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

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Date:   Tue, 19 May 2020 13:46:41 +0200
From:   Lukasz Stelmach <l.stelmach@...sung.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Dmitry Osipenko <digetx@...il.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Miao <eric.miao@...dia.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Chris Brandt <chris.brandt@...esas.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "open list\:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        Grant Likely <grant.likely@....com>
Subject: Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB

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@...linux.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@...sung.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@...xnic.net>
>>>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>>>> Reviewed-by: Nicolas Pitre <nico@...xnic.net>
>>>>> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
>>>>> Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>
>>>>> Tested-by: Dmitry Osipenko <digetx@...il.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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