Due to hardware or driver issues you might not be able to update your firmware via Ethernet and TFTP from U-Boot. Still having U-Boot on replying on the serial port, allows you to update firmware with the loadb
Update from RAM was recently added as source media for the update command, though it is not available on official DUB-22.214.171.124 (nor earlier).
Update from RAM can be done using manual U-Boot commands. The tricky part is that the eMMC must be written using mmc command, which accepts units in blocks (512-byte) instead of in bytes. So we need to transform the value of $filesize to blocks:
=> loadb <-- transfer file
=> setexpr $filesizeblks $filesize / 512
=> setexpr $filesizeblks $filesizeblks + 1
=> mmc write $loadaddr $filesizeblks
Jun 24, 2019