[PATCH] drm: document drm_mode_get_property

[Simon Ser <contact@emersion.fr>]

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.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
---
 include/uapi/drm/drm_mode.h | 52 ++++++++++++++++++++++++++++++++++---
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 98bf130feda5..dfdb595875aa 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -546,17 +546,61 @@ struct drm_mode_property_enum {
 	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 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;
 };
 
-- 
2.32.0