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

Shashank Balaji
2026-03-09T17:27:42Z

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
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 :)

Thanks,
Shashank