Linux Device Tree (DTS) Manual

Sometimes we need to modify device tree to adjust functions of the
board. For example: switch M2 slot function between SATA/PCIE, change
CAN clock rate, etc. This tutorial will teach you how to find the
right device tree file and some notes about modifying it.

Find the Device Tree

First you need to learn

How to Build Linux SDK

, after that you should understand that the configurations about
building are defined in mk files under "SDK/device/rockchip/rk3588/".

Assuming that you need to build an ITX-3588J Ubuntu firmware that
supports mipi screen, then the corresponding mk file is "itx-3588j-
BE45-A1-ubuntu.mk"

cd to "SDK/device/rockhip/rk3588", find and open "itx-3588j-
BE45-A1-ubuntu.mk", you can see the dts:

   CMD=`realpath $BASH_SOURCE`
   CUR_DIR=`dirname $CMD`

   source $CUR_DIR/itx-3588j-ubuntu.mk

   # Kernel dts
   export RK_KERNEL_DTS=rk3588-firefly-itx-3588j-mipi101-M101014-BE45-A1

"RK_KERNEL_DTS" shows the dts. No suffix here, so the full name with
suffix is "rk3588-firefly-itx-3588j-mipi101-M101014-BE45-A1.dts"

Some mk file may not have "RK_KERNEL_DTS", such as the "itx-3588j-
kylin.mk" for building kylin firmware:

   CMD=`realpath $BASH_SOURCE`
   CUR_DIR=`dirname $CMD`

   DEVICE_NAME=$(echo ${CMD} | awk -F '/' '{print $NF}')
   DEVICE_NAME=${DEVICE_NAME%\-*}
   source $CUR_DIR/${DEVICE_NAME}-ubuntu.mk

   # parameter for GPT table
   export RK_PARAMETER=parameter-kylin.txt

It is because the file sourced another file
"${DEVICE_NAME}-ubuntu.mk", "${DEVICE_NAME}" is a variable, it means
the model of the board. So the "RK_KERNEL_DTS" actually defined in
"itx-3588j-ubuntu.mk".

When you get the name of dts, then you can find it under
"SDK/kernel/arch/arm64/boot/dts/rockchip/".

Modify the Device Tree

Open "SDK/kernel/arch/arm64/boot/dts/rockchip/rk3588-firefly-itx-
3588j-mipi101-M101014-BE45-A1.dts" you will find this file is
basically empty, where are the device tree nodes?

   // SPDX-License-Identifier: (GPL-2.0+ OR MIT)
   /*
    * Copyright (c) 2021 Rockchip Electronics Co., Ltd.
    *
    */
   /dts-v1/;
   #include "rk3588-firefly-itx-3588j-mipi101-M101014-BE45-A1.dtsi"

   / {
           model = "Firefly ITX-3588J MIPI(Linux)";
           compatible = "rockchip,rk3588-firefly-itx-3588j-mipi101-M101014-BE45-A1", "rockchip,rk3588";

   };

It included another file "rk3588-firefly-itx-3588j-
mipi101-M101014-BE45-A1.dtsi", note that the suffix is dtsi, open this
dtsi file you can find many device tree nodes. This dtsi file will
also include some other files.

So device tree is not a single file but a goup of dts/dtsi files:

   The basic relationship is:
   Upper layer <--------------------> Lower layer
   A.dts include B.dtsi include C.dtsi ... N.dtsi

If you can't find a node in A.dts, then it may be in B.dtsi, or C.dtsi
... follow the inlcude relationship.

Configs in upper layer will overwrite that in lower layer. For
example: disable uart0 in C.dtsi but enable uart0 in A.dts, the
configs in A.dts will take effect, uart0 will be enabled.

A dtsi file may be included by many files, modify it will influence
many models of boards. So check it before modifying.

Device tree config instructions of functions like Ethernet, PCIE,
UART, etc. can be found under "SDK/docs/". Modifying device tree needs
basic knowledge of kernel and dts grammar, and you need to understand
what you are going to do. Improper modifications will cause abnormal
functions or even boot failure.

Compile and Download

Return to SDK root after modifying, use "build.sh" to config and
compile:

   # select mk file
   ./build.sh itx-3588j-BE45-A1-ubuntu.mk

   # compile kernel
   ./build.sh extboot

The result will be under "SDK/kernel": an image "extboot.img" and a
directory "extboot"

Download to board(choose one method):

Copy all files under SDK/kernel/extboot/ to the /boot/ of board, and
reboot the board.

Or follow the kernel part of Upgrade Firmware to download extboot.img.