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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Arnd Bergmann <arnd@arndb.de>, Arnd Bergmann <arnd@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>
Cc:     Sakari Ailus <sakari.ailus@iki.fi>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Documentation: kbuild: explain handling optional
 dependencies
In-Reply-To: <b2723c56-e2b0-4871-afbc-73cf6335ddca@app.fastmail.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20230913113801.1901152-1-arnd@kernel.org>
 <874jjwx44g.fsf@intel.com>
 <b2723c56-e2b0-4871-afbc-73cf6335ddca@app.fastmail.com>
Date:   Thu, 14 Sep 2023 18:56:42 +0300
Message-ID: <87v8ccvjd1.fsf@intel.com>
MIME-Version: 1.0
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On Thu, 14 Sep 2023, "Arnd Bergmann" <arnd@arndb.de> wrote:
> On Thu, Sep 14, 2023, at 15:42, Jani Nikula wrote:
>> On Wed, 13 Sep 2023, Arnd Bergmann <arnd@kernel.org> wrote:
>>> From: Arnd Bergmann <arnd@arndb.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

*head explodes*

> "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.

Fair enough.


BR,
Jani.


-- 
Jani Nikula, Intel