linux-kernel - Re: [possible PATCH] crypto: sahara - Use #ifdef DEBUG not IS

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Date:   Fri, 22 Mar 2019 08:07:12 -0700
From:   Joe Perches <joe@...ches.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     "David S. Miller" <davem@...emloft.net>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [possible PATCH] crypto: sahara - Use #ifdef DEBUG not
 IS_ENABLED(DEBUG)

On Fri, 2019-03-22 at 15:29 +0100, Ard Biesheuvel wrote:
> On Fri, 22 Mar 2019 at 13:43, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> > On Thu, Mar 07, 2019 at 04:15:55PM -0800, Joe Perches wrote:
> > > Normal use of IS_ENABLED is with a CONFIG_<SYMBOL> and
> > > there is no -DDEBUG in the Makefile here.
> > > 
> > > Replace the IS_ENABLED(DEBUG) with #ifdef DEBUG/#endif
> > > blocks.
> > > 
> > > Miscellanea:
> > > 
> > > o Move the sahara_state array into the function that uses it.
> > > 
> > > Signed-off-by: Joe Perches <joe@...ches.com>
> > > ---
> > >  drivers/crypto/sahara.c | 20 +++++++++-----------
> > >  1 file changed, 9 insertions(+), 11 deletions(-)
> > 
> > Even if this is correct this is way too ugly.  The original code
> > at least compiled everything regardless of macros.  Your new code
> > won't detect compile errors in debugging code unless debugging is
> > enabled.
> > 
> 
> What's wrong with IS_ENABLED(DEBUG) anyway? It may not be 'normal use'
> but it works fine.

drivers/crypto/sahara.c is the only user in the kernel tree.
So only it's abnormal use.  I rather like the concept actually.

IS_ENABLED is almost exclusively used with CONFIG_<FOO> symbols
and it could be useful to require it to be used with CONFIG_<FOO>
symbols and use some other similar mechanism for DEBUG use.

Maybe just adding a global #define in kernel.h like

#define IS_DEBUG_ENABLED	IS_ENABLED(DEBUG)

to isolate this in one place might be better.

A good thing about using IS_ENABLED or the suggested IS_DEBUG_ENABLED
would be that least gcc 5+ seems to automatically elide the uses of any
unreferenced static const char * arrays like the sahara_state uses here.

(I don't have gcc4 anymore so I couldn't check that version)

So using 'if (IS_DEBUG_ENABLED)' could simplify some existing code like
the many uses of

#ifdef DEBUG
	...
#endif

or equivalent