drm: Add drm_atomic_get_(old|new)_connector_for_encoder() helpers

Add functions to the atomic core to retrieve the old and new connectors
associated with an encoder in a drm_atomic_state. This is useful for
encoders and bridges that need to access the connector, for instance for
the drm_display_info.

The CRTC associated with the encoder can also be retrieved through the
connector state, and from it, the old and new CRTC states.

Changed in v4:
- Added to the set
Changed in v5:
- Fix up docbook (Daniel & Laurent)
Changed in v6:
- Updated commit subject (Sam)

Link to v4: https://patchwork.freedesktop.org/patch/msgid/20190508160920.144739-3-sean@poorly.run
Link to v5: https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-3-sean@poorly.run

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

[seanpaul removed WARNs from helpers and added docs to explain why
returning NULL might be valid]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611205147.181298-1-sean@poorly.run

drivers/gpu/drm/drm_atomic.c

@@ -846,6 +846,75 @@ drm_atomic_get_new_private_obj_state(struct drm_atomic_state *state,
 }
 EXPORT_SYMBOL(drm_atomic_get_new_private_obj_state);
 
+/**
+ * drm_atomic_get_old_connector_for_encoder - Get old connector for an encoder
+ * @state: Atomic state
+ * @encoder: The encoder to fetch the connector state for
+ *
+ * This function finds and returns the connector that was connected to @encoder
+ * as specified by the @state.
+ *
+ * If there is no connector in @state which previously had @encoder connected to
+ * it, this function will return NULL. While this may seem like an invalid use
+ * case, it is sometimes useful to differentiate commits which had no prior
+ * connectors attached to @encoder vs ones that did (and to inspect their
+ * state). This is especially true in enable hooks because the pipeline has
+ * changed.
+ *
+ * Returns: The old connector connected to @encoder, or NULL if the encoder is
+ * not connected.
+ */
+struct drm_connector *
+drm_atomic_get_old_connector_for_encoder(struct drm_atomic_state *state,
+					 struct drm_encoder *encoder)
+{
+	struct drm_connector_state *conn_state;
+	struct drm_connector *connector;
+	unsigned int i;
+
+	for_each_old_connector_in_state(state, connector, conn_state, i) {
+		if (conn_state->best_encoder == encoder)
+			return connector;
+	}
+
+	return NULL;
+}
+EXPORT_SYMBOL(drm_atomic_get_old_connector_for_encoder);
+
+/**
+ * drm_atomic_get_new_connector_for_encoder - Get new connector for an encoder
+ * @state: Atomic state
+ * @encoder: The encoder to fetch the connector state for
+ *
+ * This function finds and returns the connector that will be connected to
+ * @encoder as specified by the @state.
+ *
+ * If there is no connector in @state which will have @encoder connected to it,
+ * this function will return NULL. While this may seem like an invalid use case,
+ * it is sometimes useful to differentiate commits which have no connectors
+ * attached to @encoder vs ones that do (and to inspect their state). This is
+ * especially true in disable hooks because the pipeline will change.
+ *
+ * Returns: The new connector connected to @encoder, or NULL if the encoder is
+ * not connected.
+ */
+struct drm_connector *
+drm_atomic_get_new_connector_for_encoder(struct drm_atomic_state *state,
+					 struct drm_encoder *encoder)
+{
+	struct drm_connector_state *conn_state;
+	struct drm_connector *connector;
+	unsigned int i;
+
+	for_each_new_connector_in_state(state, connector, conn_state, i) {
+		if (conn_state->best_encoder == encoder)
+			return connector;
+	}
+
+	return NULL;
+}
+EXPORT_SYMBOL(drm_atomic_get_new_connector_for_encoder);
+
 /**
  * drm_atomic_get_connector_state - get connector state
  * @state: global atomic state object

include/drm/drm_atomic.h

@@ -459,6 +459,13 @@ struct drm_private_state *
 drm_atomic_get_new_private_obj_state(struct drm_atomic_state *state,
 				     struct drm_private_obj *obj);
 
+struct drm_connector *
+drm_atomic_get_old_connector_for_encoder(struct drm_atomic_state *state,
+					 struct drm_encoder *encoder);
+struct drm_connector *
+drm_atomic_get_new_connector_for_encoder(struct drm_atomic_state *state,
+					 struct drm_encoder *encoder);
+
 /**
  * drm_atomic_get_existing_crtc_state - get crtc state, if it exists
  * @state: global atomic state object

include/drm/drm_connector.h

@@ -518,6 +518,11 @@ struct drm_connector_state {
 	 * &drm_connector_helper_funcs.atomic_best_encoder or
 	 * &drm_connector_helper_funcs.best_encoder callbacks.
 	 *
+	 * This is also used in the atomic helpers to map encoders to their
+	 * current and previous connectors, see
+	 * &drm_atomic_get_old_connector_for_encoder() and
+	 * &drm_atomic_get_new_connector_for_encoder().
+	 *
 	 * NOTE: Atomic drivers must fill this out (either themselves or through
 	 * helpers), for otherwise the GETCONNECTOR and GETENCODER IOCTLs will
 	 * not return correct data to userspace.