drm: document drm_mode_get_connector

Document how to perform a GETCONNECTOR ioctl. Document the various
struct fields. Also document how to perform a forced probe, and when
should user-space do it.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4NxrTtynzPiPX4SOCzxmA1sRB8fVLfeiabVpi5j3Y@cp7-web-041.plabs.ch

include/uapi/drm/drm_mode.h

@@ -368,27 +368,95 @@ enum drm_mode_subconnector {
 #define DRM_MODE_CONNECTOR_WRITEBACK	18
 #define DRM_MODE_CONNECTOR_SPI		19
 
+/**
+ * struct drm_mode_get_connector - Get connector metadata.
+ *
+ * User-space can perform a GETCONNECTOR ioctl to retrieve information about a
+ * connector. User-space is expected to retrieve encoders, modes and properties
+ * 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_props and @count_encoders to
+ * zero, set @count_modes to 1, and set @modes_ptr to a temporary struct
+ * drm_mode_modeinfo element.
+ *
+ * To retrieve the elements, allocate arrays for @encoders_ptr, @modes_ptr,
+ * @props_ptr and @prop_values_ptr, then set @count_modes, @count_props and
+ * @count_encoders to their capacity.
+ *
+ * Performing the ioctl only twice may be racy: the number of elements may have
+ * changed with a hotplug event in-between the two ioctls. User-space is
+ * expected to retry the last ioctl until the number of elements stabilizes.
+ * The kernel won't fill any array which doesn't have the expected length.
+ *
+ * **Force-probing a connector**
+ *
+ * If the @count_modes field is set to zero, the kernel will perform a forced
+ * probe on the connector to refresh the connector status, modes and EDID.
+ * A forced-probe can be slow and the ioctl will block. A force-probe can cause
+ * flickering and temporary freezes, so it should not be performed
+ * automatically.
+ *
+ * User-space shouldn't need to force-probe connectors in general: the kernel
+ * will automatically take care of probing connectors that don't support
+ * hot-plug detection when appropriate. However, user-space may force-probe
+ * connectors on user request (e.g. clicking a "Scan connectors" button, or
+ * opening a UI to manage screens).
+ */
 struct drm_mode_get_connector {
-
+	/** @encoders_ptr: Pointer to ``__u32`` array of object IDs. */
 	__u64 encoders_ptr;
+	/** @modes_ptr: Pointer to struct drm_mode_modeinfo array. */
 	__u64 modes_ptr;
+	/** @props_ptr: Pointer to ``__u32`` array of property IDs. */
 	__u64 props_ptr;
+	/** @prop_values_ptr: Pointer to ``__u64`` array of property values. */
 	__u64 prop_values_ptr;
 
+	/** @count_modes: Number of modes. */
 	__u32 count_modes;
+	/** @count_props: Number of properties. */
 	__u32 count_props;
+	/** @count_encoders: Number of encoders. */
 	__u32 count_encoders;
 
-	__u32 encoder_id; /**< Current Encoder */
-	__u32 connector_id; /**< Id */
+	/** @encoder_id: Object ID of the current encoder. */
+	__u32 encoder_id;
+	/** @connector_id: Object ID of the connector. */
+	__u32 connector_id;
+	/**
+	 * @connector_type: Type of the connector.
+	 *
+	 * See DRM_MODE_CONNECTOR_* defines.
+	 */
 	__u32 connector_type;
+	/**
+	 * @connector_type_id: Type-specific connector number.
+	 *
+	 * This is not an object ID. This is a per-type connector number. Each
+	 * (type, type_id) combination is unique across all connectors of a DRM
+	 * device.
+	 */
 	__u32 connector_type_id;
 
+	/**
+	 * @connection: Status of the connector.
+	 *
+	 * See enum drm_connector_status.
+	 */
 	__u32 connection;
-	__u32 mm_width;  /**< width in millimeters */
-	__u32 mm_height; /**< height in millimeters */
+	/** @mm_width: Width of the connected sink in millimeters. */
+	__u32 mm_width;
+	/** @mm_height: Height of the connected sink in millimeters. */
+	__u32 mm_height;
+	/**
+	 * @subpixel: Subpixel order of the connected sink.
+	 *
+	 * See enum subpixel_order.
+	 */
 	__u32 subpixel;
 
+	/** @pad: Padding, must be zero. */
 	__u32 pad;
 };