Device tree overlays

The uboot script on Mendel is configured to load device tree blob (.dtb) files from /boot/ and overlay the existing device tree with any dtb files specified in the /boot/overlays.txt file.

For example, you can use the following steps to increase the CMA size to 512 MiB (default is 320 MiB). (All steps are performed on your Mendel device.)

First, create your device tree source (.dts) file (in this example, named cma512.dts):

// Set CMA region to 512M

/ {
    compatible = "fsl,imx8mq-phanbell";

    fragment@0 {
        target-path = "/";
        __overlay__ {
            reserved-memory {
                linux,cma {
                    size = <0 0x20000000>;

Install the device tree compiler:

sudo apt-get update

sudo apt-get install device-tree-compiler

Compile your dts file to dtb format:

dtc -I dts -O dtb -o cma512.dtbo cma512.dts

Move the cma512.dtbo file into /boot/ and edit the /boot/overlays.txt file (as root) to include this new file. The overlays.txt file should then look like this:

# List of device tree overlays to load. Format: overlay=<dtbo name, no extenstion> <dtbo2> ...

Then reboot and you're done. You can verify the CMA update like this:

cat /proc/meminfo | grep CmaTotal

It should print 524288 kB.