[PATCH RFC v2 23/24] drm: drm_hotplug_helper: new helper to implement bridge hotplug
Luca Ceresoli
luca.ceresoli at bootlin.com
Thu Oct 1 05:42:45 PDT 2026
Add a new helper to support cards implementing hotpluggable drm_bridges.
drmm_hotplug_helper_init() registers to get notified of relevant events and
react by creating a bridge (if the pipeline is complete in the hardware)
and destroying it on bridge removal.
Signed-off-by: Luca Ceresoli <luca.ceresoli at bootlin.com>
---
MAINTAINERS | 8 +
drivers/gpu/drm/display/Kconfig | 6 +
drivers/gpu/drm/display/Makefile | 2 +
drivers/gpu/drm/display/drm_hotplug_helper.c | 243 +++++++++++++++++++++++++++
include/drm/drm_hotplug_helper.h | 13 ++
5 files changed, 272 insertions(+)
diff --git a/MAINTAINERS b/MAINTAINERS
index c5ae9f2f408a..611790c175c4 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8019,6 +8019,14 @@ F: Documentation/devicetree/bindings/npu/rockchip,rk3588-rknn-core.yaml
F: drivers/accel/rocket/
F: include/uapi/drm/rocket_accel.h
+DRM BRIDGE HOTPLUG HELPER
+M: Luca Ceresoli <luca.ceresoli at bootlin.com>
+S: Maintained
+T: git https://gitlab.freedesktop.org/drm/misc/kernel.git
+F: Documentation/devicetree/bindings/display/bridge/
+F: drivers/gpu/drm/display/drm_hotplug_helper.c
+F: include/drm/drm_hotplug_helper.h
+
DRM COMPUTE ACCELERATORS DRIVERS AND FRAMEWORK
M: Oded Gabbay <ogabbay at kernel.org>
L: dri-devel at lists.freedesktop.org
diff --git a/drivers/gpu/drm/display/Kconfig b/drivers/gpu/drm/display/Kconfig
index df09cf9a8ca1..f1a6ffcd4c63 100644
--- a/drivers/gpu/drm/display/Kconfig
+++ b/drivers/gpu/drm/display/Kconfig
@@ -22,6 +22,12 @@ config DRM_BRIDGE_CONNECTOR
help
DRM connector implementation terminating DRM bridge chains.
+config DRM_HOTPLUG_HELPER
+ bool
+ select DRM_BRIDGE_CONNECTOR
+ help
+ Helper code to implement a card supporting hotpluggable bridges.
+
config DRM_DISPLAY_DP_AUX_CEC
bool "Enable DisplayPort CEC-Tunneling-over-AUX HDMI support"
select DRM_DISPLAY_DP_HELPER
diff --git a/drivers/gpu/drm/display/Makefile b/drivers/gpu/drm/display/Makefile
index 0ff4a1ad0222..ce6dbd65833b 100644
--- a/drivers/gpu/drm/display/Makefile
+++ b/drivers/gpu/drm/display/Makefile
@@ -5,6 +5,8 @@ obj-$(CONFIG_DRM_DISPLAY_DP_AUX_BUS) += drm_dp_aux_bus.o
drm_display_helper-y := drm_display_helper_mod.o
drm_display_helper-$(CONFIG_DRM_BRIDGE_CONNECTOR) += \
drm_bridge_connector.o
+drm_display_helper-$(CONFIG_DRM_HOTPLUG_HELPER) += \
+ drm_hotplug_helper.o
drm_display_helper-$(CONFIG_DRM_DISPLAY_DP_HELPER) += \
drm_dp_dual_mode_helper.o \
drm_dp_helper.o \
diff --git a/drivers/gpu/drm/display/drm_hotplug_helper.c b/drivers/gpu/drm/display/drm_hotplug_helper.c
new file mode 100644
index 000000000000..1b89b14b52aa
--- /dev/null
+++ b/drivers/gpu/drm/display/drm_hotplug_helper.c
@@ -0,0 +1,243 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2026 GE HealthCare
+ * Author: Luca Ceresoli <luca.ceresoli at bootlin.com>
+ */
+
+#include <drm/drm_bridge.h>
+#include <drm/drm_bridge_connector.h>
+#include <drm/drm_event_notifier.h>
+#include <drm/drm_hotplug_helper.h>
+#include <drm/drm_managed.h>
+#include <drm/drm_print.h>
+
+struct drm_hotplug_helper {
+ /**
+ * @drm: The DRM device we belong to
+ */
+ struct drm_device *drm;
+ /**
+ * @encoder:
+ *
+ * The encoder at the start of the bridges chain.
