[PATCH v4 0/1] printk: fix zero-valued printk timestamps

[PATCH v4 0/1] printk: fix zero-valued printk timestamps

[Tim Bird]

Some kernel messages emitted before kernel timekeeping is started
have a timestamp value of zero.  However, some platforms
have a cycle counter or timer register that is initialized
before the kernel starts (either in hardware or pre-kernel
firmware), and it can be accessed to provide uncalibrated
values before any other kernel initialization.  I refer
to this period where kernel timestamps are zero as the
printk timing "blind spot".

The following patch allows developers to set a config value
CONFIG_EARLY_CYCLES_KHZ, that will be used to provide
useful timestamps on early printk messages.
This calibration value is set as a static configuration
variable, since it must be available from the very first kernel
instruction.  Since this value is machine-specific, it cannot
be used in the .config for a generic distribution kernel.
The feature is presented here as something useful for
kernel developers working on boot-time optimization
(ie, as something that can be used temporarily during boot
time research and development work, and not necessarily
a runtime option for the average user).

Different platforms have different blind spot durations.
On an x86_64 platform, due to early tsc initialization, the
blind spot may be as small at 16 milliseconds.  On a
Raspberry Pi 4 board, it is approximately 180 milliseconds.
On a RISC V BeagleV Fire board, it can be up to 1700
milliseconds (1.7 seconds).  On some platforms, the blind spot
duration is a significant percentage of the time before user-space
initialization in the kernel (greater than 20%).

This feature uses a different mechanism to gather timestamp values.
When kernel timekeeping starts, the printk timestamps change
to a new timebase based on local_clock(), and timestamp values
reset to the baseline used for kernel timekeeping. This results in a
one-time discontinuity in timestamps between early printks and later
printks. The discontinuity should be obvious to humans, but could
possibly confuse tools.

Here is before and after dmesg output for a Raspberry PI 4 board:

before:
[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd083]
[    0.000000] Linux version 7.0.0-rc3-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) #5 SMP PREEMPT Wed Mar 11 18:30:18 MDT 2026
[    0.000000] KASLR enabled
[    0.000000] random: crng init done
[    0.000000] Machine model: Raspberry Pi 4 Model B Rev 1.5
[    0.000000] efi: UEFI not found.
...
[    0.000000] Root IRQ handler: gic_handle_irq
[    0.000000] GIC: Using split EOI/Deactivate mode
[    0.000000] rcu: srcu_init: Setting srcu_struct sizes based on contention.
[    0.000000] arch_timer: cp15 timer running at 54.00MHz (phys).
[    0.000000] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0xc743ce346, max_idle_ns: 440795203123 ns
[    0.000000] sched_clock: 56 bits at 54MHz, resolution 18ns, wraps every 4398046511102ns
[    0.000161] Console: colour dummy device 80x25
[    0.000170] printk: legacy console [tty1] enabled

after:
[    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).
[    0.000000] sched_clock: 56 bits at 54MHz, resolution 18ns, wraps every 4398046511102ns
[    0.000157] Console: colour dummy device 80x25
[    0.000165] printk: legacy console [tty1] enabled


Tim Bird (1):
  printk: fix zero-valued printk timestamps in early boot

 include/linux/early_times.h | 55 +++++++++++++++++++++++++++++++++++++
 init/Kconfig                | 30 ++++++++++++++++++++
 kernel/printk/printk.c      |  3 ++
 3 files changed, 88 insertions(+)
 create mode 100644 include/linux/early_times.h

-- 
2.43.0

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

[Tim Bird]

During early boot, printk timestamps are reported as zero before
kernel timekeeping starts (i.e. before time_init()).  This
hinders boot-time optimization efforts.  This period ranges from
17 to 1700 milliseconds on different embedded machines running Linux.

Add support for early timestamps based on processor cycle-generators
that need no kernel initialization. Use a statically-configured
frequency value for converting cycles to nanoseconds. This means
that this feature cannot be turned on in generic distro kernels.
It is intended for temporary use during kernel development and
boot-time research and optimization only.

This yields non-zero timestamps for printks from the very start
of kernel execution.  The timestamps are relative to the start of
an architecture-specific counter (e.g. tsc on x86_64 and cntvct_el0
on arm64).  Affected timestamps reflect time from cycle counter
init (usually machine power-on or virtual machine start) instead of
time from the kernel's timekeeping initialization. This results in a
discontinuity in the printk timestamp values, one time, when
kernel timekeeping starts.

