Request the GPIO
GPIOs must be allocated before use, though the current implementation does not enforce this requirement. The basic allocation function is:
int gpio_request(unsigned int gpio, const char *label);
The gpio parameter indicates which GPIO is required, while label associates a string with it that can later appear in sysfs. The usual convention applies: a zero return code indicates success; otherwise the return value will be a negative error number.
Free the GPIO
A GPIO can be returned to the system with:
void gpio_free(unsigned int gpio);
Direction of GPIO ( Input or Output )
Some GPIOs are used for output, others for input. A suitably-wired GPIO can be used in either mode, though only one direction is active at any given time. Kernel code must inform the GPIO core of how a line is to be used; that is done with these functions:
int gpio_direction_input(unsigned int gpio); int gpio_direction_output(unsigned int gpio, int value);
In either case, gpio is the GPIO number. In the output case, the value of the GPIO (zero or one) must also be specified; the GPIO will be set accordingly as part of the call. For both functions, the return value is again zero or a negative error number. The direction of (suitably capable) GPIOs can be changed at any time.
Reading from GPIO
For input GPIOs, the current value can be read with:
int gpio_get_value(unsigned int gpio);
This function returns the value of the provided gpio; it has no provision for returning an error code. It is assumed (correctly in almost all cases) that any errors will be found when gpio_direction_input() is called, so checking the return value from that function is important.
Writing to GPIO
Setting the value of output GPIOs can always be done using gpio_direction_output(), but, if the GPIO is known to be in output mode already, gpio_set_value() may be a bit more efficient:
void gpio_set_value(unsigned int gpio, int value);
Using GPIO as Interrupt
Some GPIO controllers can generate interrupts when an input GPIO changes value. In such cases, code wishing to handle such interrupts should start by determining which IRQ number is associated with a given GPIO line:
int gpio_to_irq(unsigned int gpio);
The given gpio must have been obtained with gpio_request() and put into the input mode first. If there is an associated interrupt number, it will be passed back as the return value from gpio_to_irq(); otherwise a negative error number will be returned. Once obtained in this manner, the interrupt number can be passed to request_irq() to set up the handling of the interrupt.
GPIO in Sysfs
the GPIO subsystem is able to represent GPIO lines via a sysfs hierarchy, allowing user space to query (and possibly modify) them. Kernel code can cause a specific GPIO to appear in sysfs with:
int gpio_export(unsigned int gpio, bool direction_may_change);
The direction_may_change parameter controls whether user space is allowed to change the direction of the GPIO; in many cases, allowing that control would be asking for bad things to happen to the system as a whole. A GPIO can be removed from sysfs with gpio_unexport() or given another name with gpio_export_link().
Reference : https://lwn.net/Articles/532714/