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 ---
Changes in v2: - Updates based on suggestions from Heinrich
doc/develop/devicetree/control.rst | 2 ++ doc/develop/devicetree/intro.rst | 40 ++++++++++++++++++++++++++++++ 2 files changed, 42 insertions(+)
diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst index 9f2c21034d0..b33e2a7ead6 100644 --- a/doc/develop/devicetree/control.rst +++ b/doc/develop/devicetree/control.rst @@ -152,6 +152,8 @@ 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..36e8cc0d440 100644 --- a/doc/develop/devicetree/intro.rst +++ b/doc/develop/devicetree/intro.rst @@ -2,3 +2,43 @@
Devicetree Introduction ======================= + +U-Boot uses a 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 into 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 and runtime information goes into the devicetree in +U-Boot. + +Of course it is possible to: + +- add tables into the rodata section of the U-Boot binary +- append some info to the end of U-Boot in a different format +- modify the linker script to bring in a file with some info in it +- put things in ACPI tables +- link 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.