[PATCH 3/3] doc: Add a note about why devicetree is used

[Simon Glass]

This question comes up every now and then with people coming from Linux.
Add some notes about it so we can point to it in the mailing list.

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

 doc/develop/devicetree/control.rst |  2 ++
 doc/develop/devicetree/intro.rst   | 39 ++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
index ac5f7989cc6..f98faee20d1 100644
--- a/doc/develop/devicetree/control.rst
+++ b/doc/develop/devicetree/control.rst
@@ -174,6 +174,8 @@ ways:
     $ make DEVICE_TREE=<dts-file-name>
 
 
+.. _dttweaks:
+
 Adding tweaks for U-Boot
 ------------------------
 
diff --git a/doc/develop/devicetree/intro.rst b/doc/develop/devicetree/intro.rst
index 344851327c7..e03d8e1ae66 100644
--- a/doc/develop/devicetree/intro.rst
+++ b/doc/develop/devicetree/intro.rst
@@ -2,3 +2,42 @@
 
 Devicetree Introduction
 =======================
+
+U-Boot uses devicetree for configuration. This includes the devices used by
+the board, the format of the image created with binman, which UART to use for
+the console, public keys used for secure boot and many other things.
+
+See :doc:`control` for more information.
+
+Why does U-Boot put <thing> in the devicetree?
+----------------------------------------------
+
+This question comes up a lot with people new to U-Boot, particular those coming
+from Linux who are used to quite strict rules about what can go in the
+devicetree.
+
+U-Boot uses the same devicetree as Linux but adds more things necessary for the
+bootloader environment (see :ref:`dttweaks`).
+
+U-Boot does not have a user space to provide policy and configuration. It cannot
+do what Linux does and run programs and look up filesystems to figure out how to
+boot. So configuration / runtime info go in the devicetree in U-Boot.
+
+Of course it is possible to:
+
+- add tables into the rodata
+- append some info to the end of U-Boot in a different format
+- modify the link script to bring in a file with some info in it
+- put things in ACPI tables
+- compile in a UEFI hand-off block structure and put things in there
+
+but *please don't*. In general, devicetree is the sane place to hold U-Boot's
+configuration.
+
+So please...do NOT ask why U-Boot puts <thing> in the devicetree. It is the only
+place it can go. It is a highly suitable data structure for just about anything
+that U-Boot needs to know at runtime.
+
+Note, it is possible to use platdata directly so drivers avoid devicetreee in
+SPL. But of-platdata is the modern way of avoiding devicetree overhead, so
+please use that instead.
-- 
2.32.0.432.gabb21c7263-goog