drm/privacy-screen: Add notifier support (v2)

Add support for privacy-screen consumers to register a notifier to
be notified of external (e.g. done by the hw itself on a hotkey press)
state changes.

Changes in v2:
- Drop WARN_ON(mutex_is_locked(&priv->lock)) check in
drm_privacy_screen_call_notifier_chain() it may be locked by
another thread, which would lead to a false-positive triggering
of the check

Reviewed-by: Emil Velikov <[e-mail]>
Reviewed-by: Lyude Paul <[e-mail]>
Signed-off-by: Hans de Goede <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_privacy_screen.c

···
 }
 EXPORT_SYMBOL(drm_privacy_screen_get_state);
 
+/**
+ * drm_privacy_screen_register_notifier - register a notifier
+ * @priv: Privacy screen to register the notifier with
+ * @nb: Notifier-block for the notifier to register
+ *
+ * Register a notifier with the privacy-screen to be notified of changes made
+ * to the privacy-screen state from outside of the privacy-screen class.
+ * E.g. the state may be changed by the hardware itself in response to a
+ * hotkey press.
+ *
+ * The notifier is called with no locks held. The new hw_state and sw_state
+ * can be retrieved using the drm_privacy_screen_get_state() function.
+ * A pointer to the drm_privacy_screen's struct is passed as the void *data
+ * argument of the notifier_block's notifier_call.
+ *
+ * The notifier will NOT be called when changes are made through
+ * drm_privacy_screen_set_sw_state(). It is only called for external changes.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int drm_privacy_screen_register_notifier(struct drm_privacy_screen *priv,
+					 struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&priv->notifier_head, nb);
+}
+EXPORT_SYMBOL(drm_privacy_screen_register_notifier);
+
+/**
+ * drm_privacy_screen_unregister_notifier - unregister a notifier
+ * @priv: Privacy screen to register the notifier with
+ * @nb: Notifier-block for the notifier to register
+ *
+ * Unregister a notifier registered with drm_privacy_screen_register_notifier().
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int drm_privacy_screen_unregister_notifier(struct drm_privacy_screen *priv,
+					   struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&priv->notifier_head, nb);
+}
+EXPORT_SYMBOL(drm_privacy_screen_unregister_notifier);
+
 /*** drm_privacy_screen_driver.h functions ***/
 
 static ssize_t sw_state_show(struct device *dev,
···
 		return ERR_PTR(-ENOMEM);
 
 	mutex_init(&priv->lock);
+	BLOCKING_INIT_NOTIFIER_HEAD(&priv->notifier_head);
 
 	priv->dev.class = drm_class;
 	priv->dev.type = &drm_privacy_screen_type;
···
 	device_unregister(&priv->dev);
 }
 EXPORT_SYMBOL(drm_privacy_screen_unregister);
+
+/**
+ * drm_privacy_screen_call_notifier_chain - notify consumers of state change
+ * @priv: Privacy screen to register the notifier with
+ *
+ * A privacy-screen provider driver can call this functions upon external
+ * changes to the privacy-screen state. E.g. the state may be changed by the
+ * hardware itself in response to a hotkey press.
+ * This function must be called without holding the privacy-screen lock.
+ * the driver must update sw_state and hw_state to reflect the new state before
+ * calling this function.
+ * The expected behavior from the driver upon receiving an external state
+ * change event is: 1. Take the lock; 2. Update sw_state and hw_state;
+ * 3. Release the lock. 4. Call drm_privacy_screen_call_notifier_chain().
+ */
+void drm_privacy_screen_call_notifier_chain(struct drm_privacy_screen *priv)
+{
+	blocking_notifier_call_chain(&priv->notifier_head, 0, priv);
+}
+EXPORT_SYMBOL(drm_privacy_screen_call_notifier_chain);

include/drm/drm_privacy_screen_consumer.h

···
 void drm_privacy_screen_get_state(struct drm_privacy_screen *priv,
 				  enum drm_privacy_screen_status *sw_state_ret,
 				  enum drm_privacy_screen_status *hw_state_ret);
+
+int drm_privacy_screen_register_notifier(struct drm_privacy_screen *priv,
+					 struct notifier_block *nb);
+int drm_privacy_screen_unregister_notifier(struct drm_privacy_screen *priv,
+					   struct notifier_block *nb);
 #else
 static inline struct drm_privacy_screen *drm_privacy_screen_get(struct device *dev,
 								const char *con_id)
···
 {
 	*sw_state_ret = PRIVACY_SCREEN_DISABLED;
 	*hw_state_ret = PRIVACY_SCREEN_DISABLED;
+}
+static inline int drm_privacy_screen_register_notifier(struct drm_privacy_screen *priv,
+						       struct notifier_block *nb)
+{
+	return -ENODEV;
+}
+static inline int drm_privacy_screen_unregister_notifier(struct drm_privacy_screen *priv,
+							 struct notifier_block *nb)
+{
+	return -ENODEV;
 }
 #endif
 

include/drm/drm_privacy_screen_driver.h

···
 	struct mutex lock;
 	/** @list: privacy-screen devices list list-entry. */
 	struct list_head list;
+	/** @notifier_head: privacy-screen notifier head. */
+	struct blocking_notifier_head notifier_head;
 	/**
 	 * @ops: &struct drm_privacy_screen_ops for this privacy-screen.
 	 * This is NULL if the driver has unregistered the privacy-screen.
···
 struct drm_privacy_screen *drm_privacy_screen_register(
 	struct device *parent, const struct drm_privacy_screen_ops *ops);
 void drm_privacy_screen_unregister(struct drm_privacy_screen *priv);
+
+void drm_privacy_screen_call_notifier_chain(struct drm_privacy_screen *priv);
 
 #endif