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Date: Wed, 9 Dec 2015 11:30:34 -0800
From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Linus Walleij <linus.walleij@linaro.org>
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Subject: Re: [PATCH 000/182] Rid struct gpio_chip from container_of() usage
Message-ID: <20151209193034.GE27131@dtor-ws> (sfid-20151209_203044_336639_141DA88E)
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On Wed, Dec 09, 2015 at 02:08:35PM +0100, Linus Walleij wrote:
> This removes the use of container_of() constructions from *all*
> GPIO drivers in the kernel. It is done by instead adding an
> optional void *data pointer to the struct gpio_chip and an
> accessor function, gpiochip_get_data() to get it from a driver.
> 
> WHY?
> 
> Because we want to have a proper userspace ABI for GPIO chips,
> which involves using a character device that the user opens
> and closes. While the character device is open, the underlying
> kernel objects must not go away.
> 
> Currently the GPIO drivers keep their state in the struct
> gpio_chip, and that is often allocated by the drivers, very
> often as a part of a containing per-instance state container
> struct for the driver:
> 
> struct foo_state {
>    struct gpio_chip chip;  <- OMG my state is there
> };
> 
> Drivers cannot allocate and manage this state: if a user has the
> character device open, the objects allocated must stay around
> even if the driver goes away. Instead drivers need to pass a
> descriptor to the GPIO core, and then the core should allocate
> and manage the lifecycle of things related to the device, such
> as the chardev itself or the struct device related to the GPIO
> device.

Yes, but it does not mean that the object that is being maintained by
the subsystem and that us attached to character device needs to be
gpio_chip itself. You can have something like

struct gpio_chip_chardev {
	struct cdev chardev;
	struct gpio_chip *chip;
	bool dead;
};

struct gpio_chip {
	...
	struct gpio_chip_chardev *chardev;
	...
};

You alloctae the new structure when you register/export gpio chip in
gpio subsystem core and leave all the individual drivers alone.

Thanks.

-- 
Dmitry