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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Thu, 12 Mar 2020 11:35:16 +0100
Message-ID: <CACRpkdZSooH+mXbimgT-hnaC2gO1nTi+rY7UmUhVg9bk1j+Eow@mail.gmail.com>
Subject: Re: [PATCH 2/3] gpiolib: use kref in gpio_desc
To:     Bartosz Golaszewski <brgl@bgdev.pl>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
        Khouloud Touil <ktouil@baylibre.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>
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Hi Bartosz,

I'm struggling to figure out if this is the right way to count
references for gpio descriptors.

I cleared up the situation of why we don't want to add kref
to gpio_chip in the previous message: I think we got that covered.
(If I'm not wrong about it, and I am frequently wrong.)

This mail is about contrasting the suggested gpio_desc
kref with the existing managed resources, i.e. the
devm_* mechanisms.

devm_* macros are elusive because they do not use
reference counting at all.

Instead they put every devm_* requested resource with
a destruction function on a list associated with the struct
device. Functions get put on that list when we probe a
device driver, and the list is iterated and all release functions
are called when we exit .probe() with error or after calling the
optional .remove() function on the module. (More or less.)

This means anything devm_* managed lives and dies
with the device driver attaching to the device.
Documentation/driver-api/driver-model/devres.rst

If the intention of the patch is that this action is associated
with the detachment of the driver, then we are reinventing
the wheel we already invented.

E.g. to devm_* it doesn't really matter if someone else is
using a struct gpio_desc, or not, but if the current driver
is using it, it will be kept around until that driver detaches.
No reference counting needed for that.

So is this related to your problem or do I just get things
wrong?

Yours,
Linus Walleij