[PATCH v2] emulation: fdt: Allow using U-Boot's device tree with QEMU

[Ilias Apalodimas]

Hi Tom

Thanks for roping me in.

On Tue, 15 Apr 2025 at 01:53, Tom Rini <trini at konsulko.com> wrote:
>
> On Sun, Apr 06, 2025 at 07:07:04AM +1200, Simon Glass wrote:
>
> > At present it is impossible to change the qemu_arm64 defconfig to
> > obtain a devicetree from the U-Boot build.
> >
> > This is necessary for FIT validation, for example, where the signature
> > node must be compiled into U-Boot.

I'll repeat once more, that using the DT to store whatever random data
you invent makes little sense.
No one is obliged to follow internal U-Boot ABIs. Instead, it would
make much more sense to store the data in the U-Boot binary somewhere
and retrieve them. On top of that we now have proper memory
permissions at least for arm64 and you can place certificates in
.rodata.

> >
> > A proposed change to QEMU to allow device tree additions has been
> > blocked for several years.

It hasn't been blocked, it has been rejected.

> The only known workaround is to use QEMU's
> > dumpdtb option, merge in the signature node manually, disable
> > OF_HAS_PRIOR_STAGE and then start QEMU with special arguments. This is
> > complicated enough that it is documented in U-Boot[1].
>
> This paragraph and then
>
> [snip]
> > +       Note: this work-around is necessary since Linaro has blocked

Linaro hasn't blocked anything. The QEMU maintainers  NAK'ed the patch
multiple times for a very good reason.

FWIW apart from the commit message being misleading once again, the
overall architecture is just unnecessarily complicated.

Cheers
/Ilias

> > +       addition of a feature to support additions to the QEMU devicetree:
>
> This part of the help are overly antagonistic.
>
> [snip]
> > diff --git a/dts/Kconfig b/dts/Kconfig
> > index 2e88d210bb8..53f7e246a80 100644
> > --- a/dts/Kconfig
> > +++ b/dts/Kconfig
> > @@ -205,8 +205,13 @@ config OF_HAS_PRIOR_STAGE
> >         development purposes, but it is not recommended, and likely will not
> >         even work, for production systems.
> >
> > -       Note: This option must be set in Kconfig and cannot be enabled or
> > -       disabled in the board's defconfig file.
> > +       Note: This option can be disabled for QEMU usiing QEMU_MANUAL_DTB thus
> > +       allowing QEMU to support FIT validation, where the devicetree must
> > +       include a public key.
> > +
> > +       See also this for context:
> > +
> > +       https://patchwork.kernel.org/project/qemu-devel/patch/20210926183410.256484-1-sjg@chromium.org/#24481799
>
> And we don't need more wording here. The QEMU_MANUAL_DTB option itself
> is enough.
>
> --
> Tom