linux-kernel - Re: [PATCH v8 01/25] scsi/atari_scsi: Don't select CONFIG

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Date:   Sun, 30 Dec 2018 10:06:43 -0800
From:   James Bottomley <jejb@...ux.vnet.ibm.com>
To:     LEROY Christophe <christophe.leroy@....fr>,
        Arnd Bergmann <arnd@...db.de>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Finn Thain <fthain@...egraphics.com.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        Michael Schmitz <schmitzmic@...il.com>
Subject: Re: [PATCH v8 01/25] scsi/atari_scsi: Don't select CONFIG_NVRAM

On Sun, 2018-12-30 at 18:50 +0100, LEROY Christophe wrote:
> Arnd Bergmann <arnd@...db.de> a écrit :
> > On Sat, Dec 29, 2018 at 3:51 AM Michael Schmitz
> > <schmitzmic@...il.com> wrote:
[...]
> > > (On second thought - I don't want to speculate whether there's
> > > weird compiler options that could result in the
> > > nvram_check_checksum and nvram_read_bytes symbols to still be
> > > referenced in the final link, even though
> > > IS_BUILTIN(CONFIG_NVRAM) always evaluates to false. Best
> > > leave this as-is.)
> > 
> > As far as I know, it's totally reliable with the supported
> > compilers  (gcc-4.6+). In the older compilers (e.g. 4.1), there was
> > a corner case, where it could have failed to eliminate a function
> > that was only referenced through a pointer from a discarded
> > variable, but a plain IS_ENABLED() check like the one here
> > was still ok, and lots of code relies on that.
> > 
> > Other than that, I agree either way is totally fine here, so no
> > objections to using the #ifdef.
> 
> As far as I know, kernel codying style promotes the use of  
> IS_ENABLED() etc. instead of #ifdefs when possible.

It's a preference, as with a lot of coding style stuff, which we leave
up to the maintainer.

That said, as has been pointed out, the current #ifdef has a failing
corner case when both are modular (because the code should then be
included).  The runtime macro that correctly expresses this is
IS_REACHABLE(CONFIG_NVRAM).

James