Re: [PATCH] watchdog: qcom: Add suspend/resume support

From: Brian Masney
Date: Thu Jan 17 2019 - 06:27:34 EST

On Thu, Jan 17, 2019 at 04:08:52PM +0530, Sai Prakash Ranjan wrote:
> On 1/17/2019 3:01 PM, Brian Masney wrote:
> >
> > You can use the __maybe_unused attribute to remove the #ifdef:
> >
> > static int __maybe_unused qcom_wdt_suspend(struct device *dev)
> >
> Thanks for looking into this.
> As for __maybe_unused, I think it's better to keep #ifdef rather than
> this attribute which seems to be meaning unused when actually its possible
> that it's used often(PM_SLEEP is def y). It's like saying unused when you
> are actually using it. The attribute seems like a
> hack to avoid compilation error. Please correct me if I am wrong.

That attribute suppresses a warning from the compiler if the function is
unused when PM_SLEEP is disabled. I don't consider it hackish since the
function name no longer appears outside the #ifdef. For example:

static int qcom_wdt_suspend(struct device *dev)
#endif /* CONFIG_PM_SLEEP */

static SIMPLE_DEV_PM_OPS(..., qcom_wdt_suspend, ...);

SIMPLE_DEV_PM_OPS (actually SET_SYSTEM_SLEEP_PM_OP) includes the check
for PM_SLEEP and its a noop if PM_SLEEP is disabled so this works.

Now here's the code with __maybe_unused:

static int __maybe_unused qcom_wdt_suspend(struct device *dev)

static SIMPLE_DEV_PM_OPS(..., qcom_wdt_suspend, ...);

This will still be a NOOP when power management is disabled, but have
the benefit of increased compile-time test coverage in that situation.
The symbols won't be included in the final executable. I personally
think the code a is cleaner with __maybe_unused.

This pattern is already in use across various subsystems in the kernel
for suspend and resume functions:

$ git grep __maybe_unused | egrep "_suspend|_resume" | wc -l