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

[AKASHI Takahiro]

Heinrich,

On Sat, Aug 14, 2021 at 09:21:25AM +0200, Heinrich Schuchardt wrote:
> 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>

If I correctly understand here, Simon's intent is to reassure
his opinion that a public key for capsule authentication should
be stored in a device tree, not be embedded in U-Boot binary.
So at the end, do you agree to reverting[1] Ilias' patch?

-Takahiro Akashi

[1] https://lists.denx.de/pipermail/u-boot/2021-August/456968.html


> > ---
> > 
> > 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.
> > 
>