linux-kernel - Re: [PATCH] drivers: Remove unused devm_*

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Date:	Fri, 3 Jan 2014 20:07:22 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Rashika Kheria <rashika.kheria@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kishon Vijay Abraham I <kishon@...com>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
	josh@...htriplett.org
Subject: Re: [PATCH] drivers: Remove unused devm_*_put functions

On Thu, Jan 2, 2014 at 10:07 PM, Linus Walleij <linus.walleij@...aro.org> wrote:
> On Sat, Dec 21, 2013 at 11:49 AM, Rashika Kheria
> <rashika.kheria@...il.com> wrote:
>> -/**
>> - * devm_gpiod_put - Resource-managed gpiod_put()
>> - * @desc:      GPIO descriptor to dispose of
>> - *
>> - * Dispose of a GPIO descriptor obtained with devm_gpiod_get() or
>> - * devm_gpiod_get_index(). Normally this function will not be called as the GPIO
>> - * will be disposed of by the resource management code.
>> - */
>> -void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
>> -{
>> -       WARN_ON(devres_release(dev, devm_gpiod_release, devm_gpiod_match,
>> -               &desc));
>> -}
>> -EXPORT_SYMBOL(devm_gpiod_put);
>
> Alexandre what do you think about this? Can we think of a scenario
> where explicit garbage collection is going to be needed or should we
> remove this for now?

Sorry for the delayed reply, I quickly saw the patch and then the
holidays got in the way. :)

I think all these functions should be kept. It is true that they are
seldomly used, and that the purpose of devm is to garbage-collect
resources upon driver removal, but they might (actually, probably
will) become needed by someone at some point in the future. One
example I can think of is two drivers that collaborate to share the
same GPIO line. If they acquire the GPIO through devm_gpiod_get() they
will need devm_gpiod_put() to release it so the other driver can
acquire it.

On a more general note, devm_clk_put(), devm_regulator_put(),
devm_pinctrl_put() and probably others devm_*_put() functions are
actively used in the kernel, to  support the idea that a devm removal
function makes sense. That not all the subsystem-provided functions
are used by mainline drivers does not necessary mean they should be
removed, especially if they serve a purpose. We should keep our APIs
consistent and future-proof, not to mention out-of-tree drivers that
may use them.

So this patch is a nack as far as I'm concerned, not only the GPIO
part, but the whole of it.
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