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    SubjectRe: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB
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    It was <2020-05-19 wto 13:21>, when Geert Uytterhoeven wrote:
    > Hi Russell,
    >
    > CC devicetree
    >
    > 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 ;-)

    So we can do without the mem= cmdline option which is required
    now. Sounds reasonable to me.

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