Re: [PATCH v3 2/3] kconfig: new macro IF_ENABLED()

[Andre Przywara <andre.przywara@arm.com>]

On Sun, 19 Feb 2023 12:36:28 +0100
Heinrich Schuchardt <heinrich.schuchardt@canonical.com> wrote:

Hi,

> We want to move from using #ifdef to using if in our code. A lot of our
> code using #ifdef is structured like:
> 
>     #ifdef CONFIG_FOO
>         fun(CONFIG_FOO_OPT);
>     #endif
> 
> In Kconfig you will find
> 
>     config FOO
>         bool "enable foo"
> 
>     config FOO_OPT
>         string "value for foo"
>         depends on FOO
> 
> We cannot use CONFIG_FOO_OPT in our code directly if CONFIG_FOO is not
> defined.
> 
>      if (IS_ENABLED(CONFIG_FOO)
>          fun(CONFIG_FOO_OPT);
> 
> This would result in an error due to the undefined symbol CONFIG_FOO_OPT.
> 
> The new macro IF_ENABLED(CONFIG_FOO, opt_cfg, def_val) which evaluates
> to opt_cfg if CONFIG_FOO is set to 'y' and to def_val otherwise comes to
> the rescue:
> 
>      if (IS_ENABLED(CONFIG_FOO)
>          fun(IF_ENABLED(CONFIG_FOO, CONFIG_FOO_OPT, "");

Ah, yeah, I like this one, this is indeed the most common pattern that
prevents many "#if to if" conversions.
I briefly thought about unifying those two lines, but it's probably
limiting, as it appears more flexible this way:

> 
> If CONFIG_FOO is not defined, the compiler will see
> 
>     if (0)
>         fun("");
> 
> and be happy.
> 
> If CONFIG_FOO is defined, the compiler will see
> 
>     if (1)
>         fun(CONFIG_FOO_OPT)
> 
> and be equally happy.
> 
> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Reviewed-by: Andre Przywara <andre.przywara@arm.com>

Cheers,
Andre

> ---
> v3:
> 	new patch
> ---
>  include/linux/kconfig.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
> index 2bc704e110..7fea72da1a 100644
> --- a/include/linux/kconfig.h
> +++ b/include/linux/kconfig.h
> @@ -28,6 +28,13 @@
>   */
>  #define IS_ENABLED(option)	config_enabled(option, 0)
>  
> +/*
> + * IF_ENABLED(CONFIG_FOO, opt_cfg, def_val) evalutes to opt_cfg if
> + * CONFIG_FOO is set to 'y' and to def_val otherwise.
> + */
> +#define IF_ENABLED(option, opt_cfg, def_val) \
> +	config_opt_enabled(IS_ENABLED(option), opt_cfg, def_val)
> +
>  /*
>   * U-Boot add-on: Helper macros to reference to different macros (prefixed by
>   * CONFIG_, CONFIG_SPL_, CONFIG_TPL_ or CONFIG_TOOLS_), depending on the build