drm/bridge: add transparent bridge helper

Define a helper for creating simple transparent bridges which serve the
only purpose of linking devices into the bridge chain up to the last
bridge representing the connector. This is especially useful for
DP/USB-C bridge chains, which can span across several devices, but do
not require any additional functionality from the intermediate bridges.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231203114333.1305826-2-dmitry.baryshkov@linaro.org

drivers/gpu/drm/bridge/Kconfig

@@ -12,6 +12,15 @@ config DRM_PANEL_BRIDGE
 	help
 	  DRM bridge wrapper of DRM panels
 
+config DRM_AUX_BRIDGE
+	tristate
+	depends on DRM_BRIDGE && OF
+	select AUXILIARY_BUS
+	select DRM_PANEL_BRIDGE
+	help
+	  Simple transparent bridge that is used by several non-DRM drivers to
+	  build bridges chain.
+
 menu "Display Interface Bridges"
 	depends on DRM && DRM_BRIDGE
 

drivers/gpu/drm/bridge/Makefile

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
+obj-$(CONFIG_DRM_AUX_BRIDGE) += aux-bridge.o
 obj-$(CONFIG_DRM_CHIPONE_ICN6211) += chipone-icn6211.o
 obj-$(CONFIG_DRM_CHRONTEL_CH7033) += chrontel-ch7033.o
 obj-$(CONFIG_DRM_CROS_EC_ANX7688) += cros-ec-anx7688.o

drivers/gpu/drm/bridge/aux-bridge.c

@@ -0,0 +1,140 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2023 Linaro Ltd.
+ *
+ * Author: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
+ */
+#include <linux/auxiliary_bus.h>
+#include <linux/module.h>
+
+#include <drm/drm_bridge.h>
+#include <drm/bridge/aux-bridge.h>
+
+static DEFINE_IDA(drm_aux_bridge_ida);
+
+static void drm_aux_bridge_release(struct device *dev)
+{
+	struct auxiliary_device *adev = to_auxiliary_dev(dev);
+
+	ida_free(&drm_aux_bridge_ida, adev->id);
+
+	kfree(adev);
+}
+
+static void drm_aux_bridge_unregister_adev(void *_adev)
+{
+	struct auxiliary_device *adev = _adev;
+
+	auxiliary_device_delete(adev);
+	auxiliary_device_uninit(adev);
+}
+
+/**
+ * drm_aux_bridge_register - Create a simple bridge device to link the chain
+ * @parent: device instance providing this bridge
+ *
+ * Creates a simple DRM bridge that doesn't implement any drm_bridge
+ * operations. Such bridges merely fill a place in the bridge chain linking
+ * surrounding DRM bridges.
+ *
+ * Return: zero on success, negative error code on failure
+ */
+int drm_aux_bridge_register(struct device *parent)
+{
+	struct auxiliary_device *adev;
+	int ret;
+
+	adev = kzalloc(sizeof(*adev), GFP_KERNEL);
+	if (!adev)
+		return -ENOMEM;
+
+	ret = ida_alloc(&drm_aux_bridge_ida, GFP_KERNEL);
+	if (ret < 0) {
+		kfree(adev);
+		return ret;
+	}
+
+	adev->id = ret;
+	adev->name = "aux_bridge";
+	adev->dev.parent = parent;
+	adev->dev.of_node = parent->of_node;
+	adev->dev.release = drm_aux_bridge_release;
+
+	ret = auxiliary_device_init(adev);
+	if (ret) {
+		ida_free(&drm_aux_bridge_ida, adev->id);
+		kfree(adev);
+		return ret;
+	}
+
+	ret = auxiliary_device_add(adev);
+	if (ret) {
+		auxiliary_device_uninit(adev);
+		return ret;
+	}
+
+	return devm_add_action_or_reset(parent, drm_aux_bridge_unregister_adev, adev);
+}
+EXPORT_SYMBOL_GPL(drm_aux_bridge_register);
+
+struct drm_aux_bridge_data {
+	struct drm_bridge bridge;
+	struct drm_bridge *next_bridge;
+	struct device *dev;
+};
+
+static int drm_aux_bridge_attach(struct drm_bridge *bridge,
+				 enum drm_bridge_attach_flags flags)
+{
+	struct drm_aux_bridge_data *data;
+
+	if (!(flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR))
+		return -EINVAL;
+
+	data = container_of(bridge, struct drm_aux_bridge_data, bridge);
+
+	return drm_bridge_attach(bridge->encoder, data->next_bridge, bridge,
+				 DRM_BRIDGE_ATTACH_NO_CONNECTOR);
+}
+
+static const struct drm_bridge_funcs drm_aux_bridge_funcs = {
+	.attach	= drm_aux_bridge_attach,
+};
+
+static int drm_aux_bridge_probe(struct auxiliary_device *auxdev,
+				const struct auxiliary_device_id *id)
+{
+	struct drm_aux_bridge_data *data;
+
+	data = devm_kzalloc(&auxdev->dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	data->dev = &auxdev->dev;
+	data->next_bridge = devm_drm_of_get_bridge(&auxdev->dev, auxdev->dev.of_node, 0, 0);
+	if (IS_ERR(data->next_bridge))
+		return dev_err_probe(&auxdev->dev, PTR_ERR(data->next_bridge),
+				     "failed to acquire drm_bridge\n");
+
+	data->bridge.funcs = &drm_aux_bridge_funcs;
+	data->bridge.of_node = data->dev->of_node;
+
+	return devm_drm_bridge_add(data->dev, &data->bridge);
+}
+
+static const struct auxiliary_device_id drm_aux_bridge_table[] = {
+	{ .name = KBUILD_MODNAME ".aux_bridge" },
+	{},
+};
+MODULE_DEVICE_TABLE(auxiliary, drm_aux_bridge_table);
+
+static struct auxiliary_driver drm_aux_bridge_drv = {
+	.name = "aux_bridge",
+	.id_table = drm_aux_bridge_table,
+	.probe = drm_aux_bridge_probe,
+};
+module_auxiliary_driver(drm_aux_bridge_drv);
+
+MODULE_AUTHOR("Dmitry Baryshkov <dmitry.baryshkov@linaro.org>");
+MODULE_DESCRIPTION("DRM transparent bridge");
+MODULE_LICENSE("GPL");

include/drm/bridge/aux-bridge.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2023 Linaro Ltd.
+ *
+ * Author: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
+ */
+#ifndef DRM_AUX_BRIDGE_H
+#define DRM_AUX_BRIDGE_H
+
+#if IS_ENABLED(CONFIG_DRM_AUX_BRIDGE)
+int drm_aux_bridge_register(struct device *parent);
+#else
+static inline int drm_aux_bridge_register(struct device *parent)
+{
+	return 0;
+}
+#endif
+
+#endif