Re: [PATCH] power: supply: olpc_battery: remove unnecessary CONFIG_PM_SLEEP

[Coiby Xu]

On Thu, Oct 29, 2020 at 12:09:23PM +0100, Hans de Goede wrote:
> Hi,
>
> On 10/29/20 11:59 AM, Coiby Xu wrote:
> > Hi Hans,
> >
> > Thank you for reviewing this patch!
> >
> > On Thu, Oct 29, 2020 at 11:04:36AM +0100, Hans de Goede wrote:
> > > Hi,
> > >
> > > On 10/29/20 8:41 AM, Coiby Xu wrote:
> > > > SIMPLE_DEV_PM_OPS has already took good care of CONFIG_PM_CONFIG.
> > >
> > > No it does not, when CONFIG_PM_SLEEP is not set then the
> > > SET_SYSTEM_SLEEP_PM_OPS macro which SIMPLE_DEV_PM_OPS uses
> > > is a no-op, so nothing will reference xo15_sci_resume leading to
> > > a compiler warning when CONFIG_PM_SLEEP is not set.
> > >
> > > You could drop the ifdef and add __maybe_unused to the definition
> > > of xo15_sci_resume, but that feels like needless churn, best to
> > > just keep this as is IMHO.
> >
> > Actually, this is a tree-wide change by some semi-automation scripts.
> > Thank you for pointing out the issue to prevent me from releasing
> > another ~150 emails to flood other mailing lists.
> >
> > Currently there are 929 drivers has device PM callbacks,
> >
> > $ grep -rI "\.pm = &" --include=*.c  ./|wc -l
> > 929
> >
> > I put all files having device PM callbacks into four categories
> > based on weather a file has CONFIG_PM_SLEEP or PM macro like
> > SET_SYSTEM_SLEEP_PM_OPS, here are the statistics,
> >   1. have both CONFIG_PM_SLEEP and PM_OPS macro: 213
> >   2. have CONFIG_PM_SLEEP but no PM_OPS macro: 19
> >   3. have PM macro but not CONFIG_PM_SLEEP: 347
> >   4. no PM macro or CONFIG_PM_SLEEP: 302
> >
> > Some drivers which have PM macro but not CONFIG_PM_SLEEP like
> > sound/x86/intel_hdmi_audio.c indeed use __maybe_unused to eliminate
> > the compiling warning. In 2011, there's a patch proposing to remove
> > ONFIG_PM altogether but an objection was turning CONFIG_PM on would
> > increase the kernel size [1]. So __maybe_unused also have this issue.
>
> I would expect the compiler to remove the unused function, it knows
> it is unused, that is why __maybe_unused is necessary to suppress
> the warning and compilers are pretty smart and agressive wrt remove
> unnecessary code these days.
Then __maybe_unused is a good solution and there's also convincing
reason to prefer __maybe_unused over CONFIG_PM_SLEEP according to
Arnd Bergmann [2],

> > By and large, drivers handle this by using a CONFIG_PM_SLEEP ifdef.
> >
> > Unless you can make an extremely convincing argument why not to do
> > so here, I'd like you to handle it that way instead.
>
> [adding linux-pm to Cc]
>
> The main reason is that everyone gets the #ifdef wrong, I run into
> half a dozen new build regressions with linux-next every week on
> average, the typical problems being:
>
> - testing CONFIG_PM_SLEEP instead of CONFIG_PM, leading to an unused
>   function warning
> - testing CONFIG_PM instead of CONFIG_PM_SLEEP, leading to a build
>   failure
> - calling a function outside of the #ifdef only from inside an
>   otherwise correct #ifdef, again leading to an unused function
>   warning
> - causing a warning inside of the #ifdef but only testing if that
>   is disabled, leading to a problem if the macro is set (this is
>   rare these days for CONFIG_PM as that is normally enabled)
>
> Using __maybe_unused avoids all of the above.

> Regards,
>
> Hans

[2] https://lore.kernel.org/patchwork/comment/919944/

--
Best regards,
Coiby