[PATCH v5 7/8] libfdt: Allow non-64b aligned memreserve entries

[Leo Liang]

Hi Uros,

On Wed, Dec 24, 2025 at 03:47:17PM +0000, Uros Stajic wrote:
> From: Chao-ying Fu <cfu at mips.com>
> 
> Although memreserve entries in an FDT are 64-bit aligned relative to the
> start of the FDT, we cannot guarantee that the FDT itself is 64-bit aligned
> in memory. This is especially common when using a FIT image, where the
> alignment of the embedded DTB cannot be controlled.
> 
> On systems that do not support unaligned 64-bit memory accesses, this leads
> to faults when accessing the memreserve section before the FDT is relocated.
> To resolve this, copy the 64-bit values into suitably aligned on-stack
> variables before accessing them.
> 
> Signed-off-by: Chao-ying Fu <cfu at mips.com>
> Signed-off-by: Uros Stajic <uros.stajic at htecgroup.com>
> ---
>  scripts/dtc/libfdt/fdt_ro.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/scripts/dtc/libfdt/fdt_ro.c b/scripts/dtc/libfdt/fdt_ro.c
> index d65656aaa8b..1c18aea66ab 100644
> --- a/scripts/dtc/libfdt/fdt_ro.c
> +++ b/scripts/dtc/libfdt/fdt_ro.c
> @@ -174,6 +174,7 @@ static const struct fdt_reserve_entry *fdt_mem_rsv(const void *fdt, int n)
>  
>  int fdt_get_mem_rsv(const void *fdt, int n, uint64_t *address, uint64_t *size)
>  {
> +	uint64_t u64;
>  	const struct fdt_reserve_entry *re;
>  
>  	FDT_RO_PROBE(fdt);
> @@ -181,8 +182,10 @@ int fdt_get_mem_rsv(const void *fdt, int n, uint64_t *address, uint64_t *size)
>  	if (fdt_chk_extra() && !re)
>  		return -FDT_ERR_BADOFFSET;
>  
> -	*address = fdt64_to_cpu(re->address);
> -	*size = fdt64_to_cpu(re->size);
> +	memcpy(&u64, &re->address, sizeof(u64));
> +	*address = fdt64_to_cpu(u64);
> +	memcpy(&u64, &re->size, sizeof(u64));
> +	*size = fdt64_to_cpu(u64);

There are API for unaligned access: fdt64_ld().

>  	return 0;
>  }
>  
> @@ -190,9 +193,11 @@ int fdt_num_mem_rsv(const void *fdt)
>  {
>  	int i;
>  	const struct fdt_reserve_entry *re;
> +	uint64_t u64;
>  
>  	for (i = 0; (re = fdt_mem_rsv(fdt, i)) != NULL; i++) {
> -		if (fdt64_to_cpu(re->size) == 0)
> +		memcpy(&u64, &re->size, sizeof(u64));
> +		if (fdt64_to_cpu(u64) == 0)

Ditto.

>  			return i;
>  	}
>  	return -FDT_ERR_TRUNCATED;


But other than that, this issue seemed to be discussed before
and decided to use API that does not support unaligned access.
https://github.com/dgibson/dtc/commit/a7c40409934971ac1bd934ccc411bc6932b86564

Hi Tom,

Should we accept this patch in u-boot?

Best regards,
Leo