linux-kernel - Re: [PATCH] Documentation: kbuild: explain handling optional dependencies

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Message-Id: <b2723c56-e2b0-4871-afbc-73cf6335ddca@app.fastmail.com>
Date:   Thu, 14 Sep 2023 16:57:12 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Jani Nikula" <jani.nikula@...ux.intel.com>,
        "Arnd Bergmann" <arnd@...nel.org>,
        "Masahiro Yamada" <masahiroy@...nel.org>,
        "Jonathan Corbet" <corbet@....net>
Cc:     "Sakari Ailus" <sakari.ailus@....fi>,
        "Javier Martinez Canillas" <javierm@...hat.com>,
        "Nathan Chancellor" <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        "Nicolas Schier" <nicolas@...sle.eu>, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation: kbuild: explain handling optional dependencies

On Thu, Sep 14, 2023, at 15:42, Jani Nikula wrote:
> On Wed, 13 Sep 2023, Arnd Bergmann <arnd@...nel.org> wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>>  
>> +Optional dependencies
>> +~~~~~~~~~~~~~~~~~~~~~
>> +
>> +Some drivers are able to optionally use a feature from another module
>> +or build cleanly with that module disabled, but cause a link failure
>> +when trying to use that loadable module from a built-in driver.
>> +
>> +The most common way to express this optional dependency in Kconfig logic
>> +uses the slighly counterintuitive
>> +
>> +  config FOO
>> +	bool "Support for foo hardware"
>> +	depends on BAR || !BAR
>
> 	depends on BAR || BAR=n
>
> seems to be an alternative that's about as common:
>
> $ git grep "depends on \([A-Z0-9_]\+\) || \!\1" | wc -l
> 109
> $ git grep "depends on \([A-Z0-9_]\+\) || \1=n" | wc -l
> 107
>
> Maybe worth mentioning both?

I fear that would add more confusion than it avoids:
"!BAR" is actually different from "BAR=n", but
"BAR || !BAR" is the same as "BAR || BAR=n" here, and
trying to explain this in the documentation would either
make it incorrect or unhelpfully complicated.

      Arnd