Re: [PATCH u-boot-marvell v2 8/9] arm: mvebu: spl: Use IS_ENABLED() instead of #ifdef where possible

From: "Marek Behún" <kabel@kernel.org>
To: "Pali Rohár" <pali@kernel.org>
Cc: "Stefan Roese" <sr@denx.de>,
	u-boot@lists.denx.de, "Marek Behún" <marek.behun@nic.cz>
Subject: Re: [PATCH u-boot-marvell v2 8/9] arm: mvebu: spl: Use IS_ENABLED() instead of #ifdef where possible
Date: Tue, 14 Dec 2021 13:45:36 +0100	[thread overview]
Message-ID: <20211214134536.2baeb2a0@thinkpad> (raw)
In-Reply-To: <20211214111234.ie54zex6veoirfam@pali>

On Tue, 14 Dec 2021 12:12:34 +0100
Pali Rohár <pali@kernel.org> wrote:

> On Tuesday 14 December 2021 10:45:15 Marek Behún wrote:
> > On Tue, 14 Dec 2021 10:36:00 +0100
> > Pali Rohár <pali@kernel.org> wrote:
> >   
> > > On Friday 26 November 2021 15:37:37 Marek Behún wrote:  
> > > > @@ -340,17 +333,17 @@ void board_init_f(ulong dummy)
> > > >  	timer_init();
> > > >  
> > > >  	/* Armada 375 does not support SerDes and DDR3 init yet */
> > > > -#if !defined(CONFIG_ARMADA_375)
> > > > -	/* First init the serdes PHY's */
> > > > -	serdes_phy_config();
> > > > -
> > > > -	/* Setup DDR */
> > > > -	ret = ddr3_init();
> > > > -	if (ret) {
> > > > -		debug("ddr3_init() failed: %d\n", ret);
> > > > -		hang();
> > > > +	if (!IS_ENABLED(CONFIG_ARMADA_375)) {
> > > > +		/* First init the serdes PHY's */
> > > > +		serdes_phy_config();
> > > > +
> > > > +		/* Setup DDR */
> > > > +		ret = ddr3_init();
> > > > +		if (ret) {
> > > > +			debug("ddr3_init() failed: %d\n", ret);
> > > > +			hang();
> > > > +		}
> > > >  	}
> > > > -#endif    
> > > 
> > > As written in comment above there is no SerDes and DDR3 support for
> > > Armada 375 and therefore there is no serdes_phy_config() or ddr3_init()
> > > function. So this code needs not to be compiled at all and usage of
> > > #ifdef is correct here.  
> > 
> > #ifdefs are almost never correct in C-files, for the parts of the code
> > they guard isn't put through syntactic analysis, and can therefore
> > contain bugs which we are not warned about.
> > 
> > Using if (IS_ENABLED()) almost never producess a different binary,
> > because the code is optimized away.
> > 
> > Marek  
> 
> There is no function serdes_phy_config() for Armada 375, so if you put
> it outside of #ifdef you will get compile error.

The function is always declared in
  arch/arm/mach-mvebu/include/mach/cpu.h
regardless of architecture.

Thus an error will be raised only when linking, and the compliation was
done with -O0, which I don't think anyone does.

Anyway, if we want to support -O0, this can and should be solved via
defining serdes_phy_config() as empty static inline function in the
cpu.h header, guarded by #ifdef. In header files #ifdefs are allowed,
in this manner:
  #if X
  declare function
  #else
  define that function as empty static inline
  #endif

So if you think we should support -O0, I can do this.

But the #ifdefs should really go away from real C code, that is the way
both Linux and U-Boot are trying to go for the last couple of years, if
I understand it correctly.

Marek