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

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Date:   Thu, 12 Dec 2019 14:46:16 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Marc Gonzalez <marc.w.gonzalez@...e.fr>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        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>,
        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 Thu, Dec 12, 2019 at 03:41:20PM +0100, Marc Gonzalez wrote:
> On 12/12/2019 15:17, Russell King - ARM Linux admin wrote:
> 
> > On Thu, Dec 12, 2019 at 02:53:40PM +0100, Marc Gonzalez wrote:
> >
> >> 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);
> > 
> > You leak clk here.
> 
> I don't think so ;-)
> 
> If devm_add() returns NULL, then we have called __clk_put(dev, &clk);

Okay.

However, please don't call this __clk_put().  git grep __clk_put will
tell you why.  Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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