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 <s...@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
--
2.34.0.rc2.393.gf8c9666880-goog
