linux-kernel - Re: [PATCH 1/3] kconfig.h: abstract empty functions with STUB

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Message-ID: <830dafb3-7e93-e655-464f-8cb821de8dd5@arm.com>
Date:   Fri, 18 Jan 2019 16:27:35 +0000
From:   Steven Price <steven.price@....com>
To:     Andrew Murray <andrew.murray@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     rjw@...ysocki.net, Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Grant Likely <Grant.Likely@....com>,
        Dave P Martin <Dave.Martin@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] kconfig.h: abstract empty functions with STUB_UNLESS
 macro

On 18/01/2019 16:00, Andrew Murray wrote:
> A common pattern found in header files is a function declaration dependent
> on a CONFIG_ option being enabled, followed by an empty function for when
> that option isn't enabled. This can often take up a lot of space and impact
> code readability.
> 
> Let's introduce the following STUB_UNLESS macro:
> 
> STUB_UNLESS(CONFIG_FOO, [body], prototype)
> 
> This evaluates to 'prototype' prepended with 'extern' if CONFIG_FOO is set
> to 'y'. Otherwise it will evaluate to 'prototype' prepended with 'static
> inline' followed by an empty function body. Where optional argument 'body'
> then 'body' will be used as the function body, intended to allow for simple
> return statements.
> 
> Signed-off-by: Andrew Murray <andrew.murray@....com>

Seems like it should remove a lot of boilerplate code, so looks good to me.

Reviewed-by: Steven Price <steven.price@....com>

> ---
>  include/linux/kconfig.h | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
> index cc8fa10..216a27f 100644
> --- a/include/linux/kconfig.h
> +++ b/include/linux/kconfig.h
> @@ -12,6 +12,7 @@
>  
>  #define __ARG_PLACEHOLDER_1 0,
>  #define __take_second_arg(__ignored, val, ...) val
> +#define __take_fourth_arg(__ignored, __ignored2, __ignored3, val, ...) val
>  
>  /*
>   * The use of "&&" / "||" is limited in certain expressions.
> @@ -70,4 +71,34 @@
>   */
>  #define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))
>  
> +
> +
> +/*
> + * Allow for __stub to be overloaded by making the second argument optional
> + */
> +#define __stub_overload(...)		__take_fourth_arg(__VA_ARGS__, \
> +					__stub_body, __stub_void)(__VA_ARGS__)
> +#define __stub_body(option, body, ptype) __stub(option, body, ptype)
> +#define __stub_void(option, ptype)	__stub(option, , ptype)
> +
> +/*
> + * Using the same trick as __defined we use the config option to conditionally
> + * expand to an extern function declaration or a static stub function.
> + */
> +#define __stub(option, body, ptype)	___stub( \
> +					__ARG_PLACEHOLDER_##option, body, ptype)
> +#define ___stub(arg1_or_junk, body, ptype) __take_second_arg( \
> +		arg1_or_junk __extern_ptype(ptype), __static_ptype(body, ptype))
> +#define __static_ptype(body, ptype)	static inline ptype { body; }
> +#define __extern_ptype(ptype)		extern ptype
> +
> +/*
> + * STUB_UNLESS(CONFIG_FOO, [body], prototype) evaluates to 'prototype' prepended
> + * with 'extern' if CONFIG_FOO is set to 'y'. Otherwise it will evaluate to
> + * 'prototype' prepended with 'static inline' followed by an empty function
> + * body. Where optional argument 'body' is present then 'body' will be used
> + * as the function body, intended to allow for simple return statements.
> + */
> +#define STUB_UNLESS(...) __stub_overload(__VA_ARGS__)
> +
>  #endif /* __LINUX_KCONFIG_H */
>