Hello, I happen to have almost the same problem!
My current kernel is 4.4.154-92-rockchip-g58d4b8d2fd3b on Debian 9.11 stretch.
Everything is working great so far but i can’t seem to get the HDMI-CEC to work.
According to this post a fix was already implemented into the kernel back in April.
Which kernel version actually supports HDMI-CEC?
I can’t seem to find any more recent ones than 4.4.154-92. Is it the most recent one or do i have to upgrade in a specific way other than normal dist-upgrades?
I’m also interested in Ubuntu, is there perhaps a Ubuntu version where HDMI-CEC is available?
Best regards!
Otte