Signed-off-by: Tim Bird <tim.bird@sony.com>
---
V3 -> V4
  Replace config vars with single one: CONFIG_EARLY_CYCLES_KHZ
  Replace runtime calibration with static config variable
  Remove reference to get_cycles()
  Add support for RISCV platforms
V2 -> V3
  Default CONFIG option to 'n'
  Move more code into early_times.h (reduce ifdefs in init/main.c)
  Use match64 helper routines
  Use cycles_t instead of u64 type
  Add #defines for EARLY_CYCLES_BIT and EARLY_CYCLES_MASK
  Invert if logic in adjust_early_ts()
V1 -> V2
  Remove calibration CONFIG vars
  Add 'depends on' to restrict arches (to handle ppc bug)
  Add early_ts_offset to avoid discontinuity
  Save cycles in ts_nsec, and convert on output
  Move conditional code to include file (early_times.h>
---
 include/linux/early_times.h | 55 +++++++++++++++++++++++++++++++++++++
 init/Kconfig                | 30 ++++++++++++++++++++
 kernel/printk/printk.c      |  3 ++
 3 files changed, 88 insertions(+)
 create mode 100644 include/linux/early_times.h

diff --git a/include/linux/early_times.h b/include/linux/early_times.h
new file mode 100644
index 000000000000..82bacfd0e26b
--- /dev/null
+++ b/include/linux/early_times.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _EARLY_TIMES_H
+#define _EARLY_TIMES_H
+
+#include <linux/timekeeping.h>
+#ifdef CONFIG_ARM64
+#include <asm/sysreg.h>
+#endif
+
+#ifdef CONFIG_EARLY_CYCLES_KHZ
+static inline u64 early_unsafe_cycles(void)
+{
+#if defined(CONFIG_X86_64)
+	/*
+	 * This rdtsc may happen before secure TSC is initialized, and
+	 * it is unordered. So please don't use this value for cryptography
+	 * or after SMP is initialized.
+	 */
+	return rdtsc();
+#elif defined(CONFIG_ARM64)
+	return read_sysreg(cntvct_el0);
+#elif defined(CONFIG_RISCV_TIMER)
+	u64 val;
+
+	asm volatile("rdtime %0" : "=r"(val));
+	return val;
+#else
+	return 0;
+#endif
+}
+
+#define NS_PER_KHZ	1000000UL
+
+/* returns a nanosecond value based on early cycles */
+static inline u64 early_times_ns(void)
+{
+	if (CONFIG_EARLY_CYCLES_KHZ)
+		/*
+		 * Note: the multiply must precede the division to avoid
+		 * truncation and loss of resolution
+		 * Don't use fancier MULT/SHIFT math here.  Since this is
+		 * static, the compiler can optimize the math operations.
+		 */
+		return (early_unsafe_cycles() * NS_PER_KHZ) / CONFIG_EARLY_CYCLES_KHZ;
+	return 0;
+}
+#else
+static inline u64 early_times_ns(void)
+{
+	return 0;
+}
+#endif
+
+#endif /* _EARLY_TIMES_H */
diff --git a/init/Kconfig b/init/Kconfig
index 7484cd703bc1..40c3123c2c27 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -792,6 +792,36 @@ config IKHEADERS
 	  or similar programs.  If you build the headers as a module, a module called
 	  kheaders.ko is built which can be loaded on-demand to get access to headers.
 
+config EARLY_CYCLES_KHZ
+	int "Frequency of the early times cycle counter, in KHz"
+	default 0
+	depends on PRINTK
+	depends on ARM64 || X86_64 || RISCV_TIMER
+	help
+	  Specifies the frequency value (in KHz) of the cycle counter
+	  used for early times information.
+
+	  Set this to provide timing information for printks in early boot
+	  (before the start of kernel timekeeping), that would otherwise
+	  show as 0.
+
+	  To calculate the value for your system, boot your system, and
+	  use the value expressed on the tsc or cntvct_el0 calibration line
+	  in your kernel message log:
+	  ex x86_64: tsc: Refined TSC clocksource calibration: 2095.057 MHz
+          In that case, use the value 2095057.
+	  ex arm64: arch_timer: cp15 timer running at 54.00MHz (phys).
+	  In that case, use the value of 54000.
+
+	  Note that this causes the kernel to show, for some early printks,
+	  times that are relative to processor power on, instead of
+	  relative to the start of kernel timekeeping.  When kernel
+	  timekeeping starts, the timestamps values reset, causing
+	  a discontinuity in the timestamp values.  If you see
+	  non-monotonic printk timestamps, please don't freak out.
+
+	  Set to 0 to disable this feature.
+
 config LOG_BUF_SHIFT
 	int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
 	range 12 25
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0323149548f6..ddeab81f04ef 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>
@@ -2294,6 +2295,8 @@ int vprintk_store(int facility, int level,
 	 * timestamp with respect to the caller.
 	 */
 	ts_nsec = local_clock();
+	if (unlikely(!ts_nsec))
+		ts_nsec = early_times_ns();
 
 	caller_id = printk_caller_id();
 
-- 
2.43.0