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

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Date:   Fri, 17 Feb 2023 17:19:26 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "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>
Cc:     "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 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.

      Arnd