I have noticed that default baudrate for rk3399 board RK3399_BAUDRATE defined in plat/rockchip/rk3399/rk3399_def.h is 115200 while U-boot default value and most documentation points to a value of 1500000.
This of course means that messages printed by ATF are not visible by default in this context.
The change from 1500000 to 115200 was introduced in 0c05748bdebfad9fa43a80962186438bb8fbce62,
with the following message
commit 0c05748bdebfad9fa43a80962186438bb8fbce62 Author: Caesar Wang <firstname.lastname@example.org> Date: Tue Apr 19 20:42:17 2016 +0800 rockchip: fixes for the required This patch has the following change for rk3399. * Set the uart to 115200 since the loader decide to set uart baud to 115200Hz. So the ATF also should set uart baud to 115200. [..]
However, I'm not sure it this still applies.
I'll be happy to submit a patch to update the value if it is OK.