Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB

[Lukasz Stelmach <l.stelmach@samsung.com>]

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It was <2020-05-19 wto 12:43>, when Russell King - ARM Linux admin wrote:
> On Tue, May 19, 2020 at 01:21:09PM +0200, Geert Uytterhoeven wrote:
>> On Tue, May 19, 2020 at 11:46 AM Russell King - ARM Linux admin
>> <linux@armlinux.org.uk> wrote:
>> > On Tue, May 19, 2020 at 11:44:17AM +0200, Geert Uytterhoeven wrote:
>> > > On Tue, May 19, 2020 at 10:54 AM Lukasz Stelmach <l.stelmach@samsung.com> wrote:
>> > > > It was <2020-04-29 śro 10:21>, when Geert Uytterhoeven 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@fluxnic.net>
>> > > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
>> > > > > Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
>> > > > > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
>> > > > > Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
>> > > > > Tested-by: Dmitry Osipenko <digetx@gmail.com>
>> > > > > ---
>> > > >
>> > > > [...]
>> > > >
>> > > > Apparently reading physical memory layout from DTB breaks crashdump
>> > > > kernels. A crashdump kernel is loaded into a region of memory, that is
>> > > > reserved in the original (i.e. to be crashed) kernel. The reserved
>> > > > region is large enough for the crashdump kernel to run completely inside
>> > > > it and don't modify anything outside it, just read and dump the remains
>> > > > of the crashed kernel. Using the information from DTB makes the
>> > > > decompressor place the kernel outside of the dedicated region.
>> > > >
>> > > > The log below shows that a zImage and DTB are loaded at 0x18eb8000 and
>> > > > 0x193f6000 (physical). The kernel is expected to run at 0x18008000, but
>> > > > it is decompressed to 0x00008000 (see r4 reported before jumping from
>> > > > within __enter_kernel). If I were to suggest something, there need to be
>> > > > one more bit of information passed in the DTB telling the decompressor
>> > > > to use the old masking technique to determain kernel address. It would
>> > > > be set in the DTB loaded along with the crashdump kernel.
>> > >
>> > > Shouldn't the DTB passed to the crashkernel describe which region of
>> > > memory is to be used instead?
>> >
>> > Definitely not.  The crashkernel needs to know where the RAM in the
>> > machine is, so that it can create a coredump of the crashed kernel.
>> 
>> So the DTB should describe both ;-)
>> 
>> > > Describing "to use the old masking technique" sounds a bit hackish to me.
>> > > I guess it cannot just restrict the /memory node to the reserved region,
>> > > as the crashkernel needs to be able to dump the remains of the crashed
>> > > kernel, which lie outside this region.
>> >
>> > Correct.
>> >
>> > > However, something under /chosen should work.
>> >
>> > Yet another sticky plaster...
>> 
>> IMHO the old masking technique is the hacky solution covered by
>> plasters.
>
> One line of code is not "covered by plasters".  There are no plasters.
> It's a solution that works for 99.99% of people, unlike your approach
> that has had a stream of issues over the last four months, and has
> required many reworks of the code to fix each one.  That in itself
> speaks volumes about the suitability of the approach.

As I have been working with kexec code (patches soon) I would like to
defend the DT approach a bit. It allows to avoid zImage relocation when
a decompressed kernel is larger than ~128MiB. In such case zImage isn't
small either and moving it around takes some time.

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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