linux-kernel - Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB

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Date:   Sun, 9 Sep 2018 12:07:33 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Scott Branden <scott.branden@...adcom.com>
Cc:     Grant Likely <grant.likely@...retlab.ca>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Leif Lindholm <leif.lindholm@...aro.org>,
        Alexander Graf <agraf@...e.de>,
        bcm-kernel-feedback-list@...adcom.com,
        Olof Johansson <olof@...om.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: defconfig: enable EFI_ARMSTUB_DTB_LOADER

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@...adcom.com> wrote:
> > > On 18-09-05 02:40 AM, Ard Biesheuvel wrote:
> > > > On 4 September 2018 at 19:19, Scott Branden <scott.branden@...adcom.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.

-- 
Catalin