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 d031447eebc955efcf1e018d39c015b62b969eae..3ebf6cc820e058a67f712763c341a75c671c82d1 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 76e05930f50e00f6ef5999b3f5a476215951028d..6b325de9e41ba7ee3649eaa60dfe105d6155f824 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -1143,6 +1143,14 @@ 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 */
@@ -1278,6 +1286,10 @@ 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);
 

-- 
2.51.0

Hi Luca,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 7acbe30813f04cccf7b2e8b571eb7936cfec0a87]

url:    https://github.com/intel-lab-lkp/linux/commits/Luca-Ceresoli/drm-bridge-add-drm_bridge_unplug-and-drm_bridge_enter-exit/20250927-003721
base:   7acbe30813f04cccf7b2e8b571eb7936cfec0a87
patch link:    https://lore.kernel.org/r/20250926-drm-bridge-atomic-vs-remove-v2-1-69f7d5ca1a92%40bootlin.com
patch subject: [PATCH v2 1/2] drm/bridge: add drm_bridge_unplug() and drm_bridge_enter/exit()
config: x86_64-buildonly-randconfig-002-20250927 (https://download.01.org/0day-ci/archive/20250927/202509271654.j3IsjsAJ-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250927/202509271654.j3IsjsAJ-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202509271654.j3IsjsAJ-lkp@intel.com/

All warnings (new ones prefixed by >>):

>> Warning: drivers/gpu/drm/drm_bridge.c:218 function parameter 'bridge' not described in 'drm_bridge_enter'
>> Warning: drivers/gpu/drm/drm_bridge.c:253 function parameter 'bridge' not described in 'drm_bridge_unplug'

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki