[PATCH/RFC v7] ARM: boot: Obtain start of physical memory from DTB

[Geert Uytterhoeven]

Hi Ard,

On Tue, Jul 7, 2020 at 8:50 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> On Mon, 6 Jul 2020 at 18:02, Geert Uytterhoeven <geert+renesas at glider.be> wrote:
> > Currently, the start address of physical memory is obtained by masking
> > the program counter with a fixed mask of 0xf8000000.  This mask value
> > was chosen as a balance between the requirements of different platforms.
> > However, this does require that the start address of physical memory is
> > a multiple of 128 MiB, precluding booting Linux on platforms where this
> > requirement is not fulfilled.
> >
> > Fix this limitation by obtaining the start address from the DTB instead,
> > if available (either explicitly passed, or appended to the kernel).
> > Fall back to the traditional method when needed.
> >
> > This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM
> > on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space),
> > i.e. not at a multiple of 128 MiB.
> >
> > Suggested-by: Nicolas Pitre <nico at fluxnic.net>
> > Signed-off-by: Geert Uytterhoeven <geert+renesas at glider.be>
> > Reviewed-by: Nicolas Pitre <nico at fluxnic.net>
> > Reviewed-by: Ard Biesheuvel <ardb at kernel.org>
> > Tested-by: Marek Szyprowski <m.szyprowski at samsung.com>
> > Tested-by: Dmitry Osipenko <digetx at gmail.com>
> > Cc: Lukasz Stelmach <l.stelmach at samsung.com>
> > ---
> > Marked as RFC, because:
> >   1. This is known to break crashkernel support, as the memory used by
> >      the crashkernel is not marked reserved in DT (yet),
> >   2. Russell won't apply this for v5.9 anyway,
> >
>
> Would it help if we make this behavior dependent on a simple heuristic, e.g.,
>
> if (round_up(load_address, 128M) >= dram_end)
>   use dram_start from DT
> else
>   use round_up(load_address, 128M)
>
> That way, the fix is guaranteed to only take effect for systems that
> cannot even boot otherwise, which fixes the crashkernel case, as well
> as other potential regressions due to the load address of the core
> kernel changing for existing boards.

Thanks for your suggestion!
  1. Shouldn't the calculation use round_down() instead of round_up()?
  2. Likewise, "round_down(load_address, 128M) < dram_start from DT"?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds