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

[Petr Mladek]

On Mon 2026-04-20 22:18:29, Bird, Tim wrote:
> > From: Petr Mladek <pmladek@xxxxxxxx>
> >    2. The early timestamps provided by the bogo cycles
> >       are not synchronized with timestamps from
> >       the proper time keeping.
> > 
> >       Would it help to print a disclaimer, similar to,
> >       for example, trace_printk() first use?
> >       Something like:
> > 
> > [    0.002912] **********************************************************
> > [    0.002917] ****   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE
> > [    0.002921] **
> > [    0.002935] ** Using BOGO early timestamps
> > [    0.002939] **
> > [    0.002943] ** They are not properly calibrated and might use a source
> > [    0.002949] ** with an unstable frequency.
> > [    0.002953] **
> > [    0.002957] ** They are not comparable with timestamps after
> > [    0.002961] ** the timekeeping is initialized.
> > [    0.002966] **
> > [    0.002968] ****   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE
> > [    0.002971] *******************************************************
> 
> That's a bit long, but I think a warning like this is useful.

Yeah, feel free to make it shorter. I just wanted to explain what is
going on and what is the effect.

> > [    0.002975] Booting Linux on physical CPU 0x0000000000 [0x410fd083]
> > [    0.002998] Linux version 7.0.0-rc6-v8+ (tbird@timdesk) (aarch64-linux-gnu-gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0, GNU ld (GNU
> > Binutils for Ubuntu) 2.42) #20 SMP PREEMPT Fri Apr 10 11:57:48 MDT 2026
> > [    0.003002] KASLR enabled
> > [    0.003338] random: crng init done
> > [    0.003866] Machine model: Raspberry Pi 4 Model B Rev 1.5
> > [    0.004495] efi: UEFI not found.
> > ...
> > [    0.183552] Root IRQ handler: gic_handle_irq
> > [    0.183561] GIC: Using split EOI/Deactivate mode
> > [    0.183699] rcu: srcu_init: Setting srcu_struct sizes based on contention.
> > [    0.183958] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0xc743ce346, max_idle_ns: 440795203123 ns
> > [    0.183952] arch_timer: cp15 timer running at 54.00MHz (phys).
> > [    0.183957] **********************************************************
> > [    0.183962] ****   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE
> > [    0.183967] **
> > [    0.183971] ** End of BOGO early timestamps
> > [    0.183976] **
> > [    0.183982] ****   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE
> > [    0.183989] **********************************************************
> 
> I actually worked on a notice like this, to bridge/explain the transition (for humans).  But it turned
> out that different platforms initialized their clocks differently enough that it would have
> required putting the notice in different places, which would have required architecture-specific
> #ifdefs in generic code (so I didn't do it).

I think that we could print both notices from the printk() code:

 + 1st one before the very first message
 + 2nd one before the first message with a proper timestamp from
   the local_clock().

Best Regards,
Petr