Selecting serial device for console

[Simon Glass]

Hi Fiona,

On Thu, 1 Aug 2024 at 08:07, Fiona Klute <fiona.klute at gmx.de> wrote:
>
> 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.

You should be able to update /chosen/stdout-path (in the devicetree)
to specify the UART you want, using its alias. So if you have a
'serial1' alias, you can set stdout-path to 'serial1'. This should
work in Linux too.

The code in question is serial_find_console_or_panic().

Regards,
Simon


>
> 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