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

[Samuel Holland]

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.

> +
>  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.

>  	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().

Regards,
Samuel

>  
>  	return 0;
>  }