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

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On Wed, 15 Apr 2026 at 20:41, Geert Uytterhoeven <[email protected]> wrote:
>
> Hi Roberto,
...
> Thanks for your patch!
>
> > --- /dev/null
> > +++ kernel/printk/early_times.h
> > @@ -0,0 +1,26 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _EARLY_TIMES_H
> > +#define _EARLY_TIMES_H
> > +
> > +#include <linux/timekeeping.h>
> > +
> > +/*
> > + * Fencing isn't optional here, otherwise unreliable values displaying
> > + */
> > +#if defined(CONFIG_ARM64)
> > +             #include <asm/sysreg.h>
> > +             #define __early_raw_cycles ({ u64 val; \
> > +                 asm volatile("isb; mrs %0, cntvct_el0" : "=r"(val)); val; })
> > +#elif defined(CONFIG_X86_64)
> > +             #define __early_raw_cycles ({ u64 val; \
> > +                 asm volatile("lfence; rdtsc; shl $32, %%rdx; or %%rdx, %%rax" \
> > +                     : "=a"(val) : : "rdx"); val; })
> > +#elif defined(CONFIG_RISCV_TIMER)
> > +             #define __early_raw_cycles ({ u64 val; \
> > +                 asm volatile("fence; rdtime %0" : "=r"(val)); val; })
> > +#else
>
> All of these should be handled in arch/*/include/asm/early_times.h...
>
> > +             #define __early_raw_cycles 0
>
> ... while this should be in include/asm-generic/early_times.h.

In the last release of the patch, I decided to stick with the "single
point of hacking" approach

https://github.com/robang74/uchaosys/blob/v069/cnfg/printk-early-boot-timestamps-hack-v5.patch

and I developed it in a way that can be proposed in such "exceptional
way" for this very specific case:

+#if CONFIG_PRINTK_EARLY_BOOT_TIMINGS
+#include "early_times.h"
+ /*
+ * Very few developers are using this feature and they're expecting to deal
+ * with it as a single point of change hack to be further customised by them.
+ */
+ if (unlikely(!ts_nsec))
+ ts_nsec = __early_raw_cycles;
+#endif

As you can see the include uses double quotes to indicate that the
header is local and it is not supposed to belong to the formal three:
it is where everyone of us will put it when we are going to do dirty
tricks to reach somewhat result, in the same folder of the printk.c,
out-of-the-three.

Why is this a very special case?

- First of all because just a variable is provided by the macros set.
No hypothesis, just a variable as-is because most of the few users
that might be interested in are going to watch HOW the relative
sequences of events are happening. Moreover, there is no reason to
pretend (sell) a timestamp when the timestamp sub-system is not ready
to provide that feature: fairness first, by clarity. Previously the
time skew was from zeros and something, now from two different ways of
tagging the printk.

- Second, because there is a very limited use of it due to almost
immediate timestamp readiness. Therefore those who are interested,
they were hacking with very LOW level stuff (or BIOS for those are
constraint but that evilness in x86 arch). For those people the ASM
code is fine and it is very useful to have it in the same place. And
it is in the interest of those who maintain the kernel that those
people have a single point of hack for the major arch available.
Easier is for them to test different arches, far more improbable an
under-tested patch will reach a proposal upstream stage... and trust
me about "under-tested" because I am an expert on that field... ;-)
LOL

- Third, because every assumption has been removed, even the basics
>>10 shifts. Just something that appears as a variable emerging from a
local header and these two things together should ring some bells in
the head of those who have a little experience of firmware / embedded.
Few constructs are more vicious than macros that resemble variables...
;-)

> > +             if (unlikely(!ts_nsec))
> > +                     ts_nsec = __early_raw_cycles >> 10;
>
> As there can be lots of variation in the granularity of early timers,
> I think the shift should be moved in the arch-specific __early_raw_cycles,
> so it is only done when useful.
>

I am attaching the matured V5 patch that I tested against the 5.15.202
LTS kernel. Please, sit on the approach -- include"local-stuff.h" --
and let your judgement get soothed by time and PoVs. For sure one
change is probably a necessity, moving the Kconfig part into the DEBUG
section from the INIT where it stays by now. However, moving a feature
around the menuconfig is the last and less effort change in a TODO
list.

For sake of completeness, I am attaching the V5 matured patch. Will it
be refused because of its approach? Fine, at least the idea of that
approach has a patch that can be applied to something and people can
give it a try. Which add a practical degree of judgemental because
usually we are attracted by the beauty of perfection and purity,
something that Franco Battiato entitled "Inneres Auge" (a call from
above) but as complementary we are also keen to fall in love for those
bare-simple dirty-practical solution... in the same sense that song
was intending with a convoluted and educated paraphrase... LOL

Cheers, R-

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