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

[Bird, Tim]

> -----Original Message-----
> From: Shashank Balaji <shashankbalaji02@xxxxxxxxx>
> Hi Tim,
> 
> Tested-by: Shashank Balaji <shashankbalaji02@xxxxxxxxx>

Thanks for the testing!!

> 
> ...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
> 60325c27d3cf printk: Add execution context (task name/CPU) to printk_info
> 499f86de4f8c init/main: read bootconfig header with get_unaligned_le32()
> 
> Comment below.
> 
> 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
> > @@ -46,6 +46,7 @@
> >  #include <linux/ctype.h>
> >  #include <linux/uio.h>
> >  #include <linux/sched/clock.h>
> > +#include <linux/early_times.h>
> >  #include <linux/sched/debug.h>
> >  #include <linux/sched/task_stack.h>
> >  #include <linux/panic.h>
> > @@ -75,6 +76,13 @@ EXPORT_SYMBOL(ignore_console_lock_warning);
> >
> >  EXPORT_TRACEPOINT_SYMBOL_GPL(console);
> >
> > +#ifdef CONFIG_EARLY_PRINTK_TIMES
> > +cycles_t start_cycles;
> > +u64 start_ns;
> > +u32 early_mult, early_shift;
> > +u64 early_ts_offset;
> > +#endif
> > +
> >  /*
> >   * Low level drivers may need that to know if they can schedule in
> >   * their unblank() callback or not. So let's export it.
> > @@ -639,7 +647,7 @@ static void append_char(char **pp, char *e, char c)
> >  static ssize_t info_print_ext_header(char *buf, size_t size,
> >  				     struct printk_info *info)
> >  {
> > -	u64 ts_usec = info->ts_nsec;
> > +	u64 ts_usec = adjust_early_ts(info->ts_nsec);
> >  	char caller[20];
> >  #ifdef CONFIG_PRINTK_CALLER
> >  	u32 id = info->caller_id;
> > @@ -1352,7 +1360,11 @@ static size_t print_syslog(unsigned int level, char *buf)
> >
> >  static size_t print_time(u64 ts, char *buf)
> >  {
> > -	unsigned long rem_nsec = do_div(ts, 1000000000);
> > +	unsigned long rem_nsec;
> > +
> > +	ts = adjust_early_ts(ts);
> > +
> > +	rem_nsec = do_div(ts, 1000000000);
> >
> >  	return sprintf(buf, "[%5lu.%06lu]",
> >  		       (unsigned long)ts, rem_nsec / 1000);
> > @@ -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. 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 :)

Nice catch!  I'm considering either changing the comment, or
using a union here (or maybe both).

Thanks,
 -- Tim