drm/atomic: Acquire connection_mutex lock in drm_helper_probe_single_connector_modes, v4.

mode_valid() called from drm_helper_probe_single_connector_modes()
may need to look at connector->state because what a valid mode is may
depend on connector properties being set. For example some HDMI modes
might be rejected when a connector property forces the connector
into DVI mode.

Some implementations of detect() already lock all state,
so we have to pass an acquire_ctx to them to prevent a deadlock.

This means changing the function signature of detect() slightly,
and passing the acquire_ctx for locking multiple crtc's.
For the callbacks, it will always be non-zero. To allow callers
not to worry about this, drm_helper_probe_detect_ctx is added
which might handle -EDEADLK for you.

Changes since v1:
- Always set ctx parameter.
Changes since v2:
- Always take connection_mutex when probing.
Changes since v3:
- Remove the ctx from intel_dp_long_pulse, and add
  WARN_ON(!connection_mutex) (danvet)
- Update docs to clarify the locking situation. (danvet)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1491504920-4017-1-git-send-email-maarten.lankhorst@linux.intel.com

include/drm/drm_connector.h

@@ -32,6 +32,7 @@
 struct drm_device;
 
 struct drm_connector_helper_funcs;
+struct drm_modeset_acquire_ctx;
 struct drm_device;
 struct drm_crtc;
 struct drm_encoder;
@@ -378,6 +379,11 @@ struct drm_connector_funcs {
 	 * the helper library vtable purely for historical reasons. The only DRM
 	 * core	entry point to probe connector state is @fill_modes.
 	 *
+	 * Note that the helper library will already hold
+	 * &drm_mode_config.connection_mutex. Drivers which need to grab additional
+	 * locks to avoid races with concurrent modeset changes need to use
+	 * &drm_connector_helper_funcs.detect_ctx instead.
+	 *
 	 * RETURNS:
 	 *
 	 * drm_connector_status indicating the connector's status.

include/drm/drm_crtc_helper.h

@@ -67,6 +67,9 @@ int drm_helper_crtc_mode_set_base(struct drm_crtc *crtc, int x, int y,
 int drm_helper_probe_single_connector_modes(struct drm_connector
 					    *connector, uint32_t maxX,
 					    uint32_t maxY);
+int drm_helper_probe_detect(struct drm_connector *connector,
+			    struct drm_modeset_acquire_ctx *ctx,
+			    bool force);
 void drm_kms_helper_poll_init(struct drm_device *dev);
 void drm_kms_helper_poll_fini(struct drm_device *dev);
 bool drm_helper_hpd_irq_event(struct drm_device *dev);

include/drm/drm_modeset_helper_vtables.h

@@ -747,12 +747,44 @@ struct drm_connector_helper_funcs {
 	 * This callback is used by the probe helpers in e.g.
 	 * drm_helper_probe_single_connector_modes().
 	 *
+	 * To avoid races with concurrent connector state updates, the helper
+	 * libraries always call this with the &drm_mode_config.connection_mutex
+	 * held. Because of this it's safe to inspect &drm_connector->state.
+	 *
 	 * RETURNS:
 	 *
 	 * The number of modes added by calling drm_mode_probed_add().
 	 */
 	int (*get_modes)(struct drm_connector *connector);
 
+	/**
+	 * @detect_ctx:
+	 *
+	 * Check to see if anything is attached to the connector. The parameter
+	 * force is set to false whilst polling, true when checking the
+	 * connector due to a user request. force can be used by the driver to
+	 * avoid expensive, destructive operations during automated probing.
+	 *
+	 * This callback is optional, if not implemented the connector will be
+	 * considered as always being attached.
+	 *
+	 * This is the atomic version of &drm_connector_funcs.detect.
+	 *
+	 * To avoid races against concurrent connector state updates, the
+	 * helper libraries always call this with ctx set to a valid context,
+	 * and &drm_mode_config.connection_mutex will always be locked with
+	 * the ctx parameter set to this ctx. This allows taking additional
+	 * locks as required.
+	 *
+	 * RETURNS:
+	 *
+	 * &drm_connector_status indicating the connector's status,
+	 * or the error code returned by drm_modeset_lock(), -EDEADLK.
+	 */
+	int (*detect_ctx)(struct drm_connector *connector,
+			  struct drm_modeset_acquire_ctx *ctx,
+			  bool force);
+
 	/**
 	 * @mode_valid:
 	 *
@@ -771,6 +803,10 @@ struct drm_connector_helper_funcs {
 	 * CRTC helpers will not call this function. Drivers therefore must
 	 * still fully validate any mode passed in in a modeset request.
 	 *
+	 * To avoid races with concurrent connector state updates, the helper
+	 * libraries always call this with the &drm_mode_config.connection_mutex
+	 * held. Because of this it's safe to inspect &drm_connector->state.
+         *
 	 * RETURNS:
 	 *
 	 * Either &drm_mode_status.MODE_OK or one of the failure reasons in &enum