[PATCH v2 1/7] drm/encoder: add mutex to protect the bridge chain

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

The per-encoder bridge chain is currently assumed to be static once it is
fully initialized. Work is in progress to add hot-pluggable bridges,
breaking that assumption.

With bridge removal, the encoder chain can change without notice, removing
tail bridges. This can be problematic while iterating over the chain.

Add a mutex to be taken whenever looping or changing the encoder chain.

Also add two APIs to lock/unlock the mutex without the need to manipulate
internal struct drm_encoder fields.

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

---

Changes in v2:
- Added documentation to new APIs
---
 drivers/gpu/drm/drm_encoder.c |  2 ++
 include/drm/drm_encoder.h     | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/drivers/gpu/drm/drm_encoder.c b/drivers/gpu/drm/drm_encoder.c
index 8f2bc6a28482229fd0b030a1958f87753ad7885f..3261f142baea30c516499d23dbf8d0acf5952cd6 100644
--- a/drivers/gpu/drm/drm_encoder.c
+++ b/drivers/gpu/drm/drm_encoder.c
@@ -129,6 +129,7 @@ static int __drm_encoder_init(struct drm_device *dev,
 	}
 
 	INIT_LIST_HEAD(&encoder->bridge_chain);
+	mutex_init(&encoder->bridge_chain_mutex);
 	list_add_tail(&encoder->head, &dev->mode_config.encoder_list);
 	encoder->index = dev->mode_config.num_encoder++;
 
@@ -202,6 +203,7 @@ void drm_encoder_cleanup(struct drm_encoder *encoder)
 	kfree(encoder->name);
 	list_del(&encoder->head);
 	dev->mode_config.num_encoder--;
+	mutex_destroy(&encoder->bridge_chain_mutex);
 
 	memset(encoder, 0, sizeof(*encoder));
 }
diff --git a/include/drm/drm_encoder.h b/include/drm/drm_encoder.h
index 977a9381c8ba943b4d3e021635ea14856df8a17d..449281c37e39f67a0037603762f347f5086df983 100644
--- a/include/drm/drm_encoder.h
+++ b/include/drm/drm_encoder.h
@@ -25,6 +25,7 @@
 
 #include <linux/list.h>
 #include <linux/ctype.h>
+#include <linux/mutex.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_mode.h>
 #include <drm/drm_mode_object.h>
@@ -189,6 +190,9 @@ struct drm_encoder {
 	 */
 	struct list_head bridge_chain;
 
+	/** @bridge_chain_mutex: protect bridge_chain from changes while iterating */
+	struct mutex bridge_chain_mutex;
+
 	const struct drm_encoder_funcs *funcs;
 	const struct drm_encoder_helper_funcs *helper_private;
 
@@ -319,6 +323,41 @@ static inline struct drm_encoder *drm_encoder_find(struct drm_device *dev,
 	return mo ? obj_to_encoder(mo) : NULL;
 }
 
+/**
+ * drm_encoder_chain_lock - lock the encoder bridge chain
+ * @encoder: encoder whose bridge chain must be locked
+ *
+ * Locks the mutex protecting the bridge chain from concurrent access.
+ * To be called by code modifying ot iterating over the bridge chain to
+ * prevent the list from changing while iterating over it.
+ * Call drm_encoder_chain_unlock() when done to unlock the mutex.
+ *
+ * Returns:
+ * Pointer to @encoder. Useful to lock the chain and then operate on the
+ * in the same statement, e.g.:
+ * list_first_entry_or_null(&drm_encoder_chain_lock(encoder)->bridge_chain)
+ */
+static inline struct drm_encoder *drm_encoder_chain_lock(struct drm_encoder *encoder)
+{
+	if (!WARN_ON_ONCE(!encoder))
+		mutex_lock(&encoder->bridge_chain_mutex);
+
+	return encoder;
+}
+
+/**
+ * drm_encoder_chain_unlock - unlock the encoder bridge chain
+ * @encoder: encoder whose bridge chain must be unlocked
+ *
+ * Unlocks the mutex protecting the bridge chain from concurrent access,
+ * matching drm_encoder_chain_lock().
+ */
+static inline void drm_encoder_chain_unlock(struct drm_encoder *encoder)
+{
+	if (!WARN_ON_ONCE(!encoder))
+		mutex_unlock(&encoder->bridge_chain_mutex);
+}
+
 void drm_encoder_cleanup(struct drm_encoder *encoder);
 
 /**

-- 
2.51.0