linux-kernel - Re: [PATCH v1] clk: Convert managed get functions to devm_add

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Date:   Thu, 12 Dec 2019 14:53:40 +0100
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Robin Murphy <robin.murphy@....com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v1] clk: Convert managed get functions to devm_add_action
 API

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);