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arm: riscv: qemu: Explain how to extract the generated dt

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QEMU currently generates a devicetree for use with U-Boot. Explain how to
obtain it.

Also explain how to merge it to produce a devicetree with the U-Boot
features included.

Signed-off-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Simon Glass 2021-12-16 20:59:09 -07:00 committed by Tom Rini
parent 6d7636c946
commit c3c1614537
4 changed files with 55 additions and 0 deletions
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3
doc/board/emulation/qemu-arm.rst
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@ -21,6 +21,9 @@ The 'virt' platform provides the following as the basic functionality:
Additionally, a number of optional peripherals can be added to the PCI bus.
See :doc:`../../develop/devicetree/dt_qemu` for information on how to see
the devicetree actually generated by QEMU.
Building U-Boot
---------------
Set the CROSS_COMPILE environment variable as usual, and run:

3
doc/board/emulation/qemu-riscv.rst
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@ -13,6 +13,9 @@ The QEMU virt machine models a generic RISC-V virtual machine with support for
the VirtIO standard networking and block storage devices. It has CLINT, PLIC,
16550A UART devices in addition to VirtIO and it also uses device-tree to pass
configuration information to guest software. It implements RISC-V privileged
See :doc:`../../develop/devicetree/dt_qemu` for information on how to see
the devicetree actually generated by QEMU.
architecture spec v1.10.
Building U-Boot

48
doc/develop/devicetree/dt_qemu.rst Normal file
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@ -0,0 +1,48 @@
.. SPDX-License-Identifier: GPL-2.0+
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 tou 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.

1
doc/develop/devicetree/index.rst
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@ -11,3 +11,4 @@ build-time and runtime configuration.
intro
control
dt_qemu