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 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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