Linux kernel module for Raspberry Pi enabling the use of GPIO as external interrupt sources. The driver registers a device for each gpio present on the raspberry pi header. The processes with open devices and configured file descriptors will receive a realtime SIGIO signal when an interrupt on any of gpios associated with the open devices is generated. A process with several opened devices can distiguish the source of the interrupt by reading the si_fd field in the siginfo_t parameter of the signal handler and checking which of the opened devices with that file descriptor generated the signal and consequently the interrupt. It is also possible to control the type of event that generates the interrupt (falling edge - default, rising edge, high level or low level) and configure the pins associated pull-up/down structure, through the use of ioctl commands.
/* BUILDING AND LOADING THE MODULE */
You'll need to set the environment variable CROSS to your cross-compile toolchain prefix and the KDIR variable to where your kernel source is located and run make. The Makefile has this variables set by default to my toolchain and kernel source directory under buildroot.
Once the module .ko file is on your target machine, load it using the insmod command line tool.
After loading the module the /dev/ directory should have some gpioirq_xx character devices, each one representing one gpio present on the raspberry pi header.
/* HOW TO USE IT */
In your application you will first need to define _GNU_SOURCE and insert the fcntl.h (by that order), signal.h and "gpioirq.h" headers.
You will need to follow these steps to configure your program to receive interrupts from the pins:
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Open the device(s) associated with the gpios for which you want to receive the signal/interrupt.
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Register a signal handler for SIGIO, using the sigaction function and setting the SA_SIGINFO flag in the sigaction struct. The handler should follow the int (*handler)(int, siginfo_t, void*) signature and inside it you can distinguish among the various devices by reading the si_fd field in the siginfo_t struct and comparing it to your open file descriptors.
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Use fcntl operation, F_SETSIG, for each file descriptor to specify that a realtime signal should be delivered.
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Use fcntl operation, F_SETOWN, to set your process as the owner of the file descriptor.
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Use fcntl operation, F_GETFL and F_SETFL, to enable signal-driven io by seting the O_ASYNC file status flag.
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Optionally use ioctl operations, GPIOIRQ_IOC_SETTYPE and GPIOIRQ_IOC_SETPULL, to configure the type of event that generates the interrupt (default is falling edge) or pull-up/down (default is no pull-up/down) on the pin you are using. The flags to pass as arguments to this commands are defined in gpioirq.h.
Note: The devices created can only be opened by one process at a time.
An example of how to use this devices is provided in the test_module.c program.
Any feedback or suggestions are welcome! José Martins - josemartins90@gmail.com