Re: [PATCH v4 1/1] printk: fix zero-valued printk timestamps in early boot

[David Laight <david.laight.linux@gmail.com>]

On Mon, 13 Apr 2026 17:58:34 +0000
"Bird, Tim" <Tim.Bird@sony.com> wrote:

> Hmmm,
> 
> This is a false positive warning from the compiler (see below).
> 
> I apologize for not catching it.   I did test with CONFIG_EARLY_CYCLES=0
> and I'm not sure why I didn't see the warning.
> 
> I'll see if I can silence this warning.
> 
> > -----Original Message-----
> > From: kernel test robot <lkp@intel.com>
> > Sent: Sunday, April 12, 2026 4:11 AM
> > Hi Tim,
> > 
> > kernel test robot noticed the following build warnings:
> > 
> > [auto build test WARNING on linus/master]
> > [also build test WARNING on v7.0-rc7 next-20260410]
> > [If your patch is applied to the wrong git tree, kindly drop us a note.
> > And when submitting patch, we suggest to use '--base' as documented in
> > https://git-scm.com/docs/git-format-patch#_base_tree_information]
> > 
> > url:    https://github.com/intel-lab-lkp/linux/commits/Tim-Bird/printk-fix-zero-valued-printk-timestamps-in-early-boot/20260412-134726
> > base:   linus/master
> > patch link:    https://lore.kernel.org/r/20260410203741.997410-2-tim.bird%40sony.com
> > patch subject: [PATCH v4 1/1] printk: fix zero-valued printk timestamps in early boot
> > config: arm64-allnoconfig (https://download.01.org/0day-ci/archive/20260412/202604121822.OvOLcTnO-lkp@intel.com/config)
> > compiler: aarch64-linux-gcc (GCC) 15.2.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260412/202604121822.OvOLcTnO-lkp@intel.com/reproduce)
> > 
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@intel.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202604121822.OvOLcTnO-lkp@intel.com/
> > 
> > All warnings (new ones prefixed by >>):
> > 
> >    In file included from kernel/printk/printk.c:49:
> >    include/linux/early_times.h: In function 'early_times_ns':  
> > >> include/linux/early_times.h:45:61: warning: division by zero [-Wdiv-by-zero]  
> >       45 |                 return (early_unsafe_cycles() * NS_PER_KHZ) / CONFIG_EARLY_CYCLES_KHZ;
> >          |                                                             ^
> > 
> > 
> > vim +45 include/linux/early_times.h
> > 
> >     34
> >     35	/* returns a nanosecond value based on early cycles */
> >     36	static inline u64 early_times_ns(void)
> >     37	{
> >     38		if (CONFIG_EARLY_CYCLES_KHZ)
> >     39			/*
> >     40			 * Note: the multiply must precede the division to avoid
> >     41			 * truncation and loss of resolution
> >     42			 * Don't use fancier MULT/SHIFT math here.  Since this is
> >     43			 * static, the compiler can optimize the math operations.
> >     44			 */  
> >   > 45			return (early_unsafe_cycles() * NS_PER_KHZ) / CONFIG_EARLY_CYCLES_KHZ;  
> Based on this conditional, it's not possible for CONFIG_EARLY_CYCLES_KHZ to be zero
> on this line of code.  Does GCC not catch this?
> if (0)
>    x = <some expression>/0;
> 
> So this is a false positive.

I suspect the warning is being generated before the optimiser throws
the code away as unreachable.
You might have to do:
	return (early_unsafe_cycles() * NS_PER_KHZ) / (CONFIG_EARLY_CYCLES_KHZ ?: 1);

David

> 
> >     46		return 0;
> >     47	}
> >     48	#else
> >     49	static inline u64 early_times_ns(void)
> >     50	{
> >     51		return 0;
> >     52	}
> >     53	#endif
> >     54
> > 
> > --
> > 0-DAY CI Kernel Test Service
> > https://github.com/intel/lkp-tests/wiki