linux-kernel - Re: [PATCH 0/2] of: add support for value "false" to of_property_read

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Message-ID: <CAMuHMdWUJ-nQkD-tm07uWX_TTy5Fy5JLsvedcoJ50Hi7=WnEpg@mail.gmail.com>
Date: Wed, 13 Nov 2024 11:15:44 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Josua Mayer <josua@...id-run.com>
Cc: Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, Nishanth Menon <nm@...com>, 
	Vignesh Raghavendra <vigneshr@...com>, Tero Kristo <kristo@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Jon Nettleton <jon@...id-run.com>, 
	Yazan Shhady <yazan.shhady@...id-run.com>, Rabeeh Khoury <rabeeh@...id-run.com>
Subject: Re: [PATCH 0/2] of: add support for value "false" to of_property_read_bool

Hi Josua,

On Wed, Nov 13, 2024 at 10:41 AM Josua Mayer <josua@...id-run.com> wrote:
> Am 12.11.24 um 14:46 schrieb Rob Herring:
> > On Tue, Nov 12, 2024 at 12:41 AM Josua Mayer <josua@...id-run.com> wrote:
> >> Boolean type properties are usually considered true if present and false
> >> when they do not exist. This works well for many in-tree board dts and
> >> existing drivers.
> >>
> >> When users need to overrride boolean values from included dts,
> >> /delete-property/ is recommend. This however does not work in overlays
> >> (addons).
> > As soon as someone needs to delete a non-boolean property, we're back
> > to the same problem.
>
> Properties can be reassigned any value, e.g. a driver default if needed.
> It should never be necessary to delete a property,
> since a suitable value may be specified instead.
>
> Booleans have two valid values, true and false,
> but somehow we cannot assign false, we can just delete the property.
>
> > If you want to fix it, you need to fix it for any
> > property.
> /delete-property/ is a language keyword used during compilation.
> When inspecting a .dtbo no trace of /delete-property/ is left.

That is the current behavior. But dtc could be modified to emit new
special __deleted_properties__ and __deleted_nodes__ nodes, just like
it already creates special __symbols__, __fixups__, and __local_fixups__
nodes.

> Hence we can't "fix" deleting properties through overlays.
> We can only "fix" (re-)assigning false to a boolean property.

So nothing prevents adding a way for a .dtbo to contain a list of
properties and/or nodes to delete...

> >> Geert pointed out [1] that there are several invitations for using
> >> strings "true" and "false" on boolean properties: [1], [2], [3].
>
> > There's always bad examples...
> >
> >> Add support for a string value "false" to be considered false on boolean
> >> properties by changing of_property_read_bool implementation.
>
> > Any existing s/w will treat 'foo = "false"' as true. It's an ABI.
>
> I was reading through the device-tree specification, it makes absolutely
> no mention of a boolean type.
>
> I believe of_property_read_bool should be capable of deriving false
> from a present property.
>
> What is up to now called bool in the kernel / device-tree,
> is actually of_property_present, or conversationally of_node_has_flag.

Indeed, so of_property_read_bool() is a misnomer, as it does
not read the actual value from the property, unlike all other
of_property_read_*() methods.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds