Devicetree in QEMU¶
For QEMU on ARM, RISC-V and one PPC target, the devicetree is created on-the-fly by QEMU. It is intended for use in Linux but can be used by U-Boot also, so long as any nodes/properties needed by U-Boot are merged in.
When CONFIG_OF_BOARD is enabled
Obtaining the QEMU devicetree¶
Where QEMU generates its own devicetree to pass to U-Boot you can use -dtb u-boot.dtb to force QEMU to use U-Boot’s in-tree version.
To obtain the devicetree that qemu generates, add -machine dumpdtb=qemu.dtb, e.g.:
qemu-system-arm -machine virt -machine dumpdtb=qemu.dtb qemu-system-aarch64 -machine virt -machine dumpdtb=qemu.dtb qemu-system-riscv64 -machine virt -machine dumpdtb=qemu.dtb
Merging in U-Boot nodes/properties¶
Various U-Boot features require nodes and properties in the U-Boot devicetree and at present QEMU is unaware of these. To use these you must manually merge in the appropriate pieces.
One way to do this is with dtc. This command runs dtc on each .dtb file in turn, to produce a text file. It drops the duplicate header on the qemu one. Then it joins them up and runs them through dtc to compile the output:
qemu-system-arm -machine virt -machine dumpdtb=qemu.dtb cat <(dtc -I dtb qemu.dtb) <(dtc -I dtb u-boot.dtb | grep -v /dts-v1/) | dtc - -o merged.dtb
You can then run qemu with the merged devicetree, e.g.:
qemu-system-arm -machine virt -nographic -bios u-boot.bin -dtb merged.dtb
Note that there seems to be a bug in some versions of qemu where the output of dumpdtb does not quite match what is provided to U-Boot.