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SubjectRe: [PATCH v1] clk: Convert managed get functions to devm_add_action API
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On 11/12/2019 23:28, Dmitry Torokhov wrote:

> On Wed, Dec 11, 2019 at 05:17:28PM +0100, Marc Gonzalez wrote:
>
>> What is the rationale for the devm_add_action API?
>
> For one-off and maybe complex unwind actions in drivers that wish to use
> devm API (as mixing devm and manual release is verboten). Also is often
> used when some core subsystem does not provide enough devm APIs.

Thanks for the insight, Dmitry. Thanks to Robin too.

This is what I understand so far:

devm_add_action() is nice because it hides/factorizes the complexity
of the devres API, but it incurs a small storage overhead of one
pointer per call, which makes it unfit for frequently used actions,
such as clk_get.

Is that correct?

My question is: why not design the API without the small overhead?

Proof of concept below:


diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index 0bbb328bd17f..76392dd6273b 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c
@@ -685,6 +685,20 @@ int devres_release_group(struct device *dev, void *id)
}
EXPORT_SYMBOL_GPL(devres_release_group);

+void *devm_add(struct device *dev, dr_release_t func, void *arg, size_t size)
+{
+ void *data = devres_alloc(func, size, GFP_KERNEL);
+
+ if (data) {
+ memcpy(data, arg, size);
+ devres_add(dev, data);
+ } else
+ func(dev, arg);
+
+ return data;
+}
+EXPORT_SYMBOL_GPL(devm_add);
+
/*
* Custom devres actions allow inserting a simple function call
* into the teadown sequence.
diff --git a/drivers/clk/clk-devres.c b/drivers/clk/clk-devres.c
index be160764911b..8db671823126 100644
--- a/drivers/clk/clk-devres.c
+++ b/drivers/clk/clk-devres.c
@@ -4,6 +4,11 @@
#include <linux/export.h>
#include <linux/gfp.h>

+static void __clk_put(struct device *dev, void *data)
+{
+ clk_put(*(struct clk **)data);
+}
+
static void devm_clk_release(struct device *dev, void *res)
{
clk_put(*(struct clk **)res);
@@ -11,19 +16,11 @@ static void devm_clk_release(struct device *dev, void *res)

struct clk *devm_clk_get(struct device *dev, const char *id)
{
- struct clk **ptr, *clk;
-
- ptr = devres_alloc(devm_clk_release, sizeof(*ptr), GFP_KERNEL);
- if (!ptr)
- return ERR_PTR(-ENOMEM);
+ struct clk *clk = clk_get(dev, id);

- clk = clk_get(dev, id);
- if (!IS_ERR(clk)) {
- *ptr = clk;
- devres_add(dev, ptr);
- } else {
- devres_free(ptr);
- }
+ if (!IS_ERR(clk))
+ if (!devm_add(dev, __clk_put, &clk, sizeof(clk)))
+ clk = ERR_PTR(-ENOMEM);

return clk;
}
diff --git a/include/linux/device.h b/include/linux/device.h
index e226030c1df3..5acb61ec39ab 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -970,6 +970,7 @@ void __iomem *devm_of_iomap(struct device *dev,
resource_size_t *size);

/* allows to add/remove a custom action to devres stack */
+void *devm_add(struct device *dev, dr_release_t func, void *arg, size_t size);
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);
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