linux-kernel - Re: [PATCH 2/3] riscv: Remove non-standard linux,elfcorehdr handling

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Message-ID: <CAL_JsqLU7GWDxdnR2-Yd2vbj7w=5pNr_fFocDQgPbs17EpBG0g@mail.gmail.com>
Date:   Tue, 15 Jun 2021 13:54:30 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Nick Kossifidis <mick@....forth.gr>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Frank Rowand <frowand.list@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, devicetree@...r.kernel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] riscv: Remove non-standard linux,elfcorehdr handling

On Tue, Jun 15, 2021 at 12:40 PM Nick Kossifidis <mick@....forth.gr> wrote:
>
> Στις 2021-06-15 21:17, Geert Uytterhoeven έγραψε:
> > RISC-V uses platform-specific code to locate the elf core header in
> > memory.  However, this does not conform to the standard
> > "linux,elfcorehdr" DT bindings, as it relies on a reserved memory node
> > with the "linux,elfcorehdr" compatible value, instead of on a
> > "linux,elfcorehdr" property under the "/chosen" node.
> >
> > The non-compliant code can just be removed, as the standard behavior is
> > already implemented by platform-agnostic handling in the FDT core code.
> >
> > Fixes: 5640975003d0234d ("RISC-V: Add crash kernel support")
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>
> NACK
>
> There is nothing standard about "linux,elfcorehdr", it's an

It is and it is documented which is more than we can say for
"linux,elfcorehdr" as a node.

> arm64-specific property on /chosen and it's suboptimal, it gets the
> addr/length of ELF core of the previous kernel through that property and
> then goes on to reserve that region at:
> https://elixir.bootlin.com/linux/v5.13-rc6/source/arch/arm64/mm/init.c#L155
>
> Why on earth is this cleaner than just defining a reserved-region in the
> first place (a standard binding) with and hook up a callback with
> RESERVEDMEM_OF_DECLARE for it to also initialize elfcorehdr_addr/size ?
> If you don't like the compatible string I'm ok to change it, but this
> patch breaks kdump on riscv since that region won't be reserved any more
> and kernel will corrupt it.

I might agree if we were designing this all from scratch, but we're
not. We've got powerpc doing /memreserve/ + kernel cmdline, arm64
using chosen, and RiscV a 3rd way.

What happens when/if RiscV wants to add an IMA buffer? That's no
different than this case. The 2 architectures supporting it both use
/chosen. Specifying an initrd is no different either.

Rob