+ */
+ struct drm_encoder *encoder;
+ /**
+ * @drm_event_nb: notifier to receive DRM hotplug-related events
+ */
+ struct notifier_block drm_event_nb;
+ /**
+ * @connector: the drm_connector added/removed on plug/unplug
+ */
+ struct drm_connector *connector;
+ /**
+ * @connector_mutex: Protect @connector from concurrent creation and
+ * destruction
+ */
+ struct mutex connector_mutex;
+};
+
+static bool drm_hotplug_helper_pipeline_is_complete(struct drm_hotplug_helper *hotplug_helper)
+{
+ struct drm_bridge *last_bridge __free(drm_bridge_put) =
+ drm_bridge_chain_get_last_bridge(hotplug_helper->encoder);
+
+ /* We expect at least one bridge */
+ if (!last_bridge) {
+ drm_dbg_driver(hotplug_helper->drm, "no bridges in pipeline (yet)\n");
+ return false;
+ }
+
+ struct drm_bridge *next_bridge __free(drm_bridge_put) =
+ drm_bridge_get_next(last_bridge);
+
+ /* No next bridge expected, pipeline is complete */
+ if (!next_bridge) {
+ drm_dbg_driver(hotplug_helper->drm, "pipeline complete\n");
+ return true;
+ }
+
+ /* Next bridge expected but not there now, pipeline incomplete */
+ if (next_bridge == ERR_PTR(-ENODEV)) {
+ drm_dbg_driver(hotplug_helper->drm, "pipeline not (yet) complete\n");
+ return false;
+ }
+
+ /* Unexpected error */
+ if (IS_ERR(next_bridge))
+ drm_warn(hotplug_helper->drm, "%s error %pe\n", __func__, next_bridge);
+
+ /* next_bridge is valid, but not (yet|anymore) in chain */
+ return false;
+}
+
+/**
+ * drm_hotplug_helper_connector_add - add the drm_connector
+ * @hotplug_helper: drm_hotplug_helper to add the drm_connector to
+ *
+ * Returns 0 on success or a negative error otherwise.
+ */
+static int drm_hotplug_helper_connector_add(struct drm_hotplug_helper *hotplug_helper)
+{
+ struct drm_connector *connector;
+
+ guard(mutex)(&hotplug_helper->connector_mutex);
+
+ if (drm_WARN_ON(hotplug_helper->drm, hotplug_helper->connector))
+ return -EBUSY;
+
+ connector = drm_bridge_connector_dynamic_add(hotplug_helper->drm,
+ hotplug_helper->encoder);
+ if (IS_ERR(connector))
+ return PTR_ERR(connector);
+
+ hotplug_helper->connector = connector;
+
+ return 0;
+}
+
+static void drm_hotplug_helper_connector_remove(struct drm_hotplug_helper *hotplug_helper)
+{
+ guard(mutex)(&hotplug_helper->connector_mutex);
+
+ if (drm_WARN_ON(hotplug_helper->drm, !hotplug_helper->connector))
+ return;
+
+ drm_bridge_connector_dynamic_remove(hotplug_helper->connector);
+ hotplug_helper->connector = NULL;
+}
+
+/*
+ * Propagate the attach chain and possibly add a drm_bridge_connector after
+ * a new drm_bridge is hot-plugged.
+ *
+ * The connector is added only if the pipeline is now complete. This could
+ * not be the case for various reasons:
+ *
+ * - the new bridge is just unrelated to our encoder
+ * - the new bridge is not be the next one in the pipeline
+ * - the new bridge is the next in the pipeline but the pipeline is not yet
+ * complete
+ *
+ * All these cases are normal, not an error.
