RPi CM4 Change Serial Debug Interface

[MitchGaines]

Description Hi there, I have an RPi CM4 running dunfell. I am attempting to change the serial debug interface that the system writes to. The reason for this is because I have a device that must use the UART0 interface and the boot is being interrupted (at u-boot) by the device on this interface. I would like to move all console output to the UART3 interface. However, changing the SERIAL_CONSOLES variable in my machine.conf does not seem to do the trick.

Steps to reproduce the issue:

1. Change the SERIAL_CONSOLES variable in the machine.conf to SERIAL_CONSOLES = "115200;ttyS3"
2. Rebuild image & write to device
3. Hook up a UART->USB interface to UART3 and open in screen or some other serial monitoring software

Describe the results you received: No output was sent to the UART3 interface. I hooked up to UART0 again and all console output was still sent there.

Describe the results you expected: I expected all u-boot and kernel output to be output on the UART3 interface.

Additional details (revisions used, host distro, etc.):

• dunfell
• RPi CM4 64-bit

Additional things I've tried

1. Changing the cmdline.txt from console=serial0,115200 to console=serial3,115200

2. I created the following patch, to no avail

   
   diff --git a/arch/arm/boot/dts/bcm2711-rpi-cm4.dts b/arch/arm/boot/dts/bcm2711-rpi-cm4.dts
   index 49f6ba6c4792..704f7f726fe9 100644
   --- a/arch/arm/boot/dts/bcm2711-rpi-cm4.dts
   +++ b/arch/arm/boot/dts/bcm2711-rpi-cm4.dts
   @@ -9,7 +9,7 @@
   
   chosen {
       /* 8250 auxiliary UART instead of pl011 */
   -       stdout-path = "serial1:115200n8";
   +       stdout-path = "serial3:115200n8";
   };
   
   /* Will be filled by the bootloader */

UPDATE:

I got the kernel outputting to UART3 by adding `dtoverlay=uart3` to my `/boot/config.txt` and changing my `cmdline.txt` to `console=ttyAMA2`. I'll need to make these changes in my yocto configs to persist it, but its progress. U-boot is still outputting to uart0 though. 

[MitchGaines]

I ended up applying the following patches to u-boot in order to disable serial output. These are adapted from another thread I found on a similar topic.

diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index ff69607d3c..59666d673d 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -520,3 +520,10 @@ int ft_board_setup(void *blob, struct bd_info *bd)

    return 0;
 }
+
+int board_early_init_f(void)
+{
+    gd->flags |= (GD_FLG_SILENT | GD_FLG_DISABLE_CONSOLE);
+    return 0;
+}
+
diff --git a/include/configs/rpi.h b/include/configs/rpi.h
index 4c5c1ac31f..bb9009800d 100644
--- a/include/configs/rpi.h
+++ b/include/configs/rpi.h
@@ -176,5 +176,10 @@
    ENV_MEM_LAYOUT_SETTINGS \
    BOOTENV

+#define CONFIG_SILENT_U_BOOT_ONLY
+#define CONFIG_DISABLE_CONSOLE
+#define CONFIG_SILENT_CONSOLE
+#define CONFIG_SYS_DEVICE_NULLDEV
+#define CONFIG_BOARD_EARLY_INIT_F 1

 #endif

This solves my problem in the immediate term. Unfortunately, I lose u-boot debug capability.

As stated in my update to the OP, I was able to move kernel output to another serial port.

[MitchGaines]

Going to go ahead and close this as at this point, I'm not sure its a meta-raspberrypi issue. Should probably be something more easily configurable in u-boot.