09 GPIO driver

GPIO in Linux system is controlled by the Kernel GPIO subsystem.

GPIO APIs

Include <linux/gpio.h> for control GPIO in linux driver.

gpio_is_valid

To check if a GPIO is valid.

bool gpio_is_valid(int gpio_number);
  • gpio_number : GPIO that you are planning to use
  • Return : false if it is not valid otherwise, it returns true.

gpio_request

Request the GPIO

int gpio_request(unsigned gpio, const char *label);
  • gpio : GPIO that you are planning to use
  • label : label used by the kernel for the GPIO in sysfs. You can provide any string that can be seen in /sys/kernel/debug/gpio. Do cat /sys/kernel/debug/gpio. You can see the GPIO assigned to the particular GPIO.
  • Return : 0 if success and a negative number if failure.

Request one GPIO

int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);

Request multiple GPIOs

int gpio_request_array(struct gpio *array, size_t num);

gpio_export

For debugging purposes, you can export the GPIO which is allocated to the sysfs

int gpio_export(unsigned int gpio, bool direction_may_change);
  • gpio : GPIO that you want to export
  • direction_may_change : This parameter controls whether user space is allowed to change the direction of the GPIO
    • true – Can change
    • false – Can’t change.
  • Return : zero on success, else an error.

When you export a GPIO for example gpio72 with the option direction_may_change true, you can see it in /sys/class/gpio/gpio72/direction.

Once you export the GPIO, you can see the GPIO in /sys/class/gpio/*. There you can the GPIO’s value.

gpio_unexport

Unexport the exported GPIO

void gpio_unexport(unsigned int gpio);
  • gpio : GPIO that you want to unexport

Set the direction of the GPIO

Set the GPIO direction (output or input)

int  gpio_direction_input(unsigned gpio);

int  gpio_direction_output(unsigned gpio, int value);
  • gpio : GPIO that you want to set the direction.
  • value : The value of the GPIO once the output direction is effective.
  • Return : zero on success, else an error.

Set and get value of the GPIO

Once you set the GPIO direction as an output, use this API can set the value of GPIO

void gpio_set_value(unsigned gpio, int value);
  • gpio : GPIO that you want to change the value.
  • value : value to set to the GPIO. 0 – Low, 1 – High.

Once you set the GPIO direction as an input, use this API can get the value of GPIO

int  gpio_get_value(unsigned gpio);
  • gpio : GPIO that you want to get the value.
  • Return: GPIO value. 0 – Low, 1 – High.

GPIO Interrupt (IRQ)

To use interrupt in general, we need to know the IRQ line, and IRQ line is hardware-dependent. Use this API to get the IRQ line for GPIO number

int gpio_to_irq(unsigned gpio);
  • gpio : GPIO number you want to use with interrupt input.
  • Return : IRQ line

Beside flags for use with normal interrupt, GPIO interrupt also has some flags:

  • IRQF_TRIGGER_NONE
  • IRQF_TRIGGER_RISING
  • IRQF_TRIGGER_FALLING
  • IRQF_TRIGGER_HIGH
  • IRQF_TRIGGER_LOW

gpio_free

Once you have done with the GPIO, then you can release the GPIO which you have allocated previously

void gpio_free(unsigned int gpio);
  • gpio : GPIO number you want release.

Release multiple GPIOs.

void gpio_free_array(struct gpio *array, size_t num);

Our code

The code located in folder 09_gpio_driver

# To build ko driver 
make build-09
# To clean ko driver 
make clean-09
# To upload ko driver to our board 
make upload-09

[root@luckfox root]# ls
dfc.ko     fo.ko      gd.ko      ioctrl.ko  mam.ko     ofd.ko     pf.ko      sys_fs.ko  user_app   wq.ko
[root@luckfox root]# insmod gd.ko
[root@luckfox root]# ls /dev/ | grep gd
gd_dev
[root@luckfox root]# cat /dev/g
gd_dev     gpiochip0  gpiochip1  gpiochip2  gpiochip3  gpiochip4
[root@luckfox root]# cat /dev/gd_dev
1
[root@luckfox root]# echo 0 > /dev/gd_dev
[root@luckfox root]# cat /dev/gd_dev
0
[root@luckfox root]# dmesg | tail
[  230.852866] GPIO_irqNumber = 68
[  230.852913] gd module inserted successfully.
[  246.923002] gd File Read
[  246.923035] GPIO_OUTPUT state: 1
[  246.923764] gd File Read
[  255.918031] gd File Write
[  255.918063] gd File Write
[  264.001442] gd File Read
[  264.001476] GPIO_OUTPUT state: 0
[  264.002215] gd File Read
[root@luckfox root]# dmesg | tail
[  246.923035] GPIO_OUTPUT state: 1
[  246.923764] gd File Read
[  255.918031] gd File Write
[  255.918063] gd File Write
[  264.001442] gd File Read
[  264.001476] GPIO_OUTPUT state: 0
[  264.002215] gd File Read
[  275.075895] Interrupt Occurred : GPIO_72_IN
[  275.464539] Interrupt Occurred : GPIO_72_IN
[  275.808131] Interrupt Occurred : GPIO_72_IN
[root@luckfox root]#