Usage of device-tree for blobs

[Simon Glass]

Hi Heinrich,

On Wed, 25 Aug 2021 at 02:05, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> Hello Simon,
>
> some boards like qemu-riscv64_defconfig do not use any device-tree at
> build time. A device-tree is only supplied at runtime by the prior boot
> stage (CONFIG_OF_PRIOR_STAGE=y).
>
> In doc/develop/devicetree/intro.rst you suggest to put binary blobs into
> the device-tree.
>
> Could you, please, update the documentation to explain how adding blobs
> to the device-tree works in the different scenarios depending on the
> values of:
>
> CONFIG_OF_EMBED
> CONFIG_OF_SEPARATE
> CONFIG_OF_BOARD
> CONFIG_OF_HOSTFILE
> CONFIG_OF_PRIOR_STAGE
>
> It would be especially important to understand how one can develop a
> board independent feature which works for all of these settings.

OK I will take a crack at this tomorrow.

But I think there is a disconnect here, since the only options that
matter within U-Boot are OF_SEPARATE and OF_HOSTFILE, which both use a
u-boot.dtb file. There is nothing tricky here.

The OF_BOARD business is for when the board does special things.
Presumably signing will do special things too. We cannot really know
what those things are because the board as 'opted out' of the standard
options.

>
> Please, describe CONFIG_OF_PRIOR_STAGE in
> doc/develop/devicetree/control.rst.

So I'm not allowed to delete that option? :-) It seems to me to be
extremely sparse on documentation. We need an explanation of what it
means and what effect it has on the build system, U-Boot and some
discussion of how qemu works. It seems to have been added as part of
an unrelated broadcom commit. The tags were incorrect so I doubt
anyone noticed it. Since then it has apparently proved useful
elsewhere, but no one has added more docs.

So perhaps you can help me with my doc by explaining how
OF_PRIOR_STAGE works in qmeu, why the DT in U-Boot cannot be used, and
which project actually hosts the DT that qemu provides? Armed with
that information, I might be able to make sense of it all.

Regards,
Simon