Re: [PATCH v2] docs: kconfig: Mention IS_REACHABLE as way for optional dependency

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 17/02/2025 08:51, Arnd Bergmann wrote:
> On Sat, Feb 15, 2025, at 14:17, Krzysztof Kozlowski wrote:
>> Several drivers express optional Kconfig dependency with FOO || !FOO,
>> but for many choices this is neither suitable (lack of stubs for !FOO
>> like in HWMON) nor really needed and driver can be built in even if FOO
>> is the module.  This is achieved with IS_REACHABLE, so move earlier
>> section from "imply" chapter to "Optional dependencies" and grow the
>> description.
>>
>> Cc: Masahiro Yamada <masahiroy@kernel.org>
>> Cc: Arnd Bergmann <arnd@arndb.de>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> 
> I think this needs to be more strongly worded to heavy discourage
> the use of IS_REACHABLE(). In most cases, anyone trying this has
> just given up on writing correct dependencies and works around
> it in a user-hostile way.
> 

Ack

>> +1. If pre-processor can discard entire optional code or module BAR does not
>> +   provide !BAR stubs then call can be guarded with IS_REACHABLE()::
>> +
>> +	foo_init()
>> +	{
>> +		if (IS_REACHABLE(CONFIG_BAR))
>> +			bar_register(&foo);
>> +		...
>> +	}
>> +
>> +   Drawback: this might lead to run-time debugging, when looking why
>> +   bar_register() was not called.
> 
> From this description, I wouldn't expect most readers to understand
> what you mean with "run-time debugging". The behavior that users
> are going to see is just a bug -- they turned on CONFIG_BAR=m because
> they wanted to use BAR, but then it doesn't work.

Ack


Best regards,
Krzysztof