DFU Bootloader fails to correctly allocate memory for SoftDevice update

[Tiago Costa]

Hello,

My team and I are experiencing a problem with our custom DFU Bootloader, which is based on the Experimental Bootloader Secure Serial example (SDK 13), running in a custom NRF52832 board. Our main app uses a single bank, and we're also using the S132 v4 SoftDevice, our custom Bootloader and Bootloader settings.

The whole flash process seems to work fine, whether we update the Application, the SoftDevice or the Bootloader itself. However, when our boards are flashed with an application using almost all the flash space allocated to the application, problems occur when attempting to update the Bootloader or the SoftDevice using our custom Bootloader.

When our app uses more than 307200 bytes, leaving only about 3 pages of unused space, the function nrf_dfu_find_cache() fails to safely allocate space for the temporary swap space required by the update process, leading to the write operations overwriting the Bootloader region that is executing the DFU requests. This causes the Bootloader itself to become corrupt and the system hangs (only recovers after a re-flash with the programmer).

We are wondering if this is a known problem with SDK 13 and if there is a solution to the problem. (As a workaround, we're currently limiting our application size in the linker so that it fails to compile if grows beyond the aforementioned size.)

Thank you for your attention to this matter, please let me know if we are required to provide more details regarding this issue, TC

[Tiago Costa]

Ok, I have determined that the failure due to insufficient resources is caused by the DFU_APP_DATA_RESERVED assignment, which reserves 3 pages on top of the application memory region for keeping device bonding information, correct?

I also believe that the failure described by the original question is caused by trying to flash an hex file with different linker settings that the ones configured in the already running bootloader code. That somehow causes the nrf_dfu_find_cache() to return wrong values.

[Tiago Costa]

I know what the bootloader's error is (insufficient resources for app firmware image), so I don't need to step into the code in this scenario. What I don't know is why the app binary file is larger than the limit set in the linker... because, If I specify in the linker than the app must not occupy more than 0x4A000 bytes in memory, why is the binary larger? I believe its due to metadata in the binary that is not actually to be flashed in memory. So I guess the question is, why isn't this metadata ignored when flashing the app? I mean, the firmware image is rejected because of being too large but it should fit as the actual app was compiled for that limit (otherwise, linking would fail). Shouldn't nrfutil generate an init packet with the correct app size to be occupied in memory?