[PATCH v2] platform: generic: Keep some empty space in FDT passed to next stage

[Anup Patel]

On Sun, Dec 7, 2025 at 4:29 PM Samuel Holland <samuel.holland at sifive.com> wrote:
>
> Hi Anup,
>
> On 2025-11-30 11:00 PM, Anup Patel wrote:
> > Leaving no empty space in the FDT passed to the next booting stage
> > causes the following U-Boot crash on Ventana internal platforms:
> >
> > Unhandled exception: Load access fault
> > EPC: 00000000fffa6372 RA: 00000000fffa7418 TVAL: 0001746174730068
> > EPC: 0000000080245372 RA: 0000000080246418 reloc adjusted
> >
> > SP:  00000000fef38440 GP:  00000000fef40e60 TP:  0000000000000000
> > T0:  00000000fef40a70 T1:  000000000000ff00 T2:  0000000000000000
> > S0:  00000000fffc17a8 S1:  00000000fef38d40 A0:  7375746174730068
> > A1:  00000000fffc17a8 A2:  0000000000000010 A3:  0000000000000010
> > A4:  0000000000000000 A5:  00000000fffc17b8 A6:  0000000000ff0000
> > A7:  000000000000b100 S2:  0000000000000000 S3:  0000000000000001
> > S4:  00000000fef38d40 S5:  7375746174730068 S6:  0000000000000000
> > S7:  00000000fef4eef0 S8:  00000000fef4ef90 S9:  0000000000000000
> > S10: 0000000000000000 S11: 00000000fef4efc0 T3:  00000000fef40ea8
> > T4:  0000000000ff0000 T5:  00000000fef40a60 T6:  00000000fef40a6c
> >
> > To address the above issue, keep some minimal empty space in the
> > FDT instead of no empty space.
> >
> > Fixes: bbe9a23060e9 ("platform: generic: Pack the FDT after applying fixups")
> > Signed-off-by: Anup Patel <apatel at ventanamicro.com>
> > ---
> > Changes since v1:
> > - Add Kconfig option for amount of empty space in FDT with default
> >   value as 4KB
> > ---
> >  platform/generic/Kconfig    | 5 +++++
> >  platform/generic/platform.c | 3 +++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/platform/generic/Kconfig b/platform/generic/Kconfig
> > index 25b8886b..b1808012 100644
> > --- a/platform/generic/Kconfig
> > +++ b/platform/generic/Kconfig
> > @@ -23,6 +23,11 @@ config PLATFORM_GENERIC_MINOR_VER
> >       range 0 65535
> >       default 1
> >
> > +config PLATFORM_GENERIC_FDT_EMPTY_SPACE
> > +     int "Platform FDT empty space (KB)"
> > +     range 0 1024
> > +     default 4
>
> Since this is a workaround for a specific platform, and the workaround should
> eventually be obsoleted by a U-Boot fix, please default this to zero.

Although the issue manifests on Ventana internal platforms,
the issue is not Ventana specific and can happen on any platform
where U-Boot tries to modify FDT passed by OpenSBI without
adding extra space.

>
> > +
> >  config PLATFORM_ALLWINNER_D1
> >       bool "Allwinner D1 support"
> >       depends on FDT_IRQCHIP_PLIC
> > diff --git a/platform/generic/platform.c b/platform/generic/platform.c
> > index e66f99fa..ddfc9e80 100644
> > --- a/platform/generic/platform.c
> > +++ b/platform/generic/platform.c
> > @@ -245,7 +245,10 @@ int generic_final_init(bool cold_boot)
> >       fdt_fixups(fdt);
> >       fdt_domain_fixup(fdt);
> >
> > +     /* Minimize the empty space in FDT to 4KB */
>
> This comment is no longer accurate, since the padding is not a fixed size.

Sure, I will update.

>
> >       fdt_pack(fdt);
> > +     fdt_open_into(fdt, fdt, fdt_totalsize(fdt) +
> > +                   CONFIG_PLATFORM_GENERIC_FDT_EMPTY_SPACE * 1024);
>
> This change reintroduces the problem that motivated adding fdt_pack().
> fdt_open_into() does not initialize the unused space, so once again the range
> [fdt, fdt + fdt_totalsize(fdt)) contains unitialized data.
>
> If having unitialized data in the FDT blob is not a problem on the platform
> requiring this workaround, then I suppose the code is fine. Otherwise, this
> would need a memset().

I think it is better to do memset() on additional space in fdt_open_into()
itself but that would be a separate change in libfdt.

Regards,
Anup