nRF Connect SDK DFU over BLE fails with bad State: Remote error: Unknown(1)?

Question

I am having trouble setting up DFU over BLE in nRF Connect SDK. I am using NCS 1.9.1. I have followed these instructions with my own project on both the nrf52840 devkit and a custom board: https://devzone.nordicsemi.com/guides/nrf-connect-sdk-guides/b/software/posts/ncs-dfu 
 Side note: The DFU UUID in this guide did not work for me: I used the UUID that is shown in the iOS nRF Connect Device Manager App when no devices are found: 
 
 BT_UUID_128_ENCODE ( 0x8d53dc1d , 0x1db7 , 0x4cd3 , 0x868b , 0x8a527460aa84 ) 
 
 With this UUID, I was able to connect with the device manager. However, when I start the image upgrade with "erase application settings" enabled and mode "test and confirm", I get this message: "State: Remote error: Unknown(1)". If I try again after that, the state says "UPLOADING..." until it times out. (restarting the app recreates the unknown(1) error). 
 
 When I try uploading the same firmware image to the same device via the nRF Connect for iOS app, I get more details. The log looks good until I get a message "Length of data to send exceeds MTU". This seems straightforward, but I am failing to get it resolved. 
 
 Is 252 (as stated in the above guide) the correct MTU length? If not, how do I change it? I've tried messing around with several of the CONFIG_ values, but it doesn't appear to update because I see in the nRF Connect iOS app the MTU is the default 23. 
 
 Any help appreciated, thank you! 
 
 EDIT: 
 for anyone else who comes across this problem: 
 
 - "Length of data to send exceeds MTU" is an expected message and doesn't indicate a problem 
 - The issue ended up being that I had set CONFIG_HEAP_MEM_POOL_SIZE=256, which is too small for the DFU to allocate the buffer needed to write to flash. OK values I tested are 4096 and 0(instead of allocating from heap, a static buffer is used). Optimal size is probably size of(struct flash_img_context)

dev_giraffe · Accepted Answer

AHaug said: The Unknown (1) error is sent from the remote device. The meaning depends on the method throwing this error. 
 I was able to track down the source of MGMT_ERR_EUNKNOWN (which is the error code for a lot of conditions) to img_mgmt.c line 499. It appears to fail at alloc_ctx() within img_mgmt_impl_write_image_data and causes the errstr "img_mgmt_err_str_flash_write_failed". 
 On a hunch based off of alloc_ctx()'s function brief, I increased: 
 
 CONFIG_HEAP_MEM_POOL_SIZE =256 
 to

CONFIG_HEAP_MEM_POOL_SIZE =4096 (the default for a lot of cases) 
 
 ... and the DFU suceeded! 
 
 It looks like CONFIG_HEAP_MEM_POOL_SIZE=0 also works. (in which case it looks like it would use a statically allocated buffer for alloc_ctx()) 
 
 Not sure why I had originally set it to 256, I think maybe it had been a configuration I saw in an example while I was trying to get SPI to work. Frustrating, but glad it works now. The error cases could have been a bit more descriptive - I would have thought that the alloc_ctx() failure should return 
 
 MGMT_ERR_ENOMEM.

nRF Connect SDK DFU over BLE fails with bad State: Remote error: Unknown(1)?

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