Continuing my previous post on writing an SPI driver for the BeagleBone, here’s a richly commented example C file to help implementing a generic kernel module driver for your chosen SPI device. The example is not only for the BeagleBone, you can use it for any host.

About the module

This example module shows you the following:

1. How to probe and initialize your SPI device from the kernel module
2. How to register and use interrupts on GPIO pins
3. How to use workqueues
4. How to communicate (read and write registers) with your SPI device
5. How to use the GPIO pins and LEDs on the BeagleBone
6. How to create and use sysfs files
7. How to use FIFO and ringbuffers

After loading, the module creates the following sysfs files in the /sys/kernel/misc/spikernmodex/ directory:

1. store: when you read this, the module pops and prints an entry from the ringbuffer, on writing it puts the data to a FIFO buffer and then tries to send the contents of the FIFO buffer to the SPI device
2. somereg: on reading, it prints the value of the SPI device’s (example) SOMEREG register, on writing, it puts the value to the SPI device’s SOMEREG register
3. clearringbuffer: on writing, it clears the module’s ringbuffer
4. gpio19state: on reading, returns the state (0/1) of the GPIO19 pin
5. gpio21state: on reading, returns the state (0/1) of the GPIO21 pin

The module sets an interrupt for a rising edge signal on the GPIO19 pin. When the interrupt occurs, it queues the work called READ, which reads some data from the SPI device and pushes it to the ringbuffer. When you read the sysfs file called “store”, you will get one entry from the ringbuffer (or nothing, if the ringbuffer is empty).

The reason why a workqueue is used is that the SPI commands are blocking, and we must return from an interrupt as quickly as possible (and also we can’t call a method from an interrupt which can sleep, like spi_sync()).

Notice: this module is only an example, it can be compiled and loaded as a kernel module, but it won’t do much by itself.

Installing the module

Here’s how to install the kernel module:

1. Copy spikernmodex.c to your kernel source’s drivers/misc folder.
2. Edit the folder’s Makefile, add this after the last line:

   1	obj-$(CONFIG_SPIKERNMODEX) += spikernmodex.o

3. Edit the folder’s Kconfig file, add this somewhere:

   1 2 3 4 5

   config SPIKERNMODEX     tristate "Example SPI driver by Nonoo (www.nonoo.hu)"     depends on SPI && SYSFS     help         http://dp.nonoo.hu/spi-kernel-driver-howto/

4. Compile your kernel and say “m” when the configuration script asks for building CONFIG_SPIKERNMODEX as a module or build into the kernel (or manually add CONFIG_SPIKERNMODEX=m to the end of your kernel’s .config file.