Re: [patch 20/48] x86/apic: Enable TSC coupled programming mode

[Nathan Chancellor]

On Tue, Mar 03, 2026 at 03:37:03PM +0100, Thomas Gleixner wrote:
> On Mon, Mar 02 2026 at 18:29, Nathan Chancellor wrote:
> >
> > After this change landed in -next as commit f246ec3478cf ("x86/apic:
> > Enable TSC coupled programming mode"), two of my Intel-based test
> > machines fail to boot. Unfortunately, I do not think I have any serial
> > access on these, so I have little introspective ability. Is there any
> > information I can provide or patches I can test to try and help figure
> > out what is going on here? I have attached the output of lscpu of both
> > machines, in case there is some common thread there.
> 
> Grmbl. I stared at it for a while and I have a suspicion. Can you try
> the patch below and also provide from one of the machines the output of
> 
>   dmesg | grep -i tsc

This patch works on both machines, so your suspicion seemed spot on.

Output of that dmesg commmand appears to be the same between
89f951a1e8ad and f246ec3478cf with that diff applied:

  [    0.000000] tsc: Detected 2500.000 MHz processor
  [    0.000000] tsc: Detected 2496.000 MHz TSC
  [    0.008989] TSC deadline timer available
  [    0.119139] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x23fa772cf26, max_idle_ns: 440795269835 ns
  [    0.312141] clocksource: Switched to clocksource tsc-early
  [    0.322686] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x23fa772cf26, max_idle_ns: 440795269835 ns
  [    0.322951] clocksource: Switched to clocksource tsc

If there is anything else I can provide, let me know. If it becomes a
formal patch:

Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> ---
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -404,6 +404,7 @@ static void tk_setup_internals(struct ti
>  		 */
>  		clocks_calc_mult_shift(&tk->cs_ns_to_cyc_mult, &tk->cs_ns_to_cyc_shift,
>  				       NSEC_PER_MSEC, clock->freq_khz, 3600 * 1000);
> +		tk->cs_ns_to_cyc_maxns = div_u64(clock->mask, tk->cs_ns_to_cyc_mult);
>  	}
>  }
>