Selecting serial device for console

[Fiona Klute]

Hi everyone,

how can I configure which of the available serial devices U-Boot will
use for its serial console?

Background: I have a device based on a Raspberry Pi CM4, which has 6
UARTs (UART1 is a mini UART, the others PL011) [1]. In the default
configuration (including the mainline Linux broadcom/bcm2711-rpi-cm4-io
device tree) the Bluetooth module is attached to UART0. Unless I either
completely disable UART0 (status = "disabled" in the device tree) or
reconfigure it to be a serial console (e.g. using the RPi disable-bt
DTBO) U-Boot fails to boot, I assume (it doesn't get far enough to show
messages over netconsole) because it tries and fails to set up a serial
console where there's a BT chip listening.

That happens even if another serial console is available: With the RPi
bcm2711-rpi-cm4 DTB plus "disable-bt" (disable the Bluetooth module,
reconfigure UART0 as a serial console) and "uart1" (set up a serial
console on UART1) DTBOs I can see that U-Boot discovers the serial
device on UART1 (serial at 7e215040):

U-Boot> dm uclass serial
uclass 108: serial
0   * serial at 7e201000 @ 3af3b200, seq 0
1     serial at 7e215040 @ 3af3b2d0, seq 1

However, I can't find a way to make it *use* it instead of UART0
(serial at 7e201000). I tried to set CONFIG_CONS_INDEX=1 but didn't see any
change (unfortunately the help does not describe exactly how the number
is mapped to DT UART definitions). If I completely disable UART0 in the
DT as mentioned above U-Boot uses UART1 just fine, so it's not an issue
with UART1 as such.

To make matters slightly more complicated, the specific device I have is
a RevPi Connect 4, where a vendor overlay [2] describes how to set up an
RS485 serial console using UART5. The pins defined there are wired to
the outside of the box [3], so eventually I'll actually want to use
UART5 for the serial console. Which means I really need a (build-time or
preferably DT-based) way to configure which serial device to use.

My goal is to create a single DTS which both U-Boot and Linux can use,
and get it upstreamed if possible. If there are configuration options
and they're just missing from the documentation, I'd be happy to send a
patch to add them once I have a solution.

Best regards,
Fiona

[1]
https://www.raspberrypi.com/documentation/computers/configuration.html#configure-uarts
[2]
https://gitlab.com/revolutionpi/linux/-/blob/4ae9a35871799004300f83694a24359a7b4ef8fa/arch/arm/boot/dts/overlays/revpi-connect4-overlay.dts#L281-292
[3]
https://revolutionpi.com/en/tutorials/uebersicht-revpi-connect/serielle-geraete-anschliessen-connect