Date:   Sun, 14 Nov 2021 17:02:53 -0800
From:   Drew Fustini <drew@beagleboard.org>
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     kernel test robot <lkp@intel.com>, kbuild-all@lists.01.org,
        linux-kernel@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>
Subject: Re: drivers/clocksource/timer-pistachio.c:74:22: warning: variable
 'overflow' set but not used
Message-ID: <20211115010253.GA325926@x1>
References: <202111120738.oAGjK5L8-lkp@intel.com>
 <20211114212938.GA317441@x1>
 <552daf37-c5eb-6634-7456-3fa39c74ebcb@linaro.org>
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In-Reply-To: <552daf37-c5eb-6634-7456-3fa39c74ebcb@linaro.org>
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On Sun, Nov 14, 2021 at 11:17:38PM +0100, Daniel Lezcano wrote:
> On 14/11/2021 22:29, Drew Fustini wrote:
> > On Fri, Nov 12, 2021 at 07:05:48AM +0800, kernel test robot wrote:
> >> Hi Drew,
> >>
> >> FYI, the error/warning still remains.
> >>
> >> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> >> head:   debe436e77c72fcee804fb867f275e6d31aa999c
> >> commit: a47d7ef4550d08fb428ea4c3f1a9c71674212208 clocksource/drivers/pistachio: Fix trivial typo
> >> date:   7 months ago
> >> config: mips-randconfig-r012-20210927 (attached as .config)
> >> compiler: mips-linux-gcc (GCC) 11.2.0
> >> 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
> >>         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a47d7ef4550d08fb428ea4c3f1a9c71674212208
> >>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >>         git fetch --no-tags linus master
> >>         git checkout a47d7ef4550d08fb428ea4c3f1a9c71674212208
> >>         # save the attached .config to linux build tree
> >>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross ARCH=mips 
> >>
> >> If you fix the issue, kindly add following tag as appropriate
> >> Reported-by: kernel test robot <lkp@intel.com>
> >>
> >> All warnings (new ones prefixed by >>):
> >>
> >>    drivers/clocksource/timer-pistachio.c: In function 'pistachio_clocksource_read_cycles':
> >>>> drivers/clocksource/timer-pistachio.c:74:22: warning: variable 'overflow' set but not used [-Wunused-but-set-variable]
> >>       74 |         u32 counter, overflow;
> >>          |                      ^~~~~~~~
> >>
> >>
> >> vim +/overflow +74 drivers/clocksource/timer-pistachio.c
> >>
> >>     69	
> >>     70	static u64 notrace
> >>     71	pistachio_clocksource_read_cycles(struct clocksource *cs)
> >>     72	{
> >>     73		struct pistachio_clocksource *pcs = to_pistachio_clocksource(cs);
> >>   > 74		u32 counter, overflow;
> >>     75		unsigned long flags;
> >>     76	
> >>     77		/*
> >>     78		 * The counter value is only refreshed after the overflow value is read.
> >>     79		 * And they must be read in strict order, hence raw spin lock added.
> >>     80		 */
> >>     81	
> >>     82		raw_spin_lock_irqsave(&pcs->lock, flags);
> >>     83		overflow = gpt_readl(pcs->base, TIMER_CURRENT_OVERFLOW_VALUE, 0);
> 
> overflow is set here but then never reused in the function. So a call to
> gpt_readl without getting the return value is fine.
> 
> The warning is different from 'variable is unused'

Thank you for pointing that out.  I was reading it incorrectly.

As for this function, it seems that warning is a false positive as
the act of reading TIMER_CURRENT_OVERFLOW_VALUE causes the hardware
to refresh TIMER_CURRENT_VALUE.

Thanks,
Drew