Coding: Raspi GPIO Interrupts (Slideshow)#

Move to Raspberry Pi, Cross Compiling #

Why? Use simple GPIO lines as interrupt sources.

../../../../../../_images/GPIO-Pinout-Diagram-22.png

../../../../../../_images/schematic.png

Cross Compilation #

build kernel
Build our module against that kernel. Use make parameters ARCH and CROSS_COMPILE from Raspberry Pi: Building the Kernel
Load it ⟶ fine

Cross Compilation Sucks #

No easy arm-linux-gnu glibc on Fedora, have to build on target
Copy linux/ from host to target
Won’t compile there because already configured for host

# apt install flex bison libssl-dev bc

$ make oldconfig

Short (Legacy) GPIO API Introduction #

#include <linux/gpio.h>

int gpio_request(unsigned gpio, const char *label);
void gpio_free(unsigned gpio);
int gpio_direction_input(unsigned gpio);
int gpio_direction_output(unsigned gpio, int value);
int gpio_to_irq(unsigned gpio);

Note

Check /sys/kernel/debug/gpio!

Implement New Event Type: Interrupt #

New `ioctl`: `MY_ACTIVATE_GPIO`#

Aha, third ioctl() parameter: unsigned long
Add ioctl with just a printk() on the kernel side
Write test program, test-activate-gpio.c

Interlude: `_IO*()` Macros #

# ./test-activate-gpio 0 25
ioctl: Bad address

See ioctl(): Requests
Fix
Emphasize on ioctl numbers ⟶ ABI breakage

Request GPIO #

Add new my_device method, my_device_activate_gpio(), and call right into it
- gpio_request() (and gpio_direction_input())
- gpio_free() in _destroy()

Check

# cat /sys/kernel/debug/gpio
gpiochip0: GPIOs 0-53, parent: platform/3f200000.gpio, pinctrl-bcm2835:
 gpio-25  (                    |my_driver           ) in  lo

Unload driver ⟶ gone

Request Interrupt #

gpio_to_irq()
Implement dummy handler, only printk()
request_irq()/free_irq()

Add hardware, check /proc/interrupts

Test #

Press switch ⟶ works
Hammer on it via ioctl(), and at the same time generate interrupts
- Discuss: “atomic context”
- Discuss: “sleep while atomic”