drm: document drm_mode_get_property

It's not obvious what the fields mean and how they should be used.
The most important detail is the link to drm_property.flags, which
describes how property types work.

v2: document enum drm_mode_property_enum, add ref to "Modeset Base
Object Abstraction" (Daniel)
Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210802072826.500078-1-contact@emersion.fr

Documentation/gpu/drm-kms.rst

@@ -159,6 +159,8 @@ KMS Core Structures and Functions
 .. kernel-doc:: drivers/gpu/drm/drm_mode_config.c
    :export:
 
+.. _kms_base_object_abstraction:
+
 Modeset Base Object Abstraction
 ===============================
 

include/uapi/drm/drm_mode.h

@@ -541,22 +541,74 @@ struct drm_mode_get_connector {
  */
 #define DRM_MODE_PROP_ATOMIC        0x80000000
 
+/**
+ * struct drm_mode_property_enum - Description for an enum/bitfield entry.
+ * @value: numeric value for this enum entry.
+ * @name: symbolic name for this enum entry.
+ *
+ * See struct drm_property_enum for details.
+ */
 struct drm_mode_property_enum {
 	__u64 value;
 	char name[DRM_PROP_NAME_LEN];
 };
 
+/**
+ * struct drm_mode_get_property - Get property metadata.
+ *
+ * User-space can perform a GETPROPERTY ioctl to retrieve information about a
+ * property. The same property may be attached to multiple objects, see
+ * "Modeset Base Object Abstraction".
+ *
+ * The meaning of the @values_ptr field changes depending on the property type.
+ * See &drm_property.flags for more details.
+ *
+ * The @enum_blob_ptr and @count_enum_blobs fields are only meaningful when the
+ * property has the type &DRM_MODE_PROP_ENUM or &DRM_MODE_PROP_BITMASK. For
+ * backwards compatibility, the kernel will always set @count_enum_blobs to
+ * zero when the property has the type &DRM_MODE_PROP_BLOB. User-space must
+ * ignore these two fields if the property has a different type.
+ *
+ * User-space is expected to retrieve values and enums by performing this ioctl
+ * at least twice: the first time to retrieve the number of elements, the
+ * second time to retrieve the elements themselves.
+ *
+ * To retrieve the number of elements, set @count_values and @count_enum_blobs
+ * to zero, then call the ioctl. @count_values will be updated with the number
+ * of elements. If the property has the type &DRM_MODE_PROP_ENUM or
+ * &DRM_MODE_PROP_BITMASK, @count_enum_blobs will be updated as well.
+ *
+ * To retrieve the elements themselves, allocate an array for @values_ptr and
+ * set @count_values to its capacity. If the property has the type
+ * &DRM_MODE_PROP_ENUM or &DRM_MODE_PROP_BITMASK, allocate an array for
+ * @enum_blob_ptr and set @count_enum_blobs to its capacity. Calling the ioctl
+ * again will fill the arrays.
+ */
 struct drm_mode_get_property {
-	__u64 values_ptr; /* values and blob lengths */
-	__u64 enum_blob_ptr; /* enum and blob id ptrs */
+	/** @values_ptr: Pointer to a ``__u64`` array. */
+	__u64 values_ptr;
+	/** @enum_blob_ptr: Pointer to a struct drm_mode_property_enum array. */
+	__u64 enum_blob_ptr;
 
+	/**
+	 * @prop_id: Object ID of the property which should be retrieved. Set
+	 * by the caller.
+	 */
 	__u32 prop_id;
+	/**
+	 * @flags: ``DRM_MODE_PROP_*`` bitfield. See &drm_property.flags for
+	 * a definition of the flags.
+	 */
 	__u32 flags;
+	/**
+	 * @name: Symbolic property name. User-space should use this field to
+	 * recognize properties.
+	 */
 	char name[DRM_PROP_NAME_LEN];
 
+	/** @count_values: Number of elements in @values_ptr. */
 	__u32 count_values;
-	/* This is only used to count enum values, not blobs. The _blobs is
-	 * simply because of a historical reason, i.e. backwards compat. */
+	/** @count_enum_blobs: Number of elements in @enum_blob_ptr. */
 	__u32 count_enum_blobs;
 };