LED

You can manage a led connected to a GPIO pin. The LED management is similar with the standard GPIO sysfs driver, but you have some new features like triggers (e.g. "heartbeat" LED blinks like a heart at the rate oh the CPU load) . Here are the GPIO used for the user LED for Beaglebone Black:

pinmux_userled_pins {
            pinctrl-single,pins = <0x98 0x7 0x9c 0x7>;
            linux,phandle = <0x3>;
            phandle = <0x3>;
        };

This is the led pinmux defination of pinmux@44e10800 section in the "dts" file the 0x98 and 0x9c is the offset, the 0x7,0x7 is the mode of this gpio pin.

ocp{
    ...

    gpio@481ac000 {
            compatible = "ti,omap4-gpio";
            ti,hwmods = "gpio3";
            gpio-controller;
            #gpio-cells = <0x2>;
            interrupt-controller;
            #interrupt-cells = <0x1>;
            reg = <0x481ac000 0x1000>;
            interrupts = <0x20>;
            linux,phandle = <0x16>;
            phandle = <0x16>;
        };
    }

linux,phandle and phandle is just a 32bit unique number and will be used as GPIO specifier in the next section for:

gpio-leds {
            compatible = "gpio-leds";
            pinctrl-names = "default";
            pinctrl-0 = <0x3>;

            led0 {
                label = "beaglebone:green:usr0";
                gpios = <0x16 5 0>;
                linux,default-trigger = "heartbeat";
                default-state = "off";
            };
        };