Re: [PATCH v4 5/7] arm64: Unconditionally call unflatten_device_tree()

From: Mark Rutland <mark.rutland@arm.com>
To: Rob Herring <robh@kernel.org>
Cc: Will Deacon <will@kernel.org>, Stephen Boyd <sboyd@kernel.org>,
	linux-kernel@vger.kernel.org, patches@lists.linux.dev,
	linux-um@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org, kunit-dev@googlegroups.com,
	linux-kselftest@vger.kernel.org, devicetree@vger.kernel.org,
	Frank Rowand <frowand.list@gmail.com>,
	Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v4 5/7] arm64: Unconditionally call unflatten_device_tree()
Date: Tue, 27 Feb 2024 17:34:58 +0000	[thread overview]
Message-ID: <Zd4dQpHO7em1ji67@FVFF77S0Q05N.cambridge.arm.com> (raw)
In-Reply-To: <CAL_JsqJSeSHeWV3YJE9n2NuY+s_iE6f7N5C_oguEJn7jTZ20xA@mail.gmail.com>

On Fri, Feb 23, 2024 at 11:17:02AM -0700, Rob Herring wrote:
> On Fri, Feb 23, 2024 at 3:23 AM Will Deacon <will@kernel.org> wrote:
> >
> > On Thu, Feb 22, 2024 at 05:03:17PM -0700, Rob Herring wrote:
> > > On Fri, Feb 16, 2024 at 05:05:54PM -0800, Stephen Boyd wrote:
> > > > Call this function unconditionally so that we can populate an empty DTB
> > > > on platforms that don't boot with a firmware provided or builtin DTB.
> > > > When ACPI is in use, unflatten_device_tree() ignores the
> > > > 'initial_boot_params' pointer so the live DT on those systems won't be
> > > > whatever that's pointing to. Similarly, when kexec copies the DT data
> > > > the previous kernel to the new one on ACPI systems,
> > > > of_kexec_alloc_and_setup_fdt() will ignore the live DT (the empty root
> > > > one) and copy the 'initial_boot_params' data.
> > > >
> > > > Cc: Rob Herring <robh+dt@kernel.org>
> > > > Cc: Frank Rowand <frowand.list@gmail.com>
> > > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > > Cc: Will Deacon <will@kernel.org>
> > > > Cc: Mark Rutland <mark.rutland@arm.com>
> > > > Cc: <linux-arm-kernel@lists.infradead.org>
> > > > Signed-off-by: Stephen Boyd <sboyd@kernel.org>
> > > > ---
> > > >  arch/arm64/kernel/setup.c | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > Catalin, Will, Can I get an ack on this so I can take the series via the
> > > DT tree.
> >
> > Mark had strong pretty strong objections to this in version one:
> 
> Yes, I had concerns with it as well.
> 
> > https://lore.kernel.org/all/ZaZtbU9hre3YhZam@FVFF77S0Q05N/
> >
> > and this patch looks the same now as it did then. Did something else
> > change?
> 
> Yes, that version unflattened the bootloader passed DT. Now within
> unflatten_devicetree(), the bootloader DT is ignored if ACPI is
> enabled and we unflatten an empty tree. That will prevent the kernel
> getting 2 h/w descriptions if/when a platform does such a thing. Also,
> kexec still uses the bootloader provided DT as before.

That avoids the main instance of my concern, and means that this'll boot
without issue, but IIUC this opens the door to dynamically instantiating DT
devices atop an ACPI base system, which I think in general is something that's
liable to cause more problems than it solves.

I understand that's desireable for the selftests, though I still don't believe
it's strictly necessary -- there are plenty of other things that only work if
the kernel is booted in a specific configuration.

Putting the selftests aside, why do we need to do this? Is there any other
reason to enable this?

Mark.

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