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where property pinctrl-0 sets up pins of serial@f8801900 device for "default" state. It is a handle (an address) to sub-node of pin-controller node. In this case, pinctrl-0 has two handles. uart0_pins defines pins of UART0, and uart0_test_pins defines new-add test GPIO pins.

Property testProperty test1-gpios defines a GPIO pin to GPIO0 of pin controller. Property test2-gpios defines one a GPIO pin to GPIO90 of pin controller. Linux driver can get GPIO descriptors using the propertyproperties. pctl is a handle (an address) to pin controller.

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uart0_test_pins: pinmux_uart0_test-pins {
    function = "GPIO";
    pins = "GPIO0", GPIO90";
    output-enable;
};

Manipulate GPIO in Linux kernel (driver)

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