[PATCH v7 02/11] drm/bridge: add support for refcounting

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

DRM bridges are currently considered as a fixed element of a DRM card, and
thus their lifetime is assumed to extend for as long as the card
exists. New use cases, such as hot-pluggable hardware with video bridges,
require DRM bridges to be added to and removed from a DRM card without
tearing the card down. This is possible for connectors already (used by DP
MST), it is now needed for DRM bridges as well.

As a first preliminary step, make bridges reference-counted to allow a
struct drm_bridge (along with the private driver structure embedding it) to
stay allocated even after the driver has been removed, until the last
reference is put.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

---

Changes in v7:
 - export drm_bridge_put_void
 - struct drm_bridge: use container pointer instead of container_offset
 - remove drm_bridge_is_refcounted()
 - remove all DRM_DEBUG()s
 - drm_bridge_get/put: accept NULL pointer and return the bridge pointer to
   allow pass-through calls
 - extract to separate patches:
    - the addition of drm_bridge_alloc
    - the addition of drm_bridge_get/put() to drm_bridge_add/remove()
    - the addition of drm_bridge_get/put() to drm_bridge_attach/detach()
 - fix a typo, slightly improve kerneldoc

Changes in v6:
 - use drm_warn, not WARN_ON (Jani Nikula)
 - Add devm_drm_bridge_alloc() to replace drm_bridge_init() (similar to
   drmm_encoder_alloc)
 - Remove .destroy func: deallocation is done via the struct offset
   computed by the devm_drm_bridge_alloc() macro
 - use fixed free callback, as the same callback is used in all cases
   anyway (remove free_cb, add bool is_refcounted)
 - add drm_bridge_get/put() to drm_bridge_attach/detach() (add the bridge
   to a list)
 - make some DRM_DEBUG() strings more informative

This patch was added in v5.
---
 drivers/gpu/drm/drm_bridge.c | 33 +++++++++++++++-
 include/drm/drm_bridge.h     | 91 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 123 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 96df717b2caeb41d45346ded576eaeb2806fd051..2ba0dac9bfc2dfd709d5e2457d69067c7324972c 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -199,23 +199,54 @@
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);
 
+void __drm_bridge_free(struct kref *kref)
+{
+	struct drm_bridge *bridge = container_of(kref, struct drm_bridge, refcount);
+
+	kfree(bridge->container);
+}
+EXPORT_SYMBOL(__drm_bridge_free);
+
+/**
+ * drm_bridge_put_void - wrapper to drm_bridge_put() taking a void pointer
+ *
+ * @data: pointer to @struct drm_bridge, cast to a void pointer
+ *
+ * Wrapper of drm_bridge_put() to be used when a function taking a void
+ * pointer is needed, for example as a devm action.
+ */
+void drm_bridge_put_void(void *data)
+{
+	struct drm_bridge *bridge = (struct drm_bridge *)data;
+
+	drm_bridge_put(bridge);
+}
+EXPORT_SYMBOL(drm_bridge_put_void);
+
 void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
 			      const struct drm_bridge_funcs *funcs)
 {
 	void *container;
 	struct drm_bridge *bridge;
+	int err;
 
 	if (!funcs) {
 		dev_warn(dev, "Missing funcs pointer\n");
 		return ERR_PTR(-EINVAL);
 	}
 
-	container = devm_kzalloc(dev, size, GFP_KERNEL);
+	container = kzalloc(size, GFP_KERNEL);
 	if (!container)
 		return ERR_PTR(-ENOMEM);
 
 	bridge = container + offset;
+	bridge->container = container;
 	bridge->funcs = funcs;
+	kref_init(&bridge->refcount);
+
+	err = devm_add_action_or_reset(dev, drm_bridge_put_void, bridge);
+	if (err)
+		return ERR_PTR(err);
 
 	return container;
 }
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index dae463b30542d586a595b67f7bdf5a5e898e9572..5c1e2b9cafb12eb429d1f5d3ef312e6cf9b54f47 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -840,6 +840,17 @@ struct drm_bridge {
 	const struct drm_bridge_timings *timings;
 	/** @funcs: control functions */
 	const struct drm_bridge_funcs *funcs;
+
+	/**
+	 * @container: Pointer to the private driver struct embedding this
+	 * @struct drm_bridge.
+	 */
+	void *container;
+	/**
+	 * @refcount: reference count of users referencing this bridge.
+	 */
+	struct kref refcount;
+
 	/** @driver_private: pointer to the bridge driver's internal context */
 	void *driver_private;
 	/** @ops: bitmask of operations supported by the bridge */
@@ -941,6 +952,82 @@ drm_priv_to_bridge(struct drm_private_obj *priv)
 	return container_of(priv, struct drm_bridge, base);
 }
 
+void __drm_bridge_free(struct kref *kref);
+
+/**
+ * drm_bridge_get - Acquire a bridge reference
+ * @bridge: DRM bridge
+ *
+ * This function increments the bridge's refcount.
+ *
+ * Returns:
+ * Pointer to @bridge.
+ */
+static inline struct drm_bridge *drm_bridge_get(struct drm_bridge *bridge)
+{
+	if (!bridge)
+		return bridge;
+
+	kref_get(&bridge->refcount);
+
+	return bridge;
+}
+
+/**
+ * drm_bridge_put - Release a bridge reference
+ * @bridge: DRM bridge
+ *
+ * This function decrements the bridge's reference count and frees the
+ * object if the reference count drops to zero.
+ *
+ * See also drm_bridge_put_and_clear() which is more handy in many cases.
+ *
+ * Returns:
+ * Pointer to @bridge.
+ */
+static inline struct drm_bridge *drm_bridge_put(struct drm_bridge *bridge)
+{
+	if (!bridge)
+		return bridge;
+
+	kref_put(&bridge->refcount, __drm_bridge_free);
+
+	return bridge;
+}
+
+void drm_bridge_put_void(void *data);
+
+/**
+ * drm_bridge_put_and_clear - Given a bridge pointer, clear the pointer
+ *                            then put the bridge
+ *
+ * @bridge_pp: pointer to pointer to a struct drm_bridge
+ *
+ * Helper to put a DRM bridge (whose pointer is passed), but only after
+ * setting its pointer to NULL. Useful for drivers having struct drm_bridge
+ * pointers they need to dispose of, without leaving a use-after-free
+ * window where the pointed bridge might have been freed while still
+ * holding a pointer to it.
+ *
+ * For example a driver having this private struct::
+ *
+ *     struct my_bridge {
+ *         struct drm_bridge *remote_bridge;
+ *         ...
+ *     };
+ *
+ * can dispose of remote_bridge using::
+ *
+ *     drm_bridge_put_and_clear(&my_bridge->remote_bridge);
+ */
+static inline void drm_bridge_put_and_clear(struct drm_bridge **bridge_pp)
+{
+	struct drm_bridge *bridge = *bridge_pp;
+
+	*bridge_pp = NULL;
+	drm_bridge_put(bridge);
+}
+
 void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
 			      const struct drm_bridge_funcs *funcs);
 
@@ -951,6 +1038,10 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
  * @member: the name of the &drm_bridge within @type
  * @funcs: callbacks for this bridge
  *
+ * The returned refcount is initialized to 1. This reference will be
+ * automatically dropped via devm (by calling drm_bridge_put()) when @dev
+ * is removed.
+ *
  * Returns:
  * Pointer to new bridge, or ERR_PTR on failure.
  */

-- 
2.48.1