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

[Ard Biesheuvel]

On Tue, 7 Jul 2020 at 10:39, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> 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"?
>

No.

What the code does today is round *up* to a multiple of 128 MB, and
only when that leads to a problem, we should use the DT provided
memory regions.