode_get_plane.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611213516.77904-2-leandro.ribeiro@collabora.com

include/uapi/drm/drm_mode.h

@@ -312,16 +312,48 @@ struct drm_mode_set_plane {
 	__u32 src_w;
 };
 
+/**
+ * struct drm_mode_get_plane - Get plane metadata.
+ *
+ * Userspace can perform a GETPLANE ioctl to retrieve information about a
+ * plane.
+ *
+ * To retrieve the number of formats supported, set @count_format_types to zero
+ * and call the ioctl. @count_format_types will be updated with the value.
+ *
+ * To retrieve these formats, allocate an array with the memory needed to store
+ * @count_format_types formats. Point @format_type_ptr to this array and call
+ * the ioctl again (with @count_format_types still set to the value returned in
+ * the first ioctl call).
+ */
 struct drm_mode_get_plane {
+	/**
+	 * @plane_id: Object ID of the plane whose information should be
+	 * retrieved. Set by caller.
+	 */
 	__u32 plane_id;
 
+	/** @crtc_id: Object ID of the current CRTC. */
 	__u32 crtc_id;
+	/** @fb_id: Object ID of the current fb. */
 	__u32 fb_id;
 
+	/**
+	 * @possible_crtcs: Bitmask of CRTC's compatible with the plane. CRTC's
+	 * are created and they receive an index, which corresponds to their
+	 * position in the bitmask. Bit N corresponds to
+	 * :ref:`CRTC index<crtc_index>` N.
+	 */
 	__u32 possible_crtcs;
+	/** @gamma_size: Never used. */
 	__u32 gamma_size;
 
+	/** @count_format_types: Number of formats. */
 	__u32 count_format_types;
+	/**
+	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
+	 * supported by the plane. These formats do not require modifiers.
+	 */
 	__u64 format_type_ptr;
 };