drm: Document drm_connector_helper_funcs

Nothing special, except the somewhat awkward split in probe helper
callbacks between here and drm_crtc_funcs.

v2: Suggestions from Thierry.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-25-git-send-email-daniel.vetter@ffwll.chReviewed-by: Thierry Reding <treding@nvidia.com>

include/drm/drm_modeset_helper_vtables.h

@@ -270,18 +270,114 @@ static inline void drm_encoder_helper_add(struct drm_encoder *encoder,
 
 /**
  * struct drm_connector_helper_funcs - helper operations for connectors
- * @get_modes: get mode list for this connector
- * @mode_valid: is this mode valid on the given connector? (optional)
- * @best_encoder: return the preferred encoder for this connector
- * @atomic_best_encoder: atomic version of @best_encoder
  *
- * The helper operations are called by the mid-layer CRTC helper.
+ * These functions are used by the atomic and legacy modeset helpers and by the
+ * probe helpers.
  */
 struct drm_connector_helper_funcs {
+	/**
+	 * @get_modes:
+	 *
+	 * This function should fill in all modes currently valid for the sink
+	 * into the connector->probed_modes list. It should also update the
+	 * EDID property by calling drm_mode_connector_update_edid_property().
+	 *
+	 * The usual way to implement this is to cache the EDID retrieved in the
+	 * probe callback somewhere in the driver-private connector structure.
+	 * In this function drivers then parse the modes in the EDID and add
+	 * them by calling drm_add_edid_modes(). But connectors that driver a
+	 * fixed panel can also manually add specific modes using
+	 * drm_mode_probed_add(). Finally drivers that support audio probably
+	 * want to update the ELD data, too, using drm_edid_to_eld().
+	 *
+	 * This function is only called after the ->detect() hook has indicated
+	 * that a sink is connected and when the EDID isn't overridden through
+	 * sysfs or the kernel commandline.
+	 *
+	 * This callback is used by the probe helpers in e.g.
+	 * drm_helper_probe_single_connector_modes().
+	 *
+	 * RETURNS:
+	 *
+	 * The number of modes added by calling drm_mode_probed_add().
+	 */
 	int (*get_modes)(struct drm_connector *connector);
+
+	/**
+	 * @mode_valid:
+	 *
+	 * Callback to validate a mode for a connector, irrespective of the
+	 * specific display configuration.
+	 *
+	 * This callback is used by the probe helpers to filter the mode list
+	 * (which is usually derived from the EDID data block from the sink).
+	 * See e.g. drm_helper_probe_single_connector_modes().
+	 *
+	 * NOTE:
+	 *
+	 * This only filters the mode list supplied to userspace in the
+	 * GETCONNECOTR IOCTL. Userspace is free to create modes of its own and
+	 * ask the kernel to use them. It this case the atomic helpers or legacy
+	 * CRTC helpers will not call this function. Drivers therefore must
+	 * still fully validate any mode passed in in a modeset request.
+	 *
+	 * RETURNS:
+	 *
+	 * Either MODE_OK or one of the failure reasons in enum
+	 * &drm_mode_status.
+	 */
 	enum drm_mode_status (*mode_valid)(struct drm_connector *connector,
 					   struct drm_display_mode *mode);
+	/**
+	 * @best_encoder:
+	 *
+	 * This function should select the best encoder for the given connector.
+	 *
+	 * This function is used by both the atomic helpers (in the
+	 * drm_atomic_helper_check_modeset() function) and in the legacy CRTC
+	 * helpers.
+	 *
+	 * NOTE:
+	 *
+	 * In atomic drivers this function is called in the check phase of an
+	 * atomic update. The driver is not allowed to change or inspect
+	 * anything outside of arguments passed-in. Atomic drivers which need to
+	 * inspect dynamic configuration state should instead use
+	 * @atomic_best_encoder.
+	 *
+	 * RETURNS:
+	 *
+	 * Encoder that should be used for the given connector and connector
+	 * state, or NULL if no suitable encoder exists. Note that the helpers
+	 * will ensure that encoders aren't used twice, drivers should not check
+	 * for this.
+	 */
 	struct drm_encoder *(*best_encoder)(struct drm_connector *connector);
+
+	/**
+	 * @atomic_best_encoder:
+	 *
+	 * This is the atomic version of @best_encoder for atomic drivers which
+	 * need to select the best encoder depending upon the desired
+	 * configuration and can't select it statically.
+	 *
+	 * This function is used by drm_atomic_helper_check_modeset() and either
+	 * this or @best_encoder is required.
+	 *
+	 * NOTE:
+	 *
+	 * This function is called in the check phase of an atomic update. The
+	 * driver is not allowed to change anything outside of the free-standing
+	 * state objects passed-in or assembled in the overall &drm_atomic_state
+	 * update tracking structure.
+	 *
+	 * RETURNS:
+	 *
+	 * Encoder that should be used for the given connector and connector
+	 * state, or NULL if no suitable encoder exists. Note that the helpers
+	 * will ensure that encoders aren't used twice, drivers should not check
+	 * for this.
+	 */
 	struct drm_encoder *(*atomic_best_encoder)(struct drm_connector *connector,
 						   struct drm_connector_state *connector_state);
 };