linux-kernel - Re: drivers/soc/litex/litex_soc_ctrl.c:143:34: warning: unused variable 'litex_soc_ctrl_of

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Message-ID: <20210111164334.GA1322395@ubuntu-m3-large-x86>
Date:   Mon, 11 Jan 2021 09:43:34 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Stafford Horne <shorne@...il.com>
Cc:     kernel test robot <lkp@...el.com>,
        Pawel Czarnecki <pczarnecki@...ernships.antmicro.com>,
        kbuild-all@...ts.01.org, clang-built-linux@...glegroups.com,
        linux-kernel@...r.kernel.org,
        Mateusz Holenko <mholenko@...micro.com>
Subject: Re: drivers/soc/litex/litex_soc_ctrl.c:143:34: warning: unused
 variable 'litex_soc_ctrl_of_match'

On Mon, Jan 11, 2021 at 09:30:55PM +0900, Stafford Horne wrote:
> On Thu, Jan 07, 2021 at 04:04:47AM +0800, kernel test robot wrote:
> > Hi Pawel,
> > 
> > FYI, the error/warning still remains.
> > 
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   e71ba9452f0b5b2e8dc8aa5445198cd9214a6a62
> > commit: 22447a99c97e353bde8f90c2353873f27681d57c drivers/soc/litex: add LiteX SoC Controller driver
> > date:   8 weeks ago
> > config: x86_64-randconfig-a001-20210107 (attached as .config)
> > compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project 5c951623bc8965fa1e89660f2f5f4a2944e4981a)
> > reproduce (this is a W=1 build):
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # install x86_64 cross compiling tool for clang build
> >         # apt-get install binutils-x86-64-linux-gnu
> >         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=22447a99c97e353bde8f90c2353873f27681d57c
> >         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >         git fetch --no-tags linus master
> >         git checkout 22447a99c97e353bde8f90c2353873f27681d57c
> >         # save the attached .config to linux build tree
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=x86_64 
> > 
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp@...el.com>
> > 
> > All warnings (new ones prefixed by >>):
> > 
> > >> drivers/soc/litex/litex_soc_ctrl.c:143:34: warning: unused variable 'litex_soc_ctrl_of_match' [-Wunused-const-variable]
> >    static const struct of_device_id litex_soc_ctrl_of_match[] = {
> >                                     ^
> >    1 warning generated.
> > 
> > 
> > vim +/litex_soc_ctrl_of_match +143 drivers/soc/litex/litex_soc_ctrl.c
> > 
> >    142	
> >  > 143	static const struct of_device_id litex_soc_ctrl_of_match[] = {
> >    144		{.compatible = "litex,soc-controller"},
> >    145		{},
> >    146	};
> >    147	
> > 
> 
> I don't use clang but GCC, and I cannot reproduce this warning.
> 
> $ make drivers/soc/litex/litex_soc_ctrl.o 
>   CALL    scripts/checksyscalls.sh
>   CALL    scripts/atomic/check-atomics.sh
>   DESCEND  objtool
>   CC      drivers/soc/litex/litex_soc_ctrl.o
> 
> Also, I can see litex_soc_ctrl_of_match is used.  I am not sure what is going on
> here.
> 
> -Stafford
> 

You need W=1

$ make -skj"$(nproc)" W=1 olddefconfig drivers/soc/litex/litex_soc_ctrl.o
drivers/soc/litex/litex_soc_ctrl.c:143:34: warning: ‘litex_soc_ctrl_of_match’ defined but not used [-Wunused-const-variable=]
  143 | static const struct of_device_id litex_soc_ctrl_of_match[] = {
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~

$ rg "CONFIG_OF|CONFIG_LITEX_SOC_CONTROLLER" .config
1124:# CONFIG_OF is not set
4673:CONFIG_LITEX_SOC_CONTROLLER=y

This variable is used in two places in that file, in the
MODULE_DEVICE_TABLE macro and the of_match_ptr macro. When CONFIG_OF is
disabled, of_match_ptr evaluates to NULL. When the file is built into
the kernel image, MODULE_DEVICE_TABLE evaluates to nothing, leaving this
variable defined but unused in the final preprocessed source.

Hope that helps!

Cheers,
Nathan