[U-Boot] CONFIG_SYS_FDT_PAD: Why do we need this?

[Simon Glass]

Hi Dirk,

On Sat, Mar 24, 2012 at 11:59 PM, Dirk Behme <dirk.behme at googlemail.com> wrote:
>
> In boot_relocate_fdt() [1] we increase the size of the loaded fdt by a
> predefined, hard coded CONFIG_SYS_FDT_PAD. The basic idea seem to be to
> create some space for adding additional information later, like the
> bootargs.
>
> I wonder why we have to do this by a hard coded, predefined value (which is
> most probably always to large)? Why can't we increase 'totalsize' in the
> fdt_header just if we add anything? Individually by the size of the element
> added, at runtime? E.g. by anything like [2]?
>
> I debugged a system were
>
> #define CONFIG_SYS_FDT_PAD 0x3000
>
> was used. These 0x3000 are added to totalsize before something is added at
> all. Then, if an element is added, always the whole 0x3000 bytes have to be
> memmoved additionally. And in the end only ~0x100 additional space was
> really needed.

I don't really know the answer, but I guess that just increasing the
FDT is dangerous, since we don't know how big it can grow before
running into something else. This way we set a maximum size that it
can grow to and will produce an error if it is exceeded.

There is also the practical problem that we would need to check for
FDT_ERR_NOSPACE, expand the space, and then retry. A bit of a
hassle...

I suppose you could use fdt_pack() afterwards, although I don't see a
command for this at present.

Regards,
Simon

>
> Best regards
>
> Dirk
>
> [1]
> http://git.denx.de/cgi-bin/gitweb.cgi?p=u-boot.git;a=blob;f=common/image.c;h=8c644b7dacdc9b7a196a5238f4dc5d94bd615d07;hb=HEAD#l1285
>
> [2]
>
> static int _fdt_splice_struct(void *fdt, void *p,
>                              int oldlen, int newlen)
> {
>        int delta = newlen - oldlen;
>        int err;
>
>        if ((err = _fdt_splice(fdt, p, oldlen, newlen)))
>                return err;
>
>        fdt_set_size_dt_struct(fdt, fdt_size_dt_struct(fdt) + delta);
>        fdt_set_off_dt_strings(fdt, fdt_off_dt_strings(fdt) + delta);
> +       fdt_set_totalsize(fdt, fdt_totalsize(fdt) + delta);
>        return 0;
> }
>
> in
>
> http://git.denx.de/cgi-bin/gitweb.cgi?p=u-boot.git;a=blob;f=lib/libfdt/fdt_rw.c;h=5c27a677e3536ed7320ee399113666975e5d545b;hb=HEAD#l127
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