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

[Bird, Tim]

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@xxxxxxxxx>
> 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@xxxxxxxxx/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@xxxxxxxxx/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@xxxxxxxxx>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202604121822.OvOLcTnO-lkp@xxxxxxxxx/
> 
> 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.

>     46		return 0;
>     47	}
>     48	#else
>     49	static inline u64 early_times_ns(void)
>     50	{
>     51		return 0;
>     52	}
>     53	#endif
>     54
> 
> --
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