[PATCH 3/3] doc: Make a start on docs for fdt command

Simon Glass sjg at chromium.org
Wed Jul 13 14:07:00 CEST 2022

Add some information about the 'fdt addr' subcommand, to get things

Signed-off-by: Simon Glass <sjg at chromium.org>

 doc/usage/cmd/fdt.rst | 69 +++++++++++++++++++++++++++++++++++++++++++
 doc/usage/index.rst   |  1 +
 2 files changed, 70 insertions(+)
 create mode 100644 doc/usage/cmd/fdt.rst

diff --git a/doc/usage/cmd/fdt.rst b/doc/usage/cmd/fdt.rst
new file mode 100644
index 00000000000..07fed732e45
--- /dev/null
+++ b/doc/usage/cmd/fdt.rst
@@ -0,0 +1,69 @@
+.. SPDX-License-Identifier: GPL-2.0+
+fdt command
+    fdt addr [-cq] [addr [len]]
+The fdt command provides access to flat device tree blobs in memory. It has
+many subcommands, some of which are not documented here.
+    Select the control FDT (otherwise the working FDT is used).
+    Don't display errors
+The control FDT is the one used by U-Boot itself to control various features,
+including driver model. This should only be changed if you really know what you
+are doing, since once U-Boot starts it maintains pointers into the FDT from the
+various driver model data structures.
+The working FDT is the one passed to the Operating System when booting. This
+can be freely modified, so far as U-Boot is concerned, since it does not affect
+U-Boot's operation.
+fdt addr
+With no arguments, this shows the address of the current working or control
+If the `addr` argument is provided, then this sets the address of the working or
+control FDT to the provided address.
+If the `len` argument is provided, then the device tree is expanded to that
+size. This can be used to make space for more nodes and properties. It is
+assumed that there is enough space in memory for this expansion.
+Get the control address and copy that FDT to free memory::
+    => fdt addr -c
+    Control fdt: 0aff9fd0
+    => cp.b 0aff9fd0 10000 10000
+    => md 10000 4
+    00010000: edfe0dd0 5b3d0000 78000000 7c270000  ......=[...x..'|
+The second word shows the size of the FDT. Now set the working FDT to that
+address and expand it to 0xf000 in size::
+    => fdt addr 10000 f000
+    => md 10000 4
+    00010000: edfe0dd0 00f00000 78000000 7c270000  ...........x..'|
+Return value
+The return value $? indicates whether the command succeeded.
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 8b98629d6bf..16a3db5c000 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -43,6 +43,7 @@ Shell commands
+   cmd/fdt

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