See original article by Thomas Petazzoni at bootlin.com
Enabling the I2C bus
An introduction to the Device Tree
In order to enable the I2C bus, we’ll need to modify the Device Tree, so we’ll first need to give a few details about what Device Tree is. If you read again our previous blog post in this series, we already mentioned the Device Tree. As part of the Buildroot build process, a file called stm32mp157c-dk2.dtb is produced, and this file is used at boot time by the Linux kernel: it is the Device Tree.
...
At this point, we won’t give much more generic details about the Device Tree, as it’s an entire topic on its own. For additional details, you could check the Device Tree for Dummies presentation from your author (slides, video) or the devicetree.org web site.
I2C controllers in the Device Tree
Zooming in to the topic of I2C, we can see that arm/boot/dts/stm32mp157c.dtsi describes 6 I2C controllers through six different nodes in the Device Tree:
...
For now, this doesn’t describe any device on the bus, but should be sufficient to have the bus enabled in Linux. The question now is how to make this modification in our Device Tree in the proper way ?
Changing the Linux kernel source code
When Buildroot builds each package, it extracts its source code in output/build/<package>-<version>, so the source code of our Linux kernel has been extracted in output/build/linux-custom/. One could therefore be tempted to make his code changes directory in output/build/linux-custom/, but this has a number of major drawbacks:
...
If that is the case, then reflash output/images/sdcard.img on your SD card, and run the new system on the board. It’s now time to test the I2C bus!
Testing the I2C bus
After booting the new system on your Discovery board and logging in as root, let’s have a look at all I2C related devices:
...
Good, this matches the expected value according to the BME280 datasheet, so it seems like communication with our I2C device is working, let’s move on to enabling the BME280 sensor driver.
Enabling the sensor driver
As discussed earlier, this BME280 sensor already has a driver in the upstream Linux kernel, in the IIO subsystem. IIO stands for Industrial Input/Output, and this subsystems contains a lot of drivers for various ADCs, sensors and other types of measurement/acquisition devices. In order to use this driver for our BME280 device, we will essentially have to do two things:
- Enable the driver in our Linux kernel configuration, so that the driver code gets built as part of our kernel image
- Describe the BME280 device in our Device Tree so that the Linux kernel knows we have one such device, and how it is connected to the system
Adjusting the kernel configuration
In the previous blog post, we explained that the Linux kernel configuration used to build the kernel for the STM32 Discovery board was located at board/stmicroelectronics/stm32mp157-dk/linux.config. Obviously, we are not going to edit this file manually: we need to run the standard Linux kernel configuration tools.
...
We’re all set for the kernel configuration!
Describing the BME280 in the Device Tree
We now need to tell the Linux kernel that we have a BME280 sensor and how it is connected to the system, which is done by adding more details into our Device Tree. We have already enabled the I2C5 bus, and we now need to describe one device connected to it: this gets done by creating a child node of the I2C controller node.
...
&i2c5 {
status = "okay";
clock-frequency = <100000>;
pinctrl-names = "default", "sleep";
pinctrl-0 = <&i2c5_pins_a>;
pinctrl-1 = <&i2c5_pins_sleep_a>;
pressure@76 {
compatible = "bosch,bme280";
reg = <0x76>;
};
};
Re-building the kernel
Let’s now ask Buildroot to rebuild the Linux kernel, with our Device Tree change and kernel configuration change. Instead of rebuilding from scratch, we’ll just ask Buildroot to restart the build of the Linux kernel, which will be much faster:
...
To regenerate the SD card image, write it on your SD card, and boot your system.
Testing the sensor
After booting the system, if we check /sys/bus/i2c/devices, a new entry has appeared:
...