[PATCH RFC 1/3] drm/drm_bridge: adjust bridge module's refcount

[Jyri Sarha <jsarha@xxxxxx>]

Store the module that provides the bridge and adjust its reference
count accordingly. The bridge module unload should not be allowed
while the bridge is attached. To do this safely we need to add
reference counting to drm_bridge objects. This has impact to both
bridge drivers and the drivers using the bridges.

Signed-off-by: Jyri Sarha <jsarha@xxxxxx>
---
 drivers/gpu/drm/drm_bridge.c | 78 +++++++++++++++++++++++++++++++++++++++-----
 include/drm/drm_bridge.h     | 10 +++++-
 2 files changed, 78 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 0ee052b..4b0ff45 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -24,6 +24,7 @@
 #include <linux/err.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/slab.h>
 
 #include <drm/drm_bridge.h>
 
@@ -60,6 +61,39 @@
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);
 
+static void drm_bridge_release(struct kref *ref)
+{
+	struct drm_bridge *bridge = container_of(ref, struct drm_bridge, kref);
+
+	kfree(bridge);
+}
+
+struct drm_bridge *drm_bridge_alloc(struct module *module)
+{
+	struct drm_bridge *bridge = kzalloc(sizeof(*bridge), GFP_KERNEL);
+
+	if (!bridge)
+		return NULL;
+
+	bridge->module = module;
+	kref_init(&bridge->kref);
+
+	return bridge;
+}
+EXPORT_SYMBOL(drm_bridge_alloc);
+
+void drm_bridge_get(struct drm_bridge *bridge)
+{
+	kref_get(&bridge->kref);
+}
+EXPORT_SYMBOL(drm_bridge_get);
+
+void drm_bridge_put(struct drm_bridge *bridge)
+{
+	kref_put(&bridge->kref, drm_bridge_release);
+}
+EXPORT_SYMBOL(drm_bridge_put);
+
 /**
  * drm_bridge_add - add the given bridge to the global bridge list
  *
@@ -70,6 +104,8 @@
  */
 int drm_bridge_add(struct drm_bridge *bridge)
 {
+	drm_bridge_get(bridge);
+
 	mutex_lock(&bridge_lock);
 	list_add_tail(&bridge->list, &bridge_list);
 	mutex_unlock(&bridge_lock);
@@ -88,6 +124,8 @@ void drm_bridge_remove(struct drm_bridge *bridge)
 	mutex_lock(&bridge_lock);
 	list_del_init(&bridge->list);
 	mutex_unlock(&bridge_lock);
+
+	drm_bridge_put(bridge);
 }
 EXPORT_SYMBOL(drm_bridge_remove);
 
@@ -108,18 +146,35 @@ void drm_bridge_remove(struct drm_bridge *bridge)
  */
 int drm_bridge_attach(struct drm_device *dev, struct drm_bridge *bridge)
 {
+	int ret = 0;
+
 	if (!dev || !bridge)
 		return -EINVAL;
 
+	mutex_lock(&bridge_lock);
+
 	if (bridge->dev)
-		return -EBUSY;
+		ret = -EBUSY;
+	/* Check that the bridge is still part of the list */
+	else if (list_empty(&bridge->list))
+		ret = -ENOENT;
+	else if (!try_module_get(bridge->module))
+		ret = -ENOENT;
+	else
+		bridge->dev = dev;
 
-	bridge->dev = dev;
+	mutex_unlock(&bridge_lock);
 
-	if (bridge->funcs->attach)
-		return bridge->funcs->attach(bridge);
+	if (ret)
+		return ret;
 
-	return 0;
+	if (bridge->funcs->attach) {
+		ret = bridge->funcs->attach(bridge);
+		if (ret)
+			module_put(bridge->module);
+	}
+
+	return ret;
 }
 EXPORT_SYMBOL(drm_bridge_attach);
 
@@ -144,6 +199,8 @@ void drm_bridge_detach(struct drm_bridge *bridge)
 	if (bridge->funcs->detach)
 		bridge->funcs->detach(bridge);
 
+	module_put(bridge->module);
+
 	bridge->dev = NULL;
 }
 EXPORT_SYMBOL(drm_bridge_detach);
@@ -312,15 +369,17 @@ void drm_bridge_enable(struct drm_bridge *bridge)
 
 #ifdef CONFIG_OF
 /**
- * of_drm_find_bridge - find the bridge corresponding to the device node in
- *			the global bridge list
+ * of_drm_get_bridge - find the bridge corresponding to the device node in
+ *		       the global bridge list and increase its refcount.
+ *                     It is caller responsibility to call drm_bridge_put()
+ *                     when it no longer needs the object.
  *
  * @np: device node
  *
  * RETURNS:
  * drm_bridge control struct on success, NULL on failure
  */
-struct drm_bridge *of_drm_find_bridge(struct device_node *np)
+struct drm_bridge *of_drm_get_bridge(struct device_node *np)
 {
 	struct drm_bridge *bridge;
 
@@ -328,6 +387,7 @@ struct drm_bridge *of_drm_find_bridge(struct device_node *np)
 
 	list_for_each_entry(bridge, &bridge_list, list) {
 		if (bridge->of_node == np) {
+			drm_bridge_get(bridge);
 			mutex_unlock(&bridge_lock);
 			return bridge;
 		}
@@ -336,7 +396,7 @@ struct drm_bridge *of_drm_find_bridge(struct device_node *np)
 	mutex_unlock(&bridge_lock);
 	return NULL;
 }
-EXPORT_SYMBOL(of_drm_find_bridge);
+EXPORT_SYMBOL(of_drm_get_bridge);
 #endif
 
 MODULE_AUTHOR("Ajay Kumar <ajaykumar.rs@xxxxxxxxxxx>");
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 530a1d6..cd39b7c 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -25,6 +25,7 @@
 
 #include <linux/list.h>
 #include <linux/ctype.h>
+#include <linux/module.h>
 #include <drm/drm_mode_object.h>
 #include <drm/drm_modes.h>
 
@@ -192,15 +193,22 @@ struct drm_bridge {
 #ifdef CONFIG_OF
 	struct device_node *of_node;
 #endif
+	struct kref kref;
+	struct module *module;
+
 	struct list_head list;
 
 	const struct drm_bridge_funcs *funcs;
 	void *driver_private;
 };
 
+struct drm_bridge *drm_bridge_alloc(struct module *module);
+void drm_bridge_get(struct drm_bridge *bridge);
+void drm_bridge_put(struct drm_bridge *bridge);
+
 int drm_bridge_add(struct drm_bridge *bridge);
 void drm_bridge_remove(struct drm_bridge *bridge);
-struct drm_bridge *of_drm_find_bridge(struct device_node *np);
+struct drm_bridge *of_drm_get_bridge(struct device_node *np);
 int drm_bridge_attach(struct drm_device *dev, struct drm_bridge *bridge);
 void drm_bridge_detach(struct drm_bridge *bridge);
 
-- 
1.9.1

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