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
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