feat(core): mapper for magic bootloader values.

To trigger bootloaders that use a magic value in RAM to trigger
bootloader mode, add a mapping retained memory driver that maps
write/read of boot mode values to a special magic value stored
in the actually backing RAM.

app/CMakeLists.txt

@@ -23,6 +23,7 @@ zephyr_syscall_header(${APPLICATION_SOURCE_DIR}/include/drivers/ext_power.h)
 # Add your source file to the "app" target. This must come after
 # find_package(Zephyr) which defines the target.
 target_include_directories(app PRIVATE include)
+add_subdirectory(src/boot)
 target_sources(app PRIVATE src/stdlib.c)
 target_sources(app PRIVATE src/activity.c)
 target_sources(app PRIVATE src/behavior.c)

app/Kconfig

@@ -474,6 +474,8 @@ endmenu
 
 menu "Advanced"
 
+rsource "src/boot/Kconfig"
+
 menu "Initialization Priorities"
 
 if USB_DEVICE_STACK

app/dts/bindings/retained_mem/zmk,bootmode-to-magic-mapper.yaml

@@ -0,0 +1,9 @@
+# Copyright (c) 2024 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: |
+  Driver for mapping bootloader boot mode to a magic value
+
+compatible: "zmk,bootmode-to-magic-mapper"
+
+include: base.yaml

app/src/boot/CMakeLists.txt

@@ -0,0 +1,2 @@
+
+target_sources_ifdef(CONFIG_ZMK_BOOTMODE_TO_MAGIC_VALUE_MAPPER app PRIVATE bootmode_to_magic_mapper.c)

app/src/boot/Kconfig

@@ -0,0 +1,31 @@
+
+config ZMK_BOOTMODE_TO_MAGIC_VALUE_MAPPER
+    bool "Magic Value Mapper"
+    default y
+    depends on DT_HAS_ZMK_BOOTMODE_TO_MAGIC_MAPPER_ENABLED
+
+if ZMK_BOOTMODE_TO_MAGIC_VALUE_MAPPER
+
+choice ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE
+    prompt "Magic Value Bootloader Type"
+
+config ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_UNKNOWN
+    bool "Unknown"
+
+config ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_TINYUF2
+    bool "tinyuf2"
+
+config ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_ADAFRUIT_BOSSA
+    bool "Adafruit BOSSA"
+
+config ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_ADAFRUIT_NRF52
+    bool "Adafruit nRF52"
+
+endchoice
+
+config ZMK_BOOTMODE_BOOTLOADER_MAGIC_VALUE
+    hex
+    default 0xf01669ef if ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_TINYUF2 || ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_ADAFRUIT_BOSSA || BOOTLOADER_BOSSA_ADAFRUIT_UF2
+    default 0x57 if ZMK_BOOTMODE_MAGIC_VALUE_BOOTLOADER_TYPE_ADAFRUIT_NRF52
+
+endif

app/src/boot/bootmode_to_magic_mapper.c

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2024 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define DT_DRV_COMPAT zmk_bootmode_to_magic_mapper
+
+#include <zephyr/device.h>
+#include <zephyr/retention/bootmode.h>
+#include <zephyr/retention/retention.h>
+#include <zephyr/drivers/retained_mem.h>
+
+static const struct device *magic_dev = DEVICE_DT_GET(DT_CHOSEN(zmk_magic_boot_mode));
+#define MAGIC_DEST_ONE_BYTE (DT_REG_SIZE(DT_CHOSEN(zmk_magic_boot_mode)) == 1)
+
+#if MAGIC_DEST_ONE_BYTE
+typedef uint8_t magic_val_t;
+#else
+typedef uint32_t magic_val_t;
+#endif
+
+static const magic_val_t bootloader_magic_value = CONFIG_ZMK_BOOTMODE_BOOTLOADER_MAGIC_VALUE;
+
+static ssize_t btmm_ram_size(const struct device *dev) { return (ssize_t)1; }
+
+static int btmm_ram_read(const struct device *dev, off_t offset, uint8_t *buffer, size_t size) {
+    if (size != 1) {
+        return -ENOTSUP;
+    }
+
+    magic_val_t val;
+    int ret = retention_read(magic_dev, 0, (uint8_t *)&val, sizeof(val));
+    if (ret < 0) {
+        return ret;
+    }
+
+    *buffer = (val == bootloader_magic_value) ? BOOT_MODE_TYPE_BOOTLOADER : BOOT_MODE_TYPE_NORMAL;
+
+    return 0;
+}
+
+static int btmm_ram_write(const struct device *dev, off_t offset, const uint8_t *buffer,
+                          size_t size) {
+    if (size != 1) {
+        return -ENOTSUP;
+    }
+
+    magic_val_t val = (*buffer == BOOT_MODE_TYPE_BOOTLOADER) ? bootloader_magic_value : 0;
+
+    return retention_write(magic_dev, 0, (uint8_t *)&val, sizeof(val));
+}
+
+static int btmm_ram_clear(const struct device *dev) { return retention_clear(magic_dev); }
+
+static const struct retained_mem_driver_api btmm_api = {
+    .size = btmm_ram_size,
+    .read = btmm_ram_read,
+    .write = btmm_ram_write,
+    .clear = btmm_ram_clear,
+};
+
+DEVICE_DT_INST_DEFINE(0, NULL, NULL, NULL, NULL, POST_KERNEL, 0, &btmm_api);