samples: basic: custom devicetree binding demo

This sample shows how to define a custom devicetree binding to use
GPIO pins for a specific purpose.

Signed-off-by: Martin Jäger <martin@libre.solar>

samples/basic/custom_dts_binding/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(gpio_custom_dts_binding)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/basic/custom_dts_binding/README.rst

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+.. _gpio-custom-dts-binding-sample:
+
+GPIO with custom devicetree binding
+###################################
+
+Overview
+********
+
+In Zephyr, all hardware-specific configuration is described in the devicetree.
+
+Consequently, also GPIO pins are configured in the devicetree and assigned to a specific purpose
+using a compatible.
+
+This is in contrast to other embedded environments like Arduino, where e.g. the direction (input /
+output) of a GPIO pin is configured in the application firmware.
+
+For typical use cases like LEDs or buttons, the existing :dtcompatible:`gpio-leds` or
+:dtcompatible:`gpio-keys` compatibles can be used.
+
+This sample demonstrates how to use a GPIO pin for other purposes with a custom devicetree binding.
+
+We assume that a load with high current demands should be switched on or off via a MOSFET. The
+custom devicetree binding for the power output controlled via a GPIO pin is specified in the file
+:zephyr_file:`samples/basic/custom_dts_binding/dts/bindings/power-switch.yaml`. The gate driver for
+the MOSFET would be connected to the pin as specified in the ``.overlay`` file in the boards
+folder.
+
+Building and Running
+********************
+
+For each board that should be supported, a ``.overlay`` file has to be defined
+in the ``boards`` subfolder.
+
+Afterwards, the sample can be built and executed for the ``<board>`` as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/basic/custom_dts_binding
+   :board: <board>
+   :goals: build flash
+   :compact:
+
+For demonstration purposes, some boards use the GPIO pin of the built-in LED.
+
+Sample output
+=============
+
+The GPIO pin should be switched to active level after one second.
+
+The following output is printed:
+
+.. code-block:: console
+
+   Initializing pin with inactive level.
+   Waiting one second.
+   Setting pin to active level.

samples/basic/custom_dts_binding/boards/nucleo_l073rz.overlay

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+/*
+ * Copyright (c) 2023 Martin Jäger / Libre Solar
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	load_switch: load_switch {
+		compatible = "power-switch";
+                /* using built-in LED pin for demonstration */
+		gpios = <&gpioa 5 GPIO_ACTIVE_HIGH>;
+	};
+};

samples/basic/custom_dts_binding/dts/bindings/power-switch.yaml

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+# Copyright (c) 2023 Libre Solar Technologies GmbH
+# SPDX-License-Identifier: Apache-2.0
+
+description: GPIO pin to switch a power output on or off
+
+compatible: "power-switch"
+
+properties:
+  gpios:
+    type: phandle-array
+    required: true
+    description: |
+      The GPIO connected to the gate driver for the MOSFET.

samples/basic/custom_dts_binding/prj.conf

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+CONFIG_GPIO=y

samples/basic/custom_dts_binding/sample.yaml

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+sample:
+  name: GPIO with custom devicetree binding
+tests:
+  sample.basic.custom_dts_binding:
+    tags: gpio devicetree
+    platform_allow: nucleo_l073rz
+    integration_platforms:
+      - nucleo_l073rz
+    depends_on: gpio
+    harness: console
+    harness_config:
+      type: multi_line
+      regex:
+        - "Initializing pin with inactive level."
+        - "Waiting one second."
+        - "Setting pin to active level."

samples/basic/custom_dts_binding/src/main.c

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+/*
+ * Copyright (c) 2023 Libre Solar Technologies GmbH
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdio.h>
+
+#include <zephyr/devicetree.h>
+#include <zephyr/drivers/gpio.h>
+#include <zephyr/kernel.h>
+
+#if !DT_NODE_EXISTS(DT_NODELABEL(load_switch))
+#error "Overlay for power output node not properly defined."
+#endif
+
+static const struct gpio_dt_spec load_switch =
+	GPIO_DT_SPEC_GET_OR(DT_NODELABEL(load_switch), gpios, {0});
+
+void main(void)
+{
+	int err;
+
+	if (!gpio_is_ready_dt(&load_switch)) {
+		printf("The load switch pin GPIO port is not ready.\n");
+		return;
+	}
+
+	printf("Initializing pin with inactive level.\n");
+
+	err = gpio_pin_configure_dt(&load_switch, GPIO_OUTPUT_INACTIVE);
+	if (err != 0) {
+		printf("Configuring GPIO pin failed: %d\n", err);
+		return;
+	}
+
+	printf("Waiting one second.\n");
+
+	k_sleep(K_MSEC(1000));
+
+	printf("Setting pin to active level.\n");
+
+	err = gpio_pin_set_dt(&load_switch, 1);
+	if (err != 0) {
+		printf("Setting GPIO pin level failed: %d\n", err);
+	}
+}