Re: [PATCH 1/6] disk: part_efi: Size the partition entry array from the block size

On 2026-07-31T12:39:59, Alexey Charkov <[email protected]> wrote:
> disk: part_efi: Size the partition entry array from the block size
>
> gpt_fill_header() computes last_usable_lba and first_usable_lba from
> hardcoded block counts that silently assumes 512-byte sectors and the
> default number of partition entries.
>
> The partition entry array holds GPT_ENTRY_NUMBERS entries of 128 bytes
> each, so the number of blocks it needs depends on both the entry count
> and the block size. write_gpt_table() derives that count correctly and
> writes the backup array at last_usable_lba + 1, so whenever the two
> disagree the backup array no longer immediately precedes the backup GPT
> header. Furthermore, on a device with 4096-byte native sectors, such as
> UFS flash, and the default 128 entries, the array is 4 blocks rather than
> 32. Current code reserves 34 blocks at each end of the disk for any block
> size and wastes about 114 KiB at each end.
>
> Add a new helper gpt_pte_blocks() and use it in all four places which
> currently calculate the number of blocks each in its own way, so the
> layout written by gpt_fill_header() and the extent written by
> write_gpt_table() cannot drift apart again. With this, first_usable_lba
> on 4096-byte sectors is 6, last_usable_lba is lba - 6, and the backup
> array occupies lba - 5 .. lba - 2, immediately preceding the backup
> header, as the UEFI specification describes.
>
> Note that this changes the 512-byte layout too for boards that do not use
> the default entry count. CONFIG_EFI_PARTITION_ENTRIES_NUMBERS is
> "default 56 if ARCH_SUNXI" and is set to 64 by a number of Rockchip
> defconfigs:
>
>   entries  array blocks  first_usable_lba  last_usable_lba
>        56            14        34 -> 16    lba - 34 -> lba - 16
>        64            16        34 -> 18    lba - 34 -> lba - 18
>       128            32        34 -> 34    lba - 34 -> lba - 34
>
> Existing partition tables stay readable either way, since is_gpt_valid()
> locates the entry array from the on-disk partition_entry_lba. Only newly
> written tables change. Partitions given without an explicit start= will
> now be placed lower on those boards; on sunxi first_usable_lba lands on
> the 8 KiB SPL offset, so such boards should keep specifying start=
> explicitly.
>
> While here, report both LBAs when the requested layout does not fit, since
> the existing "Partitions layout exceeds disk size" debug message gives no
> clue as to by how much, making it less helpful in debugging.
>
> Co-developed-by: Anton Burticica <[email protected]>
> Signed-off-by: Anton Burticica <[email protected]>
> Signed-off-by: Alexey Charkov <[email protected]>
>
> disk/part_efi.c | 47 +++++++++++++++++++++++++++++++++++------------
>  1 file changed, 35 insertions(+), 12 deletions(-)

Reviewed-by: Simon Glass <[email protected]>