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

[Luca Ceresoli]

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 at kernel.org>
Link: https://lore.kernel.org/all/20250106-vigorous-talented-viper-fa49d9@houat/
Signed-off-by: Luca Ceresoli <luca.ceresoli at 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