Pi4 enable uart4 without using boot/config.txt

Question

I need to be able to enable uart4 on a Pi4 using dtoverlay, without adding it to /boot/config.txt, and have /dev/ttyAMA2 available immediately. The dtoverlay part is easy, sudo dtoverlay uart4, but /dev/ttyAMA2 does not show up.

Why? Because I am setting up automated configuration of a device and while I'm writing dtoverlay=uart4 to /boot/config.txt, I need to be able to immediately access that serial port without a reboot, in order to flash firmware to a device on that port.

dtoverlay -l shows the overlay. I just don't understand how to get /dev/ttyAMA2 to show up as a device.

Answer 1

When you add an overlay using the dtoverlay command, it only applies to the current running session. It does not persist across reboots, so it won't be available immediately after a reboot.

To make the overlay persistent, you need to add it to the /boot/config.txt file. However, if you don't want to modify that file, you can create a temporary overlay file that includes your desired overlay and then load it using the dtoverlay command. Here's how you can do it:

Create a new file with your overlay configuration. For example, you can create a file named uart4-overlay.dts with the following contents:

/dts-v1/;
/plugin/;
/ {
  fragment@0 {
    target = <&uart4>;
    overlay {
      status = "okay";
    };
  };
};

Compile the overlay file by running this command:

dtc -@ -I dts -O dtb -o uart4.dtbo uart4-overlay.dts

Load the overlay using the dtoverlay command: sudo dtoverlay uart4.dtbo After running these commands, /dev/ttyAMA2 should become available as a device.

Answer 2

It is far from clear WHY you don't want to use config.txt.

You can enable a dtoverlay on the fly with the following command:-

sudo dtoverlay uart4

You will need to repeat this every time you boot.

The serial port will be assigned the next available /dev/ttyAMA This will normally be /dev/ttyAMA1

See https://raspberrypi.stackexchange.com/a/107780/8697