ZynqMP boot: no messages from SPL other than "Debug uart enabled"

[Major A]

Hi everyone,

Please forgive me if this issue has already been discussed somewhere, I 
haven't been able to find the solution after searching and playing 
around for the past week.

I have a ZynqMP board (Xilinx ZCU102 V1.1) and would like to install my 
own Linux on it, based on the U-Boot SPL.  After playing around with the 
Xilinx version of U-Boot and various sources for ATF as well as PMUFW, 
I've now settled on mainstream U-Boot (from git, master branch) as the 
code I'd like to use.  There's an issue there already: if I run

   make DEVICE_TREE="zynqmp-zcu102-rev1.0" xilinx_zynqmp_virt_defconfig

then the default device tree in .config ends up being 
"zynqmp-zcu100-revC", there's no sign of my DEVICE_TREE parameter making 
it into .config .  In any case, I fixed this manually, then I also 
enabled early UART output and UART debugging.

After each build, I copy spl/boot.bin to the SD card and try to boot the 
ZCU102.

The real issue is that, whatever I do, whichever version or 
configuration of U-Boot I compile, I get a message "Debug uart enabled" 
from the early UART code, but then nothing.  Everyone else on the 
internet seems to see at least a few more lines of output, usually 
starting with "U-Boot SPL 2020.04-rc3-00108-gdb41d985f6" or similar 
(this string was taken from the boot.bin I copied to the SD card, so 
it's there!), whereas I see nothing at all.

If I turn early UART off, then I don't even get the "Debug uart enabled" 
message.  Simply nothing.  Also no complaints about bl31 or PMUFW in 
case I deliberately built without them.

The board works fine with Petalinux, and it passes all hardware tests, 
so it should be OK.  I'm monitoring all four UARTs exposed through the 
USB device interface, just in case something is routed to the wrong 
UART.  But again, nothing.

I'm stuck, I'd appreciate any help.

Cheers,

   András