+ */
+static void drm_hotplug_helper_try_complete(struct drm_hotplug_helper *hotplug_helper)
+{
+ int err;
+
+ /*
+ * drm_connector already present, the new bridge must be for
+ * another card
+ */
+ if (hotplug_helper->connector)
+ return;
+
+ /* Propagate the attach call chain to newly hotplugged bridge(s) */
+ struct drm_bridge *last_bridge __free(drm_bridge_put) =
+ drm_bridge_chain_get_last_bridge(hotplug_helper->encoder);
+ err = last_bridge->funcs->attach(last_bridge, hotplug_helper->encoder,
+ DRM_BRIDGE_ATTACH_NO_CONNECTOR);
+ if (err)
+ return;
+
+ /* Add the connector if the pipeline is now complete */
+ if (drm_hotplug_helper_pipeline_is_complete(hotplug_helper))
+ drm_hotplug_helper_connector_add(hotplug_helper);
+}
+
+static int drm_hotplug_helper_handle_event(struct notifier_block *nb,
+ unsigned long event, void *data)
+{
+ struct drm_hotplug_helper *hotplug_helper =
+ container_of(nb, struct drm_hotplug_helper, drm_event_nb);
+
+ switch (event) {
+ case DRM_MIPI_DSI_ATTACHED:
+ /* One or more bridges hot-plugged, try adding the drm_bridge_connector */
+ drm_hotplug_helper_try_complete(hotplug_helper);
+ break;
+ case DRM_BRIDGE_DETACHED:
+ {
+ /*
+ * A bridge was unplugged, remove the drm_bridge_connector
+ * if it's part of the same pipeline
+ */
+ struct drm_bridge *bridge = (struct drm_bridge *)data;
+
+ if (hotplug_helper->connector &&
+ bridge->encoder == hotplug_helper->encoder)
+ drm_hotplug_helper_connector_remove(hotplug_helper);
+ break;
+ }
+ default:
+ }
+
+ return NOTIFY_DONE;
+}
+
+static void drm_hotplug_helper_fini(struct drm_device *dev, void *res)
+{
+ struct drm_hotplug_helper *hotplug_helper = (struct drm_hotplug_helper *)res;
+
+ drm_hotplug_helper_connector_remove(hotplug_helper);
+}
+
+static void drm_hotplug_helper_notifier_unregister(struct drm_device *dev, void *res)
+{
+ struct notifier_block *nb = (struct notifier_block *)res;
+
+ drm_event_notifier_unregister(nb);
+}
+
+/**
+ * drmm_hotplug_helper_init - Initialise the hotplug helper for an encoder
+ * @drm: the DRM device
+ * @encoder: the encoder where the bridge chain starts
+ *
+ * Register to receive hotplug-related events and react to them:
+ * - when a new bridge appears, check if the pipeline is now complete in
+ * the hardware, and if it is add a drm_bridge_connector which will add a
+ * drm_connector
+ * - when a bridge dispears, remove the drm_bridge_connector which will
+ * remove the drm_connector
+ *
+ * Returns a pointer to the new &drm_hotplug_helper on success, or a
+ * negative error pointer otherwise.
+ */
+struct drm_hotplug_helper *drmm_hotplug_helper_init(struct drm_device *drm,
+ struct drm_encoder *encoder)
+{
+ struct drm_hotplug_helper *hotplug_helper;
+ int ret;
+
+ hotplug_helper = drmm_kzalloc(drm, sizeof(*hotplug_helper), GFP_KERNEL);
+ if (!hotplug_helper)
+ return ERR_PTR(-ENOMEM);
+
+ mutex_init(&hotplug_helper->connector_mutex);
+ hotplug_helper->drm = drm;
+ hotplug_helper->encoder = encoder;
+ hotplug_helper->drm_event_nb.notifier_call = drm_hotplug_helper_handle_event;
+
+ if (drm_hotplug_helper_pipeline_is_complete(hotplug_helper)) {
+ ret = drm_hotplug_helper_connector_add(hotplug_helper);
+ if (ret)
+ return ERR_PTR(ret);
+ }
+
+ ret = drmm_add_action_or_reset(drm, drm_hotplug_helper_fini, hotplug_helper);
+ if (ret)
+ return ERR_PTR(ret);
+
+ ret = drm_event_notifier_register(&hotplug_helper->drm_event_nb);
+ if (ret)
+ return ERR_PTR(ret);
+
+ ret = drmm_add_action_or_reset(drm, drm_hotplug_helper_notifier_unregister,
+ &hotplug_helper->drm_event_nb);
+ if (ret)
+ return ERR_PTR(ret);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(drmm_hotplug_helper_init);
diff --git a/include/drm/drm_hotplug_helper.h b/include/drm/drm_hotplug_helper.h
new file mode 100644
index 000000000000..26779a0b6554
--- /dev/null
+++ b/include/drm/drm_hotplug_helper.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2026 GE HealthCare
+ * Author: Luca Ceresoli <luca.ceresoli at bootlin.com>
+ */
+
+#ifndef __DRM_HOTPLUG_HELPER_H__
+#define __DRM_HOTPLUG_HELPER_H__
+
+struct drm_hotplug_helper *drmm_hotplug_helper_init(struct drm_device *drm,
+ struct drm_encoder *encoder);
+
+#endif /* __DRM_HOTPLUG_HELPER_H__ */
--
2.55.0
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