[PATCH v2 3/3] doc: Add a note about why devicetree is used

Heinrich Schuchardt xypron.glpk at gmx.de
Sat Aug 14 09:21:25 CEST 2021


On 8/2/21 2:57 AM, 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 at chromium.org>

Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>

> ---
>
> 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.
>



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