device tree, change fixed link ethernet phy speed runtime - linux

Question

I'm working with a custom IMX8 board with a phy that is not supported by the Linux kernel (it's a clause 45 automotive oabr transceiver).
The phy is actually working, and its mdio bus and digital IO's are controlled with an userspace application.
To acheive this i had to bind it in the device tree with the fixed-link property as below.
...
port#0 {
reg = <0x00>;
label = "oabr";
phy-mode = "rgmii";
fixed-link {
speed = <1000>;
full-duplex;
pause;
asym-pause;
};
};
...
Now... the question is, i would like to change the speed of the phy from 1000 to 100, i'm able to do it if configure the phy AND change the device tree, but this implies a reboot of the device to load a new dts file.
Is there a way to get it working runtime?
Thanks a lot,
Marco

Answer 1

In order to add or load a device tree blob in runtime, the only way is to use Linux overlay.
But the problem is that NXP does not support it in its linux-imx kernel , you can see their post about it here.
If you don't make the change permanently to your main device tree file used for the image, I can advise you of doing this:
Create another dts (Ex: new-phy.dts) that includes the main dts and add your override node there
Add the new dtb name to your ${MACHINE}.conf KERNEL_DEVICETREE variable:
KERNEL_DEVICETREE += "freescale/new-phy.dtb"
Now you need to choose it once you boot from u-boot CLI, like:
u-boot> setenv fdt_file new-phy.dtb
u-boot> saveenv
u-boot> boot
Or, you can set it in your u-boot/configs/${MACHINE}_defconfig:
CONFIG_DEFAULT_FDT_FILE="new-phy.dtb"
Otherwise, you can try to add the overlay support for the kernel you are using.
Toradex has SOMs and EVKs based on IMX8M and they are working with overlays, you can take a look here and try to understand what they did to support it.