[RFC PATCH] devres: better type safety with devm_*_action()

[Rasmus Villemoes]

With a little MacroMagic(tm), we can allow users to pass a pointer to
a function that actually takes the type of the data argument, instead
of forcing the function to have prototype void (*)(void *). Of course,
we must still accept such functions.

This can provide a little more type safety in that we get fewer
implicit casts to and from void* - as a random example,
gpio_mockup_dispose_mappings in drivers/gpio/gpio-mockup.c could take
a "struct gpio_mockup_chip *chip" directly.

Moreover, when the action is some "external" API, there will in many
cases no longer be a need for a trivial local wrapper -
e.g. drivers/watchdog/cadence_wdt.c could just use
clk_disable_unprepare directly and avoid defining
cdns_clk_disable_unprepare.

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 drivers/base/devres.c  | 32 +++++++++++++++++++++++---------
 include/linux/device.h | 36 ++++++++++++++++++++++++++++++++----
 2 files changed, 55 insertions(+), 13 deletions(-)

diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index fb9d5289a620..97ebd26bc44a 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c
@@ -728,15 +728,25 @@ static void devm_action_release(struct device *dev, void *res)
 }
 
 /**
- * devm_add_action() - add a custom action to list of managed resources
+ * __devm_add_action() - add a custom action to list of managed resources
  * @dev: Device that owns the action
  * @action: Function that should be called
  * @data: Pointer to data passed to @action implementation
  *
  * This adds a custom action to the list of managed resources so that
  * it gets executed as part of standard resource unwinding.
+ *
+ * Do not call directly, but use the the macro wrapper
+ * devm_add_action, whose "prototype" is
+ *
+ * devm_add_action(struct device *dev, void (*action)(T *), T *data)
+ *
+ * This allows use of type-correct callbacks and can avoid wrapping
+ * external APIs. For example, if the data item is a "struct clk *", one
+ * can use clk_disable_unprepare directly as the action instead of
+ * creating a local wrapper taking a "void *" argument.
  */
-int devm_add_action(struct device *dev, void (*action)(void *), void *data)
+int __devm_add_action(struct device *dev, void (*action)(void *), void *data)
 {
 	struct action_devres *devres;
 
@@ -751,18 +761,20 @@ int devm_add_action(struct device *dev, void (*action)(void *), void *data)
 	devres_add(dev, devres);
 	return 0;
 }
-EXPORT_SYMBOL_GPL(devm_add_action);
+EXPORT_SYMBOL_GPL(__devm_add_action);
 
 /**
- * devm_remove_action() - removes previously added custom action
+ * __devm_remove_action() - removes previously added custom action
  * @dev: Device that owns the action
  * @action: Function implementing the action
  * @data: Pointer to data passed to @action implementation
  *
  * Removes instance of @action previously added by devm_add_action().
  * Both action and data should match one of the existing entries.
+ *
+ * Use the macro wrapper devm_remove_action, see __devm_add_action for details.
  */
-void devm_remove_action(struct device *dev, void (*action)(void *), void *data)
+void __devm_remove_action(struct device *dev, void (*action)(void *), void *data)
 {
 	struct action_devres devres = {
 		.data = data,
@@ -772,10 +784,10 @@ void devm_remove_action(struct device *dev, void (*action)(void *), void *data)
 	WARN_ON(devres_destroy(dev, devm_action_release, devm_action_match,
 			       &devres));
 }
-EXPORT_SYMBOL_GPL(devm_remove_action);
+EXPORT_SYMBOL_GPL(__devm_remove_action);
 
 /**
- * devm_release_action() - release previously added custom action
+ * __devm_release_action() - release previously added custom action
  * @dev: Device that owns the action
  * @action: Function implementing the action
  * @data: Pointer to data passed to @action implementation
@@ -783,8 +795,10 @@ EXPORT_SYMBOL_GPL(devm_remove_action);
  * Releases and removes instance of @action previously added by
  * devm_add_action().  Both action and data should match one of the
  * existing entries.
+ *
+ * Use the macro wrapper devm_release_action, see __devm_add_action for details.
  */
-void devm_release_action(struct device *dev, void (*action)(void *), void *data)
+void __devm_release_action(struct device *dev, void (*action)(void *), void *data)
 {
 	struct action_devres devres = {
 		.data = data,
@@ -795,7 +809,7 @@ void devm_release_action(struct device *dev, void (*action)(void *), void *data)
 			       &devres));
 
 }
-EXPORT_SYMBOL_GPL(devm_release_action);
+EXPORT_SYMBOL_GPL(__devm_release_action);
 
 /*
  * Managed kmalloc/kfree
diff --git a/include/linux/device.h b/include/linux/device.h
index ba660731bd25..c924612bfefd 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -250,11 +250,39 @@ void __iomem *devm_of_iomap(struct device *dev,
 			    resource_size_t *size);
 
 /* allows to add/remove a custom action to devres stack */
-int devm_add_action(struct device *dev, void (*action)(void *), void *data);
-void devm_remove_action(struct device *dev, void (*action)(void *), void *data);
-void devm_release_action(struct device *dev, void (*action)(void *), void *data);
 
-static inline int devm_add_action_or_reset(struct device *dev,
+/*
+ * +0 forces the expression to decay to a function pointer. We cannot
+ * just put an & in front as callers may already include that.
+ */
+#define devm_action_typecheck(action, data)				\
+	static_assert(__same_type(action + 0, void (*)(void *)) ||	\
+		      __same_type(action + 0, void (*)(typeof(data))))
+
+#define devm_add_action(dev, action, data) ({				\
+	devm_action_typecheck(action, data);				\
+	__devm_add_action(dev, (void (*)(void *))action, data);		\
+})
+#define devm_remove_action(dev, action, data) ({			\
+	devm_action_typecheck(action, data);				\
+	__devm_remove_action(dev, (void (*)(void *))action, data);	\
+})
+#define devm_release_action(dev, action, data) ({			\
+	devm_action_typecheck(action, data);				\
+	__devm_release_action(dev, (void (*)(void *))action, data);	\
+})
+
+
+int __devm_add_action(struct device *dev, void (*action)(void *), void *data);
+void __devm_remove_action(struct device *dev, void (*action)(void *), void *data);
+void __devm_release_action(struct device *dev, void (*action)(void *), void *data);
+
+#define devm_add_action_or_reset(dev, action, data) ({			\
+	devm_action_typecheck(action, data);				\
+	__devm_add_action_or_reset(dev, (void (*)(void *))action, data); \
+})
+
+static inline int __devm_add_action_or_reset(struct device *dev,
 					   void (*action)(void *), void *data)
 {
 	int ret;
-- 
2.29.2