Restore OTA CRC integrity check and add hard reset after BLE DFU

[4np]

Improve BLE OTA reliability: fix discarded CRC and add hard reset after DFU

Disclaimer: This investigation and the resulting code changes were performed with the assistance of Claude Code (Anthropic). All changes have been reviewed and verified by the author.

Description of Change

Two improvements to the BLE OTA DFU path, verified on RAK4631 (nRF52840, S140 6.1.1) with MeshCore 1.14.1 via the iOS nRF DFU app. The serial/USB DFU path is unaffected.

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1. OTA CRC was silently discarded (dfu_init.c, dfu_single_bank.c, dfu_init.h)

dfu_init_postvalidate() computed and validated the image CRC but never returned it to the caller. m_image_crc stayed 0 after every OTA, so bank_0_crc = 0 was persisted to bootloader settings. Because bootloader_app_is_valid() skips the CRC check when bank_0_crc is zero, the boot-time integrity check was permanently disabled for any OTA-flashed image.

Fix: added uint16_t * p_crc_out to dfu_init_postvalidate() and updated the call site in dfu_single_bank.c to pass &m_image_crc. The validated CRC is now stored in bank_0_crc and verified on every subsequent boot.

// before
err_code = dfu_init_postvalidate((uint8_t *)mp_storage_handle_active->block_id, m_image_size);

// after
err_code = dfu_init_postvalidate((uint8_t *)mp_storage_handle_active->block_id, m_image_size, &m_image_crc);

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2. No hard reset after BLE OTA (main.c)

After image activation, the bootloader jumped directly to the application without NVIC_SystemReset(). This left nRF52840 peripheral registers and radio state in a post-DFU condition. Additionally, sd_softdevice_vector_table_base_set() silently fails when the SoftDevice is already disabled, and its SRAM fallback at 0x20000000 can be zeroed by the application's .bss initialisation before the SD is re-enabled.

GPREGRET is already cleared to 0 earlier in check_dfu_mode(), so a hard reset produces a clean boot directly into the application without re-entering DFU mode.

Fix: added NVIC_SystemReset() after BLE OTA teardown.

// before
if (_ota_dfu) {
    sd_softdevice_disable();
    usb_teardown();
}

// after
if (_ota_dfu) {
    sd_softdevice_disable();
    usb_teardown();
    NVIC_SystemReset(); // clean reset; GPREGRET is already 0, boots straight to app
}

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3. GCC 12–15 compatibility (Makefile)

Extended the existing GCC version workaround to cover GCC 15, and added -Wno-array-bounds and -Wno-unterminated-string-initialization to suppress false-positive warnings on memory-mapped address dereferences in SDK headers and intentional FAT 8.3 filename fields in ghostfat.c.

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Building and flashing

I used the following commands to build and flash the bootloader:

xcode-select --install
brew install --cask gcc-arm-embedded
pip3 install adafruit-nrfutil intelhex
git submodule update --init --recursive
python3 -m venv .venv
source .venv/bin/activate
pip install intelhex adafruit-nrfutil
make BOARD=wiscore_rak4631_board flash-dfu SERIAL=/dev/cu.usbmodem2101

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Testing

• BLE OTA of MeshCore 1.14.1 on RAK4631 via iOS nRF DFU app: application boots successfully
  after OTA completes.
• Serial DFU path (adafruit-nrfutil dfu serial) unaffected.
• Consecutive OTA cycles complete and boot correctly.