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

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Date:   Thu, 17 Jan 2019 06:27:30 -0500
From:   Brian Masney <masneyb@...tation.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        linux-watchdog@...r.kernel.org,
        Guenter Roeck <groeck@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Doug Anderson <dianders@...omium.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] watchdog: qcom: Add suspend/resume support

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:

	#ifdef CONFIG_PM_SLEEP
	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
767

Brian