I was able to compile the ubuntu-4.9 branch (using gcc-5.4.1) but I didn’t manage to boot this kernel
Starting kernel ...
uboot time: 5238283 us
domain-0 init dvfs: 4
efuse_pw_en: 0x3
WARNING! efuse bits is disabled
Enable A53 JTAG to AO
INFO: HDCP22 key read fail!
INFO: p1d 0
INFO: pd1 0
while the plain vanilla mainline kernel boots fine at the same time (but has other deficiencies).
The u-boot version is
U-Boot 2015.01-g0f90c8e (Mar 04 2017 - 09:13:33)
aarch64-linux-gnu-gcc (Ubuntu/Linaro 4.8.4-2ubuntu1~14.04.1) 4.8.4
GNU ld (GNU Binutils for Ubuntu) 2.24
How can I perform a non-destructive update to the 4.9 kernel instead of reflashing the eMMC storage?