samples: add debug monitor sample

Implemented a sample that shows configuration and basic usage of monitor
mode debugging feature.

Signed-off-by: Piotr Jasiński <piotr.jasinski@nordicsemi.no>

samples/subsys/debug/debugmon/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(debugmon)
+
+target_sources(app PRIVATE src/main.c)

samples/subsys/debug/debugmon/README.rst

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+.. _debugmon-sample:
+
+Debug monitor
+#############
+
+Overview
+********
+
+The Debug Monitor sample shows a basic configuration of debug monitor feature.
+
+
+The source code shows how to:
+
+#. Configure registers to enable degugging in debug monitor mode
+#. Specify custom interrupt to be executed when entering a breakpoint
+
+.. _debugmon-sample-requirements:
+
+Requirements
+************
+
+Your board must:
+
+#. Support Debug Monitor feature (available on Cortex-M processors with the exception of Cortex-M0)
+#. Have an LED connected via a GPIO pin (these are called "User LEDs" on many of
+   Zephyr's :ref:`boards`).
+#. Have the LED configured using the ``led0`` devicetree alias.
+
+Building and Running
+********************
+
+Build and flash Debug Monitor as follows, changing ``reel_board`` for your board:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/debug/debugmon
+   :board: reel_board
+   :goals: build flash
+   :compact:
+
+After flashing the board enters a breakpoint and executes debug monitor exception code.
+The LED starts to blink, indicating that even though the processor spins in debug monitor
+interrupt, other higher priority interrupts continue to execute.

samples/subsys/debug/debugmon/prj.conf

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+CONFIG_CORTEX_M_DEBUG_MONITOR_HOOK=y

samples/subsys/debug/debugmon/sample.yaml

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+sample:
+  name: debug monitor sample
+tests:
+  sample.debug.debugmon:
+    build_only: true
+    platform_allow: reel_board
+    tags: debug

samples/subsys/debug/debugmon/src/main.c

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+/*
+ * Copyright (c) 2022 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <zephyr/drivers/gpio.h>
+#include <zephyr/sys/printk.h>
+#include <zephyr/arch/arm/aarch32/cortex_m/cmsis.h>
+#include <zephyr/arch/arm/aarch32/exc.h>
+
+#define LED0_NODE DT_ALIAS(led0)
+static const struct gpio_dt_spec led = GPIO_DT_SPEC_GET(LED0_NODE, gpios);
+
+void timer_handler(struct k_timer *timer)
+{
+	gpio_pin_toggle_dt(&led);
+}
+K_TIMER_DEFINE(led_timer, timer_handler, NULL);
+
+int debug_mon_enable(void)
+{
+	/*
+	 * Cannot enable monitor mode if C_DEBUGEN bit is set. This bit can only be
+	 * altered from debug access port. It is cleared on power-on-reset.
+	 */
+	bool is_in_halting_mode = (CoreDebug->DHCSR & CoreDebug_DHCSR_C_DEBUGEN_Msk) == 1;
+
+	if (is_in_halting_mode) {
+		return -1;
+	}
+
+	/* Enable monitor mode debugging by setting MON_EN bit of DEMCR */
+	CoreDebug->DEMCR |= CoreDebug_DEMCR_MON_EN_Msk;
+	return 0;
+}
+
+/*
+ * Interrupt handler that will be called each time the device
+ * enters a breakpoint while debugging in monitor mode.
+ * With CONFIG_CORTEX_M_DEBUG_MONITOR_HOOK enabled this exception is set to lowest
+ * possible priority (IRQ_PRIO_LOWEST).
+ */
+void z_arm_debug_monitor(void)
+{
+	/*
+	 * Implement the logic or use a ready implementation available for your platform
+	 * (ie. SEGGER implementation for their debug probes)
+	 */
+	printk("Entered debug monitor interrupt\n");
+
+	/* Spin in breakpoint. Other, higher-priority interrupts will continue to execute */
+	while (true)
+		;
+}
+
+void main(void)
+{
+	/* Set up led and led timer */
+	if (!device_is_ready(led.port)) {
+		printk("Device not ready\n");
+		return;
+	}
+
+	int err = gpio_pin_configure_dt(&led, GPIO_OUTPUT_ACTIVE);
+
+	if (err) {
+		printk("Error configuring LED\n");
+		return;
+	}
+	k_timer_start(&led_timer, K_NO_WAIT, K_SECONDS(1));
+
+	/* Set up debug monitor */
+	err = debug_mon_enable();
+	if (err) {
+		printk("Error enabling monitor mode:\n"
+			"Cannot enable DBM when CPU is in Debug mode");
+		return;
+	}
+
+	/* Enter a breakpoint */
+	__asm("bkpt");
+}