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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 4 Jun 2021 20:33:39 +0200
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Subject: Re: [PATCH -fixes] riscv: Fix BUILTIN_DTB for sifive and microchip soc
To:     Vitaly Wool <vitaly.wool@konsulko.com>
Cc:     Alexandre Ghiti <alex@ghiti.fr>, Rob Herring <robh+dt@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.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 7:45 PM Vitaly Wool <vitaly.wool@konsulko.com> wrote:
> On Fri, Jun 4, 2021 at 3:18 PM Arnd Bergmann <arnd@arndb.de> 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.
>
> can't quite agree here. If we take XIP, it does make sense to have
> BUILTIN_DTB, since 1) this will not be a generic kernel anyway 2) we
> may want to skip the bootloader altogether or at least make it as thin
> as possible and 3) copying device tree binaries from bootloader to RAM
> as opposed to having it handy compiled in the kernel will be just a
> waste of RAM.

Indeed, it does make sense in combination with XIP. Maybe there could
be a Kconfig option that depends on CONFIG_EXPERT and that can
be used to guard non-generic options like this?

       Arnd