drm: Add helper to create a connector for a chain of bridges

Most bridge drivers create a DRM connector to model the connector at the
output of the bridge. This model is historical and has worked pretty
well so far, but causes several issues:

- It prevents supporting more complex display pipelines where DRM
connector operations are split over multiple components. For instance a
pipeline with a bridge connected to the DDC signals to read EDID data,
and another one connected to the HPD signal to detect connection and
disconnection, will not be possible to support through this model.

- It requires every bridge driver to implement similar connector
handling code, resulting in code duplication.

- It assumes that a bridge will either be wired to a connector or to
another bridge, but doesn't support bridges that can be used in both
positions very well (although there is some ad-hoc support for this in
the analogix_dp bridge driver).

In order to solve these issues, ownership of the connector needs to be
moved to the display controller driver.

To avoid code duplication in display controller drivers, add a new
helper to create and manage a DRM connector backed by a chain of
bridges. All connector operations are delegating to the appropriate
bridge in the chain.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-21-laurent.pinchart@ideasonboard.com

Documentation/gpu/drm-kms-helpers.rst

@@ -145,6 +145,12 @@ Bridge Operations
 .. kernel-doc:: drivers/gpu/drm/drm_bridge.c
    :doc: bridge operations
 
+Bridge Connector Helper
+-----------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_bridge_connector.c
+   :doc: overview
+
 
 Bridge Helper Reference
 -------------------------
@@ -155,6 +161,12 @@ Bridge Helper Reference
 .. kernel-doc:: drivers/gpu/drm/drm_bridge.c
    :export:
 
+Bridge Connector Helper Reference
+---------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_bridge_connector.c
+   :export:
+
 Panel-Bridge Helper Reference
 -----------------------------
 

drivers/gpu/drm/Makefile

@@ -39,7 +39,8 @@ obj-$(CONFIG_DRM_VRAM_HELPER) += drm_vram_helper.o
 drm_ttm_helper-y := drm_gem_ttm_helper.o
 obj-$(CONFIG_DRM_TTM_HELPER) += drm_ttm_helper.o
 
-drm_kms_helper-y := drm_crtc_helper.o drm_dp_helper.o drm_dsc.o drm_probe_helper.o \
+drm_kms_helper-y := drm_bridge_connector.o drm_crtc_helper.o drm_dp_helper.o \
+		drm_dsc.o drm_probe_helper.o \
 		drm_plane_helper.o drm_dp_mst_topology.o drm_atomic_helper.o \
 		drm_kms_helper_common.o drm_dp_dual_mode_helper.o \
 		drm_simple_kms_helper.o drm_modeset_helper.o \

drivers/gpu/drm/drm_bridge.c

@@ -79,6 +79,12 @@
  * requires no intervention from the driver. For other drivers, the relevant
  * DRM bridge chain functions shall be called manually.
  *
+ * Bridges also participate in implementing the &drm_connector at the end of
+ * the bridge chain. Display drivers may use the drm_bridge_connector_init()
+ * helper to create the &drm_connector, or implement it manually on top of the
+ * connector-related operations exposed by the bridge (see the overview
+ * documentation of bridge operations for more details).
+ *
  * &drm_bridge, like &drm_panel, aren't &drm_mode_object entities like planes,
  * CRTCs, encoders or connectors and hence are not visible to userspace. They
  * just provide additional hooks to get the desired output at the end of the

include/drm/drm_bridge_connector.h

+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2019 Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+ */
+
+#ifndef __DRM_BRIDGE_CONNECTOR_H__
+#define __DRM_BRIDGE_CONNECTOR_H__
+
+struct drm_connector;
+struct drm_device;
+struct drm_encoder;
+
+void drm_bridge_connector_enable_hpd(struct drm_connector *connector);
+void drm_bridge_connector_disable_hpd(struct drm_connector *connector);
+struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
+						struct drm_encoder *encoder);
+
+#endif /* __DRM_BRIDGE_CONNECTOR_H__ */