[PATCH v6 03/25] arm: riscv: qemu: Explain how to extract the generated dt

[Heinrich Schuchardt]

On 12/2/21 16:58, Simon Glass wrote:
> 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.

Information like the number of CPUs and the memory must be accurate in
the devicetree and match the call parameter of QEMU.

This worked without this series and must work afterwards. How do you
achieve this?

Best regards

Heinrich


>
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>
> (no changes since v5)
>
> Changes in v5:
> - Merge RISC-V and ARM patches since they are similar
>
>   doc/board/emulation/qemu-arm.rst   |  3 ++
>   doc/board/emulation/qemu-riscv.rst |  3 ++
>   doc/develop/devicetree/dt_qemu.rst | 48 ++++++++++++++++++++++++++++++
>   doc/develop/devicetree/index.rst   |  1 +
>   4 files changed, 55 insertions(+)
>   create mode 100644 doc/develop/devicetree/dt_qemu.rst
>
> diff --git a/doc/board/emulation/qemu-arm.rst b/doc/board/emulation/qemu-arm.rst
> index 8bd7b60cdb4..ff64ec08c3e 100644
> --- a/doc/board/emulation/qemu-arm.rst
> +++ b/doc/board/emulation/qemu-arm.rst
> @@ -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:
> diff --git a/doc/board/emulation/qemu-riscv.rst b/doc/board/emulation/qemu-riscv.rst
> index 4b8e104a215..3409fff8117 100644
> --- a/doc/board/emulation/qemu-riscv.rst
> +++ b/doc/board/emulation/qemu-riscv.rst
> @@ -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
> diff --git a/doc/develop/devicetree/dt_qemu.rst b/doc/develop/devicetree/dt_qemu.rst
> new file mode 100644
> index 00000000000..1392a2cae97
> --- /dev/null
> +++ b/doc/develop/devicetree/dt_qemu.rst
> @@ -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.
> diff --git a/doc/develop/devicetree/index.rst b/doc/develop/devicetree/index.rst
> index b5b33dfea0f..fc2fb41b1bb 100644
> --- a/doc/develop/devicetree/index.rst
> +++ b/doc/develop/devicetree/index.rst
> @@ -12,3 +12,4 @@ build-time and runtime configuration.
>      intro
>      control
>      dt_update
> +   dt_qemu
>