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

[Arnd Bergmann]

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@xxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>

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.

> +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.

      Arnd