Re: [PATCH v3] printk: fix zero-valued printk timestamps in early boot

[Petr Mladek <pmladek@suse.com>]

On Tue 2026-03-10 02:27:27, Shashank Balaji wrote:
> Hi Tim,
> 
> Tested-by: Shashank Balaji <shashankbalaji02@gmail.com>
> 
> ...on top of rc3 on an AMD Ryzen 7 4800H laptop. This patch conflicts
> with these commits with trivial fixes:
> 
> 032a730268a3 init/main.c: wrap long kernel cmdline when printing to logs
> 60325c27d3cfq printk: Add execution context (task name/CPU) to printk_info
> 499f86de4f8c init/main: read bootconfig header with get_unaligned_le32()

Good to know.

> On Tue, Feb 10, 2026 at 04:47:41PM -0700, Tim Bird wrote:
> > During early boot, printk timestamps are reported as zero before
> 	<snip>
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 1d765ad242b8..5afd31c3345c 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2242,6 +2254,8 @@ int vprintk_store(int facility, int level,
> >  	 * timestamp with respect to the caller.
> >  	 */
> >  	ts_nsec = local_clock();
> > +	if (!ts_nsec)
> > +		ts_nsec = early_cycles();
> 
> ts_nsec goes on to be stored in a struct printk_info's ts_nsec which is
> documented to be "timestamp in nanoseconds":
> 
> 	/*
> 	 * Meta information about each stored message.
> 	 *
> 	 * All fields are set by the printk code except for @seq, which is
> 	 * set by the ringbuffer code.
> 	 */
> 	struct printk_info {
> 		u64	seq;		/* sequence number */
> 		u64	ts_nsec;	/* timestamp in nanoseconds */
> 		u16	text_len;	/* length of text message */
> 		u8	facility;	/* syslog facility */
> 		u8	flags:5;	/* internal record flags */
> 		u8	level:3;	/* syslog level */
> 		u32	caller_id;	/* thread id or processor id */
> 	#ifdef CONFIG_PRINTK_EXECUTION_CTX
> 		u32	caller_id2;	/* caller_id complement */
> 		/* name of the task that generated the message */
> 		char	comm[TASK_COMM_LEN];
> 	#endif
> 
> 		struct dev_printk_info	dev_info;
> 	};
> 
> Since with this patch, ts_nsec can either be a timestamp in ns or a
> cycle count, the comment should be updated.

Yup, great catch!

> Ideally, I'd like the member
> name to be changed as well to reflect the new semantic. I'm thinking 
> ts_raw or ts_ns_or_cyc... naming is hard :)

Hmm, we could not change it easily because it would break user space
tools for reading kernel crash dump.

Alternative solution would be usign an union.

	union {
		u64	ts_nsec;
		u64	ts_cycles;
	};

Best Regards,
Petr