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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Sun, 9 Sep 2018 17:55:55 +0200
Message-ID: <CAKv+Gu_y9fzRR8DLS2gVmcW8U4jZAnxgQUjxrOO=QS=0NPq5Zw@mail.gmail.com>
Subject: Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Scott Branden <scott.branden@broadcom.com>,
        Grant Likely <grant.likely@secretlab.ca>,
        Arnd Bergmann <arnd@arndb.de>,
        Will Deacon <will.deacon@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Leif Lindholm <leif.lindholm@linaro.org>,
        Alexander Graf <agraf@suse.de>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>,
        Olof Johansson <olof@lixom.net>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On 9 September 2018 at 13:07, Catalin Marinas <catalin.marinas@arm.com> wrote:
> On Wed, Sep 05, 2018 at 11:04:36AM -0700, Scott Branden wrote:
>> On 18-09-05 11:00 AM, Grant Likely wrote:
>> > On Wed, Sep 5, 2018 at 6:27 PM Scott Branden <scott.branden@broadcom.com> wrote:
>> > > On 18-09-05 02:40 AM, Ard Biesheuvel wrote:
>> > > > On 4 September 2018 at 19:19, Scott Branden <scott.branden@broadcom.com> wrote:
>> > > > > Rather than introduce EFI_ARMSTUB_DTB_LOADER, why not have
>> > > > > the efistub use CONFIG_OF to determine whether it supports dtb= or not?
>> > > > >
>> > > > > That way ACPI-only distros disable devicetree support entirely.
>> > > > >
>> > > > Unfortunately, CONFIG_OF cannot be disabled on arm64 even on ACPI-only builds.
>> > > OF shouldn't be automatically selected in the arm64/Kconfig.  It should
>> > > have a config parmaeter like other archs as mips and arm.  I can
>> > > submit a patch so it functions the same way as other archs so it
>> > > is not always selected.  It will be good to add a USE_OF config
>> > > options like the other archs (or simply remove the select from the
>> > > Kconfig and choose OF directly in the defconfig. This will have
>> > > the added benefit of doing away with OF support when its not
>> > > needed on an ARM64 platform.  ACPI is already not automatically
>> > > selected for all ARM64 platforms, nor should devicetree.
>> > We don't do that on Arm because a devicetree is always required at
>> > boot time. Even on ACPI systems a tiny DTB is used containing just a
>> > /chosen node for passing the kernel command line and the initrd
>> > location.
>>
>> Seems bizarre DTB is not needed for x86 to boot from UEFI with ACPI
>> support?  I'll look into it further at some point in order to remove such
>> anomaly.  There should be no need for such devicetree reliance.
>
> I'd say don't waste time on this, the patch would not get merged ;). As
> Grant said, we use a tiny dtb to pass the command line, initrd to the
> kernel. You'd have to invent an alternative (setup_header, ATAGs) and I
> really don't see the point of increased complexity just because of some
> philosophical arguments against OF.
>

Not just that: we also need of_match_table support for ACPI's PRP0001
device (which is a horrible hack in itself, but currently supported in
Linux/arm64 *and* Linux/x86 nonetheless)