linux-kernel - Re: [PATCH] sfc: use IS_ENABLED() checks for CONFIG_SFC

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Message-ID: <Y/IktbtY0rz+SVuX@corigine.com>
Date:   Sun, 19 Feb 2023 14:31:33 +0100
From:   Simon Horman <simon.horman@...igine.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Edward Cree <ecree.xilinx@...il.com>,
        Arnd Bergmann <arnd@...nel.org>,
        Martin Habets <habetsm.xilinx@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alejandro Lucero <alejandro.lucero-palau@....com>,
        Jonathan Cooper <jonathan.s.cooper@....com>,
        Netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH] sfc: use IS_ENABLED() checks for CONFIG_SFC_SRIOV

On Fri, Feb 17, 2023 at 05:19:26PM +0100, Arnd Bergmann wrote:
> On Fri, Feb 17, 2023, at 17:13, Edward Cree wrote:
> > On 17/02/2023 09:56, Arnd Bergmann wrote:
> >> From: Arnd Bergmann <arnd@...db.de>
> >> 
> >> One local variable has become unused after a recent change:
> >> 
> >> drivers/net/ethernet/sfc/ef100_nic.c: In function 'ef100_probe_netdev_pf':
> >> drivers/net/ethernet/sfc/ef100_nic.c:1155:21: error: unused variable 'net_dev' [-Werror=unused-variable]
> >>   struct net_device *net_dev = efx->net_dev;
> >>                      ^~~~~~~
> >> 
> >> The variable is still used in an #ifdef. Replace the #ifdef with
> >> an if(IS_ENABLED()) check that lets the compiler see where it is
> >> used, rather than adding another #ifdef.
> >
> > So we've had Leon telling us[1] to use __maybe_unused, and you're
> >  saying to use IS_ENABLED() instead.  Which is right?
> > (And does it make any difference to build time?  I'm assuming the
> >  compiler is smart enough that this change doesn't affect text
> >  size...?)
> > -ed
> 
> Both are correct, but I prefer the IS_ENABLED() change because it
> improves build coverage. The resulting object code should be the
> same, as the dead-code-elimination in gcc takes care of removing
> it the same way.
> 
> If you use the __maybe_uninitialized annotation, you still need
> an extra fix to initialize the ef100_probe_netdev_pf() return
> code.

FWIIW, IS_ENABLED() is the approach that is more familiar to me.
Though I have nothing in particular against other approaches.

Questions of consistency aside, this patch does look good to
me and does appear to address the build problem in question - on x86_64.

Reviewed-by: Simon Horman <simon.horman@...igine.com>