On 7/25/21 6:44 PM, Simon Glass wrote:
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@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
%s/devicetree/devicetrees/ or 'uses a'
+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
%s/in/into/ ?
+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.
Do you mean 'configuration and runtime' or 'runtime configuration'?
%s/info go in/information goes into/
+Of course it is possible to:
+- add tables into the rodata
%s/rodata/rodata section of the U-Boot binary/
+- 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
%s/link/linker/
+- put things in ACPI tables
%s/in/into/ ?
+- compile in a UEFI hand-off block structure and put things in there
%s/compile/link/ ?
+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
%s/So please.../So, please, /
Best regards
Heinrich
+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.