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

[Samuel Holland]

Hi Anup,

On 2025-12-07 11:53 PM, Anup Patel wrote:
> On Sun, Dec 7, 2025 at 4:29 PM Samuel Holland <samuel.holland at sifive.com> wrote:
>> 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.

It's still a U-Boot bug, no? Is your plan to keep the workaround indefinitely?

>>> +
>>>  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.

I don't have an opinion on whether it is better to change libfdt, but we need to
add the memset() in at least one place to avoid regressing here.

Regards,
Samuel