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

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Message-ID: <288deac4-5387-4492-9e9c-cabf77b9c0ba@solid-run.com>
Date: Wed, 13 Nov 2024 09:41:05 +0000
From: Josua Mayer <josua@...id-run.com>
To: Rob Herring <robh@...nel.org>
CC: 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>, Geert Uytterhoeven
	<geert@...ux-m68k.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

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.

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

>
>
>> 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.


sincerely
Josua Mayer