Hi,
I have used prebuilt kernel images on Hikey 620 applying this (below mentioned steps) patches for HDMI
download and extract HDMI binaries:
$ wget https://dl.google.com/dl/android/aosp/linaro-hikey-20170523-4b9ebaff.tgz
$ tar xzf linaro-hikey-20170523-4b9ebaff.tgz
$ ./extract-linaro-hikey.sh
It worked for me.
Now, I have built the kernel image and dtb using below steps
$ git clone kernel/hikey-linaro - Git at Google
$ cd hikey-linaro
$ git checkout -b android-hikey-linaro-4.9 origin/android-hikey-linaro-4.9
$ make ARCH=arm64 hikey_defconfig
$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-android- -j24
But HDMI is not working. Should I make any changes in Kernel
