linux-kernel - Re: [PATCH 2/2] pm: allow drivers to drop #ifdef and __maybe

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Date:   Thu, 8 Apr 2021 23:29:45 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Paul Cercueil <paul@...pouillou.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 2/2] pm: allow drivers to drop #ifdef and __maybe_unused
 from pm callbacks

On Thu, Apr 8, 2021 at 11:00 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Drivers typically surround suspend and resume callbacks with #ifdef
> CONFIG_PM(_SLEEP) or mark them as __maybe_unused in order to avoid
> -Wunused-const-variable warnings.
>
> With this commit, drivers will be able to remove #ifdef CONFIG_PM(_SLEEP)
> and __maybe_unsed because unused functions are dropped by the compiler
> instead of the preprocessor.
>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

I tried this before and could not get it to work right.

>
> -#ifdef CONFIG_PM_SLEEP
> +#define pm_ptr(_ptr)           PTR_IF(IS_ENABLED(CONFIG_PM), _ptr)
> +#define pm_sleep_ptr(_ptr)     PTR_IF(IS_ENABLED(CONFIG_PM_SLEEP), _ptr)
> +
>  #define SET_SYSTEM_SLEEP_PM_OPS(suspend_fn, resume_fn) \
> -       .suspend = suspend_fn, \
> -       .resume = resume_fn, \
> -       .freeze = suspend_fn, \
> -       .thaw = resume_fn, \
> -       .poweroff = suspend_fn, \
> -       .restore = resume_fn,
> -#else
> -#define SET_SYSTEM_SLEEP_PM_OPS(suspend_fn, resume_fn)
> -#endif
> +       .suspend  = pm_sleep_ptr(suspend_fn), \
> +       .resume   = pm_sleep_ptr(resume_fn), \
> +       .freeze   = pm_sleep_ptr(suspend_fn), \
> +       .thaw     = pm_sleep_ptr(resume_fn), \
> +       .poweroff = pm_sleep_ptr(suspend_fn), \
> +       .restore  = pm_sleep_ptr(resume_fn),

The problem that I think you inevitably hit is that you run into a missing
declaration for any driver that still uses an #ifdef around a static
function.

The only way I can see us doing this is to create a new set of
macros that behave like the version you propose here but leave
the old macros in place until the last such #ifdef has been removed.

       Arnd