linux-kernel - Re: [RFC/PATCH 1/3] pm: make PM macros more smart

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Message-ID: <20131220202336.GB14693@psi-dev26.jf.intel.com>
Date:	Fri, 20 Dec 2013 12:23:36 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Pavel Machek <pavel@....cz>
Cc:	rjw@...ysocki.net, len.brown@...el.com,
	sarah.a.sharp@...ux.intel.com, gregkh@...uxfoundation.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org, santosh.shilimkar@...com
Subject: Re: [RFC/PATCH 1/3] pm: make PM macros more smart

On Fri, Dec 20, 2013 at 08:55:27PM +0100, Pavel Machek wrote:
> On Sun 2013-12-15 11:25:08, David Cohen wrote:
> > On Sun, Dec 15, 2013 at 06:51:12PM +0100, Pavel Machek wrote:
> > > On Thu 2013-12-12 21:18:23, David Cohen wrote:
> > > > This patch makes SET_SYSTEM_SLEEP_PM_OPS() and SET_RUNTIME_PM_OPS() more
> > > > smart.
> > > > 
> > > > Despite those macros check for '#ifdef CONFIG_PM_SLEEP/RUNTIME' to avoid
> > > > setting the callbacks when such #ifdef's aren't defined, they don't
> > > > handle compiler to avoid messages like that:
> > > > 
> > > > drivers/usb/host/xhci-plat.c:200:12: warning: ???xhci_plat_suspend??? defined but not used [-Wunused-function]
> > > > drivers/usb/host/xhci-plat.c:208:12: warning: ???xhci_plat_resume??? defined but not used [-Wunused-function]
> > > > 
> > > > As result, those macros get rid of #ifdef's when setting callbacks but
> > > > not when implementing them.
> > > > 
> > > > With this patch, drivers using SET_*_PM_OPS() macros don't need to #ifdef
> > > > the callbacks implementation as well.
> > > 
> > > Well... Interesting trickery, but it means that resulting kernel
> > > will be bigge due to the dead functions no?
> > 
> > Actually, it doesn't get bigger. Before sending the patch I did this
> > dummy test app:
> > 
> > --------------------------------------------------------------------------------
> > #include <stdio.h>
> > 
> > #define USE_IT_OR_LOOSE_IT(fn)  ((void *)((unsigned long)(fn) - (unsigned long)(fn)))
> > 
> > #ifdef MAKE_ME_NULL
> > static int func1(int a)
> > {
> >         printf("Hey!!\n");
> >         return 0;
> > }
> > #endif
> 
> I thought that point of this patch series was getting rid of the
> #ifdefs around the function...? Now I'm confused.

Maybe you're misinterpreting the test :)

This #ifdef is used to make this same test code to replicate both
scenarios according to -DMAKE_ME_NULL (just pay attention to actual
resulting code after #ifdef's are tested. the #ifdef here is nor related
to actual #ifdef on kernel). Here are both scenarios:

(1) Not using my trickery (which needs the function to not be present).
(2) Using my trickery (which needs to function to stay).

With -DMAKE_ME_NULL we replicate (2), then the function *is* there but
gcc gets rid of it on resulting binary without warnings if used with -O2.

Without -DMAKE_ME_NULL we replicate (1). The #ifdef will fail and then
remove the function which is an obvious scenario the function won't be
part of resulting binary.

If we use -S option to have human readable resulting assembly code
(which is kind of 1:1 for resulting binary), we can compare the result
of (1) and (2) and check they are pretty similar.
This proves gcc behaves as expected with my patch: do not need #ifdef
and do not generate dead codes to resulting binary.

> 
> > struct global_data {
> >         int (*func)(int);
> > };
> > 
> > static struct global_data gd = {
> > #ifdef MAKE_ME_NULL
> >         .func = USE_IT_OR_LOOSE_IT(func1),
> 
> If you have ifdef around the function, why do you need magic here? Why
> not

This #ifdef is necessary to prevent the function to be used when it
doesn't exist due to above #ifdef. But once again: don't misinterpret
the #ifdefs in this test app with the ones in kernel. They are not
related at all. If it's still confusing you just make 2 test apps
without #ifdeds out of this one where one keeps the code inside #ifdefs
and the other doesn't.

> 
> 	.func = func1
> 
> ?
> 
> Basically the warning tells you that you want the ifdef around the
> function, too... (Otherwise you waste space). That seems like good
> warning.

Just check my first explanation.

Br, David Cohen

> 
> 									Pavel
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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