CPC GPIO Driver#

The CPC GPIO Driver is a component part of the CPC GPIO Expander and acts as a backend for the Linux GPIO subsystem and as a frontend for the CPC GPIO Bridge.

Table of Contents#

• Installation

  • Kernel headers

    • Raspbian

    • Ubuntu

    • Other distributions

  • Building

  • Loading (Manual)

  • Loading (Automatic)

• Debugging

  • Enable logging

Installation#

Kernel headers#

This driver has been tested under v5.10 - v6.1 and may or may not be compatible with newer releases of the Linux Kernel.

Raspbian#

On Raspbian, the package manager only fetches the latest kernel headers.

This means you must keep your OS up to date to match the running kernel version:

sudo apt update, sudo apt dist-upgrade -y and reboot.

And then fetch the latest kernel headers:

sudo apt install raspberrypi-kernel-headers

If you do not wish to update your OS, you can manually download old kernel headers and install them:

sudo apt install raspberrypi-kernel-headers_xxxx_xxxx.deb

The matching tag release/kernel version can be determined in different ways.

Ubuntu#

On Ubuntu, you can fetch the current kernel headers matching your currently running kernel:

sudo apt install linux-headers-$(uname -r)

Other distributions#

Other distributions should have similar ways of acquiring their kernel headers but might use different package managers.

Building#

Invoke make to build the driver:

make

This generates a kernel module (cpc_gpio.ko) which can then be loaded manually (see Loading (Manual) below) or automatically (see Loading (Automatic) below).

Loading (Manual)#

This step manually loads the driver, meaning it will not be running after a reboot:

sudo insmod cpc_gpio.ko

You may then unload the driver:

sudo rmmod cpc-gpio

Or simply reboot.

Loading (Automatic)#

These steps ensures the driver loads on subsequent boots:

Copy driver to appropriate folder:

cp `pwd`/cpc_gpio.ko /lib/modules/`uname -r`/kernel/drivers/gpio

Generate dependencies:

sudo depmod

Add the driver to be loaded at boot time and reboot:

echo "cpc-gpio" | sudo tee -a /etc/modules

You may then unload the driver and remove it from subsequents boots:

sudo rmmod cpc-gpio

Then remove cpc-gpio from /etc/modules and reboot.

Optionally, you may also remove cpc_gpio.ko from /lib/modules/`uname -r`/kernel/drivers/gpio but it is not required.

Debugging#

Enable logging#

In the Kbuild file, add the following DEBUG flag:

EXTRA_CFLAGS := -DDEBUG

Rebuild and reload the driver.

Run dmesg.

You should see debugging logs during GPIO transactions.