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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 4 Jun 2021 20:34:53 +0200
Message-ID: <CAK8P3a0fsYwMOioFd7UqzVT0oDSKAA=cjo2o781juO1v88sLAw@mail.gmail.com>
Subject: Re: [PATCH -fixes] riscv: Fix BUILTIN_DTB for sifive and microchip soc
To:     Palmer Dabbelt <palmer@dabbelt.com>
Cc:     Alexandre Ghiti <alex@ghiti.fr>, Rob Herring <robh+dt@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        DTML <devicetree@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Jun 4, 2021 at 5:51 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
> On Fri, 04 Jun 2021 06:08:05 PDT (-0700), Arnd Bergmann wrote:
> > On Fri, Jun 4, 2021 at 2:06 PM Alexandre Ghiti <alex@ghiti.fr> wrote:
> >>
> >> Fix BUILTIN_DTB config which resulted in a dtb that was actually not built
> >> into the Linux image: in the same manner as Canaan soc does, create an object
> >> file from the dtb file that will get linked into the Linux image.
> >>
> >> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> >
> > Along the same lines as the comment that Jisheng Zhang made on the fixed
> > address, building a dtb into the kernel itself fundamentally breaks generic
> > kernel images.
> >
> > I can understand using it on K210, which is extremely limited and wouldn't
> > run a generic kernel anyway, but for normal platforms like microchip and
> > sifive, it would be better to disallow CONFIG_BUILTIN_DTB in Kconfig
> > and require a non-broken boot loader.
>
> When we first added BUILTIN_DTB we actually had a compatibility
> mechanism in there.  There isn't enough in the ISA to handle board
> compatibility, but we were hoping to get something to deal with that.
> It didn't pan out so we dropped the compatibility mechanism, which is
> how we ended up here.
>
> Maybe the right thing to do here is to add some sort of "be compatible
> with the platform spec" Kconfig, which we could then use to disallow all
> these features that result in non-portable kernels?

Yes, I should have read your email before I replied with the same
suggestion to Vitaly ;-)

       Arnd