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The aim of this document is to explain how to use GPIO of SP7350 in Linux. SP7350 has 106 general purpose IO (GPIO) pins. This section explains how to modify device-tree source file to set up GPIO pins as digital input or output pins.

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Defining GPIO in device-tree source file

Every single device has a node in device-tree source (dts) file in Linux. Property pinctrl-0 (or pinctrl-1, pinctrl-2,… if a device has more states) is used to point at pin configuration node within pin controller (node pinctrl@f8800080 in SP7350). Pin configuration nodes (sub-nodes in node pinctrl@f8800080 ) define the actual pins assignment.

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

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Manipulating GPIO in Linux kernel (driver)

In above example, property test1-gpios define one GPIO pin to GPIO0 of pin controller and property test2-gpios define one GPIO pin to GPIO90 of pin controller, respectively. Linux driver can use function devm_gpiod_get to get GPIO descriptor with the property. Use gpiod_set_value to set value to GPIO or use gpiod_get_value to get value from GPIO. Refer to the following C example code.

struct gpio_desc *test1_gpio;
struct gpio_desc *test2_gpio;
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:
// Get test1_gpio.
test1_gpio = devm_gpiod_get(&pdev->dev, "test1", GPIOD_OUT_HIGH);
if (!IS_ERR(test1_gpio)) {
    printk(KERN_INFO "test1_gpio is at GPIO[%d].\n", desc_to_gpio(test1_gpio));
}

// Get test2_gpio.
test2_gpio = devm_gpiod_get(&pdev->dev, "test2", GPIOD_OUT_LOW);
if (!IS_ERR(test2_gpio)) {
    printk(KERN_INFO "test2_gpio is at GPIO[%d].\n", desc_to_gpio(test2_gpio));
}
:
:
gpiod_set_value(test1_gpio, 0);
gpiod_set_value(test2_gpio, 1);
:

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Manipulating GPIO in Linux user-space (using sysfs)

Users can setup and access GPIO using sysfs interface. Path of gpio of sysfs is /sys/class/gpio. Enter the folder and use ls command to list contents:

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where exportand unexportare write-only control interface. gpiochip0is a GPIO controller.

Example 1: Set up GPIO0 as output pin.

First, use echo command to export GPIO0:

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/sys/class/gpio # echo 1 > GPIO0/value
/sys/class/gpio # cat GPIO0/value
1
/sys/class/gpio # echo 0 > GPIO0/value
/sys/class/gpio # cat GPIO0/value
0
/sys/class/gpio #

Example 2：Setup GPIO0 as input port.

/sys/class/gpio # echo 0 > export
/sys/class/gpio # ls
GPIO0 		export     gpiochip0  unexport
/sys/class/gpio # echo in > GPIO0/direction
/sys/class/gpio # cat GPIO0/direction
in
/sys/class/gpio # cat GPIO0/value
1
/sys/class/gpio #

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