How do I add an i2c device to the device tree?

Question

I am trying to add an mpu 9250 to the device tree of my RPi 3 A+ so that my custom driver's probe function will be called. I have followed this documentation about the Raspberry Pi device tree and this documentation describing the required properties for binding the mpu9250. Here is what I have so far:

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2708";

    fragment@0 {
        target = <&i2c1>;
        __overlay__ {
            status = "okay";
            mpu9250@68 {
                compatible = "invensense,mpu9250";
                reg = <0x68>;
                interrupt-parent = <&gpio3>;
                interrupts = <21 IRQ_TYPE_EDGE_RISING>;
                status = "okay";
                };
        };
    };
};

I build it on the target RPi 3 A+ with this command:

dtc -@ -I dts -O dtb -o mpu_9250.dtbo mpu_9250.dts

and here is the result:

Error: mpu_9250.dts:19.25-26 syntax error
FATAL ERROR: Unable to parse input tree

I think it is complaining about IRQ_TYPE_EDGE_RISING. When I change IRQ_TYPE_EDGE_RISING to a 1 then build, I get:

mpu_9250.dtbo: Warning (reg_format): "reg" property in /fragment@0/__overlay__/mpu9250@68 has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
mpu_9250.dtbo: Warning (unit_address_vs_reg): Node /fragment@0 has a unit name, but no reg property
mpu_9250.dtbo: Warning (pci_device_reg): Failed prerequisite 'reg_format'
mpu_9250.dtbo: Warning (pci_device_bus_num): Failed prerequisite 'reg_format'
mpu_9250.dtbo: Warning (simple_bus_reg): Failed prerequisite 'reg_format'
mpu_9250.dtbo: Warning (avoid_default_addr_size): Relying on default #address-cells value for /fragment@0/__overlay__/mpu9250@68
mpu_9250.dtbo: Warning (avoid_default_addr_size): Relying on default #size-cells value for /fragment@0/__overlay__/mpu9250@68
dtc: livetree.c:521: get_node_by_phandle: Assertion `(phandle != 0) && (phandle != -1)' failed.
Aborted (core dumped)

How do I fix these issues? What resources should I turn to?

I know that a driver already exists for the device, allowing access to it thru sysfs, and about i2c-dev but they don't suit my purposes.

Thanks

Answer 1

Use the Hex number of the IRQ_TYPE_EDGE_RISING.

interrupts = <21 0x1>;