Modern Computer: Embedded Systems

ECE 320 · Week 6 · Lecture

The kernel learns the board from the device tree

• The device tree is data, not code: nodes for buses and devices, each with properties.
• The running tree is visible under /proc/device-tree, one directory per node.
• A device on an I2C bus cannot announce itself; unless a node describes it, no driver binds it.
• An overlay adds or changes nodes in the base tree without rebuilding it.

ECE 320 · Week 6 · Lecture

The I2C pins on the header

ECE 320 · Week 6 · Lecture

Three labels a stock driver needs

The driver's string comes from its binding document; the address comes from the part's datasheet.

ECE 320 · Week 6 · Lecture

The shape of an overlay

/dts-v1/;
/plugin/;

&i2c_bus_label {
	status = "okay";

	mydev@48 {
		compatible = "vendor,mydev";
		reg = <0x48>;
		status = "okay";
	};
};

ECE 320 · Week 6 · Lecture

Compile it, then enable it with ldto

dtc -@ -I dts -O dtb -o mydev.dtbo mydev.dts
ldto list                  # the stock overlays
ldto info <stock>          # shows the overlay directory
sudo cp mydev.dtbo <that directory>/
sudo ldto enable mydev     # applied live, until reboot
ldto active                # what is applied now
sudo ldto disable mydev

• ldto and dtc are on the image; -@ keeps the labels the overlay refers to.

ECE 320 · Week 6 · Lecture

Proving it bound: /sys before and after

• ls /sys/bus/i2c/devices/ lists devices by bus number and address, such as 1-0048.
• Each device directory has a driver link once a driver binds; no link, no bind.
• A bound driver may also add its own class directory, for example under /sys/class/.
• Take the "before" listing first: it is the only proof the overlay made the difference.