[PATCH v2 04/12] x86: Make sure the SPL image ends on a suitable boundary

[Bin Meng]

On Mon, Jan 25, 2021 at 1:06 AM Simon Glass <sjg at chromium.org> wrote:
>
> The part of U-Boot that actually ends up in u-boot-nodtb.bin is not built
> with any particular alignment. It ends at the start of the BSS section.
> The BSS section selects its own alignment, which may larger.

may be

> This means that there can be a gap of a few bytes between the image
> ending and BSS starting.
>
> Since u-boot.bin is build by joining u-boot-nodtb.bin and u-boot.dtb (with
> perhaps some padding for BSS), the expected result is not obtained. U-Boot
> uses the end of BSS to find the devicetree, so this means that it cannot
> be found.
>
> Add 32-byte alignment of BSS so that the image size is correct and
> appending the devicetree will place it at the end of BSS.
>
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>
> Example SPL output without this patch:
>
> Sections:
> Idx Name          Size      VMA       LMA       File off  Algn
>   0 .text         000142a1  fef40000  fef40000  00001000  2**4
>                   CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
>   1 .u_boot_list  000014a4  fef542a8  fef542a8  000152a8  2**3
>                   CONTENTS, ALLOC, LOAD, RELOC, DATA
>   2 .rodata       0000599c  fef55760  fef55760  00016760  2**5
>                   CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
>   3 .data         00000970  fef5b100  fef5b100  0001c100  2**5
>                   CONTENTS, ALLOC, LOAD, RELOC, DATA
>   4 .binman_sym_table 00000020  fef5ba70  fef5ba70  0001ca70  2**2
>                   CONTENTS, ALLOC, LOAD, DATA
>   5 .bss          00000060  fef5baa0  fef5baa0  00000000  2**5
>                   ALLOC
>
> You can see that .bss is aligned to 2**5 (32 bytes). This is because of
> the mallinfo struct in dlmalloc.c:
>
>  17 .bss.current_mallinfo 00000028  00000000  00000000  000004c0  2**5
>                   ALLOC
>
> In this case the size of u-boot-spl-nodtb.bin is 0x1ba90. This matches up
> with the _image_binary_end symbol:
>
> fef5ba90 g       .binman_sym_table      00000000 _image_binary_end
>
> But BSS starts 16 bytes later, at 0xfef5baa0, due to the 32-byte
> alignment. So we must align _image_binary_end to a 32-byte boundary. This
> forces the binary size to be 0x1baa0, i.e. ending at the start of bss, as
> expected.
>
> Note that gcc reports __BIGGEST_ALIGNMENT__ of 16 on this build, even
> though it generates an object file with a member that requests 32-byte
> alignment.
>
> The current_mallinfo struct is 40 bytes in size. Increasing the struct to
> 68 bytes (i.e. just above a 64-byte boundary) does not cause the alignment
> to go above 32 bytes. So it seems that 32 bytes is the maximum alignment
> at present.
>

The above information is very useful for people to understand such
tricky changes.

I can put such in the commit message when applying.

> Changes in v2:
> - Add comment to .lds file
> - Add more notes to the commit
>
>  arch/x86/cpu/u-boot-spl.lds | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>

Reviewed-by: Bin Meng <bmeng.cn at gmail.com>