[PATCH 1/2] drm/bridge: add drm_bridge_unplug() and drm_bridge_enter/exit()

Luca Ceresoli
2025-08-08T13:53:00Z

To allow DRM bridges to be removable, add synchronization functions
allowing to tell when a bridge hardware has been physically unplugged and
to mark a critical section that should not be entered after that.

This is inspired by the drm_dev_unplugged/enter/exit() functions for struct
drm_device.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20250106-vigorous-talented-viper-fa49d9@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
drivers/gpu/drm/drm_bridge.c | 58 ++++++++++++++++++++++++++++++++++++++++++++
include/drm/drm_bridge.h | 12 +++++++++
2 files changed, 70 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index c3bfcd735a3c426a147bf0a7427b3d2cd0df3524..486bc48881c6a05ebe3c911754a40530f0d08e3e 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -27,6 +27,7 @@
#include <linux/media-bus-format.h>
#include <linux/module.h>
#include <linux/mutex.h>
+#include <linux/srcu.h>

#include <drm/drm_atomic_state_helper.h>
#include <drm/drm_bridge.h>
@@ -200,6 +201,63 @@
static DEFINE_MUTEX(bridge_lock);
static LIST_HEAD(bridge_list);

+DEFINE_STATIC_SRCU(drm_bridge_unplug_srcu);
+
+/**
+ * drm_bridge_enter - Enter DRM bridge critical section
+ * @dev: DRM bridge
+ * @idx: Pointer to index that will be passed to the matching drm_bridge_exit()
+ *
+ * This function marks and protects the beginning of a section that should not
+ * be entered after the bridge has been unplugged. The section end is marked
+ * with drm_bridge_exit(). Calls to this function can be nested.
+ *
+ * Returns:
+ * True if it is OK to enter the section, false otherwise.
+ */
+bool drm_bridge_enter(struct drm_bridge *bridge, int *idx)
+{
+ *idx = srcu_read_lock(&drm_bridge_unplug_srcu);
+
+ if (bridge->unplugged) {
+ srcu_read_unlock(&drm_bridge_unplug_srcu, *idx);
+ return false;
+ }
+
+ return true;
+}
+EXPORT_SYMBOL(drm_bridge_enter);
+
+/**
+ * drm_bridge_exit - Exit DRM bridge critical section
+ * @idx: index returned by drm_bridge_enter()
+ *
+ * This function marks the end of a section that should not be entered after
+ * the bridge has been unplugged.
+ */
+void drm_bridge_exit(int idx)
+{
+ srcu_read_unlock(&drm_bridge_unplug_srcu, idx);
+}
+EXPORT_SYMBOL(drm_bridge_exit);
+
+/**
+ * drm_bridge_unplug - unplug a DRM bridge
+ * @dev: DRM bridge
+ *
+ * This tells the bridge has been physically unplugged and no operations on
+ * device resources must be done anymore. Entry-points can use
+ * drm_bridge_enter() and drm_bridge_exit() to protect device resources in
+ * a race free manner.
+ */
+void drm_bridge_unplug(struct drm_bridge *bridge)
+{
+ bridge->unplugged = true;
+
+ synchronize_srcu(&drm_bridge_unplug_srcu);
+}
+EXPORT_SYMBOL(drm_bridge_unplug);
+
static void __drm_bridge_free(struct kref *kref)
{
struct drm_bridge *bridge = container_of(kref, struct drm_bridge, refcount);
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 620e119cc24c3491c2be5f08efaf51dfa8f708b3..0c6869ebb2be9b89499ee5c42692fd94ca5b3161 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -1095,6 +1095,14 @@ struct drm_bridge {
*/
struct kref refcount;

+ /**
+ * @unplugged:
+ *
+ * Flag to tell if the bridge has been unplugged.
+ * See drm_bridge_enter() and drm_bridge_unplug().
+ */
+ bool unplugged;
+
/** @driver_private: pointer to the bridge driver's internal context */
void *driver_private;
/** @ops: bitmask of operations supported by the bridge */
@@ -1226,6 +1234,10 @@ drm_priv_to_bridge(struct drm_private_obj *priv)
return container_of(priv, struct drm_bridge, base);
}

+bool drm_bridge_enter(struct drm_bridge *bridge, int *idx);
+void drm_bridge_exit(int idx);
+void drm_bridge_unplug(struct drm_bridge *bridge);
+
struct drm_bridge *drm_bridge_get(struct drm_bridge *bridge);
void drm_bridge_put(struct drm_bridge *bridge);

--
2.50.1