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

[Ard Biesheuvel]

On Tue, 7 Jul 2020 at 11:40, Ard Biesheuvel <ardb at kernel.org> wrote:
>
> On Tue, 7 Jul 2020 at 11:35, Ard Biesheuvel <ardb at kernel.org> wrote:
> >
> > On Tue, 7 Jul 2020 at 10:58, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> > >
> > > Hi Ard,
> > >
> > > On Tue, Jul 7, 2020 at 9:45 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> > > > On Tue, 7 Jul 2020 at 10:39, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> > > > > 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.
> > >
> > >                 mov     r4, pc
> > >                 and     r4, r4, #0xf8000000
> > >
> > > Surely this is rounding down, isn't it?
> > >
> >
> > Yes you are right.
> >
> > >                 add     r4, r4, #TEXT_OFFSET
> > >
> > > Followed by adding a small number (typically 0x00008000).
> > >
> > > On RZA2MEVB with 64 MiB of RAM, the result lies below dram_start.
> >
> > Yes, but in the general case, this is not true. Platforms that manage
> > to boot using the current arrangement will do so by putting the
> > decompressor above the first 128 MB aligned boundary covered by DRAM
> > (and lose access to any memory below it via the linear mapping, but
> > this memory could still be used via a no-map reserved-memory node
> > AFAIK.)
> >
> > > BTW, how to obtain dram_end? From DT again? Do we trust it, as we
> > > apparently cannot trust dram_start in some configurations.
> > >
> > > Do I need more coffee?
> > >
> >
> > Maybe we both do :-)
> >
> > AIUI, the reason we cannot trust dram_start is because of the
> > crashkernel case, i.e., the kernel may have deliberately been put high
> > up in memory, and the expectation is that the load address is derived
> > by rounding down the load address of the decompressor.
> >
> > Hence my suggestion to round *up* and compare with dram_end: if
> > round_up(load_address, 128M) >= dram_end holds, it is guaranteed that
> > no address exists in memory from which we could round down and arrive
> > at a valid DRAM address. This would mean that your change will only
> > affect platforms that were unable to boot to begin with, and not
> > affect any other weird configurations including crashkernels etc
>
> Uhm maybe not ...
>
> Time to get that coffee...

OK  so we know that the memory base should be a 16 MB aligned value >=
dram_start. This holds for the crashkernel as well, although in that
case, the memory base is much higher than dram_start, and not right
above it.

So how about if we *only* use the DT dram_start as the memory base if
it is aligned to 16 MB, and if rounding down from the load_address
produces an address that is below it? That should cover your use case,
but very conservatively, reducing the likelihood of regressions.