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From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Thu, 18 Jan 2024 09:45:49 +0100
Message-ID: <CAMuHMdXg1Y7mwHKTYi_j7a_XGdMJ7Aa7u5dEv5+xsLe8=BMaRw@mail.gmail.com>
Subject: Re: [PATCH 4/6] of: Create of_root if no dtb provided by firmware
To: Rob Herring <robh@kernel.org>
Cc: Stephen Boyd <sboyd@kernel.org>, Frank Rowand <frowand.list@gmail.com>, 
	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, Simon Glass <sjg@chromium.org>
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Hi Rob,

On Wed, Jan 17, 2024 at 6:41=E2=80=AFPM Rob Herring <robh@kernel.org> wrote=
:
> On Tue, Jan 16, 2024 at 05:18:15PM -0800, Stephen Boyd wrote:
> > Quoting Rob Herring (2024-01-15 12:32:30)
> > > On Fri, Jan 12, 2024 at 12:07:47PM -0800, Stephen Boyd wrote:
> > > > diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> > > > index da9826accb1b..9628e48baa15 100644
> > > > --- a/drivers/of/Kconfig
> > > > +++ b/drivers/of/Kconfig
> > > > @@ -54,9 +54,14 @@ config OF_FLATTREE
> > > >       select CRC32
> > > >
> > > >  config OF_EARLY_FLATTREE
> > > > -     bool
> > > > +     bool "Functions for accessing Flat Devicetree (FDT) early in =
boot"
> > >
> > > I think we could instead just get rid of this kconfig option. Or
> > > always enable with CONFIG_OF (except on Sparc). The only cost of
> > > enabling it is init section functions which get freed anyways.
> >
> > Getting rid of it is a more massive change. It can be the default and
> > kept hidden instead? If it can't be selected on Sparc then it should be
> > hidden there anyway.
>
> The easier option is certainly fine for this series. I just don't want
> it visible.
>
> > > >       select DMA_DECLARE_COHERENT if HAS_DMA && HAS_IOMEM
> > > >       select OF_FLATTREE
> > > > +     help
> > > > +       Normally selected by platforms that process an FDT that has=
 been
> > > > +       passed to the kernel by the bootloader.  If the bootloader =
does not
> > > > +       pass an FDT to the kernel and you need an empty devicetree =
that
> > > > +       contains only a root node to exist, then say Y here.
> > > >
> > > >  config OF_PROMTREE
> > > >       bool
> > [...]
> > > > @@ -195,6 +191,17 @@ static inline int of_node_check_flag(const str=
uct device_node *n, unsigned long
> > > >       return test_bit(flag, &n->_flags);
> > > >  }
> > > >
> > > > +/**
> > > > + * of_have_populated_dt() - Has DT been populated by bootloader
> > > > + *
> > > > + * Return: True if a DTB has been populated by the bootloader and =
it isn't the
> > > > + * empty builtin one. False otherwise.
> > > > + */
> > > > +static inline bool of_have_populated_dt(void)
> > > > +{
> > > > +     return of_root !=3D NULL && !of_node_check_flag(of_root, OF_E=
MPTY_ROOT);
> > >
> > > Just a side comment, but I think many/all callers of this function co=
uld
> > > just be removed.
> > >
> > > I don't love new flags. Another possible way to handle this would be
> > > checking for "compatible" being present in the root node. I guess thi=
s
> > > is fine as-is for now at least.
> >
> > Ok. I can add a check for a compatible property. That's probably better
> > anyway. Should there be a compatible property there to signal that this
> > DT isn't compatible with anything? I worry about DT overlays injecting =
a
> > compatible string into the root node, but maybe that is already
> > prevented.
>
> I worry about DT overlays injecting anything...
>
> I don't think it is explicitly forbidden, but I have asked that any
> general purpose interface to apply overlays be restricted to nodes
> explicitly allowed (e.g. downstream of a connector node). For now, the
> places (i.e. drivers) overlays are applied are limited.
>
> We could probably restrict the root node to new nodes only and no new
> or changed properties.

Changing (<wild dream>or appending to</wild dream>) the root
"compatible" and/or "model" properties is useful in case of large
extension boards, though.  This is also the case for DTBs created from
a base DTB and one or more overlays using fdtoverlay.

For the latter, see also the following threads, where you weren't
(but probably should have been) CCed:

[1] "[PATCH v9 2/2] arm64: boot: Support Flat Image Tree"
     https://lore.kernel.org/all/20231202035511.487946-3-sjg@chromium.org
[2] "Proposal: FIT support for extension boards / overlays"
    https://lore.kernel.org/all/CAPnjgZ06s64C2ux1rABNAnMv3q4W++sjhNGCO_uPMH=
_9sTF7Mw@mail.gmail.com

Gr{oetje,eeting}s,

                        Geert

--=20
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k=
org

In personal conversations with technical people, I call myself a hacker. Bu=
t
when I'm talking to journalists I just say "programmer" or something like t=
hat.
                                -- Linus Torvalds