timekeeping

Linux 6.18.37 LTS: kernel/time/timekeeping.c
Linux 6.18.37 LTS: kernel/time/clocksource.c
Linux 6.18.37 LTS: kernel/time/tick-common.c

/* Flag for if there is a persistent clock on this platform */
static bool persistent_clock_exists;

/*
 * timekeeping_init - Initializes the clocksource and common timekeeping values
 */
void __init timekeeping_init(void)
{
	struct timespec64 wall_time, boot_offset, wall_to_mono;
	struct timekeeper *tk = &tk_core.timekeeper;
	struct clocksource *clock;
	unsigned long flags;

	read_persistent_wall_and_boot_offset(&wall_time, &boot_offset);
	if (timespec64_valid_settod(&wall_time) &&
	    timespec64_to_ns(&wall_time) > 0) {
		persistent_clock_exists = true;
	} else if (timespec64_to_ns(&wall_time) != 0) {
		pr_warn("Persistent clock returned invalid value");
		wall_time = (struct timespec64){0};
	}

	if (timespec64_compare(&wall_time, &boot_offset) < 0)
		boot_offset = (struct timespec64){0};

	/*
	 * We want set wall_to_mono, so the following is true:
	 * wall time + wall_to_mono = boot time
	 */
	wall_to_mono = timespec64_sub(boot_offset, wall_time);

	raw_spin_lock_irqsave(&timekeeper_lock, flags);
	write_seqcount_begin(&tk_core.seq);
	ntp_init();

	clock = clocksource_default_clock();
	if (clock->enable)
		clock->enable(clock);
	tk_setup_internals(tk, clock);

 * Employ the latch technique; see @raw_write_seqcount_latch.
 *
 * So if a NMI hits the update of base[0] then it will use base[1]
 * which is still consistent. In the worst case this can result is a
 * slightly wrong timestamp (a few nanoseconds). See
 * @ktime_get_mono_fast_ns.
 */
static void update_fast_timekeeper(const struct tk_read_base *tkr,
				   struct tk_fast *tkf)
{
	struct tk_read_base *base = tkf->base;

	/* Force readers off to base[1] */
	raw_write_seqcount_latch(&tkf->seq);

	/* Update base[0] */
	memcpy(base, tkr, sizeof(*base));

	/* Force readers back to base[0] */
	raw_write_seqcount_latch(&tkf->seq);

	/* Update base[1] */
	memcpy(base + 1, base, sizeof(*base));
}

static __always_inline u64 fast_tk_get_delta_ns(struct tk_read_base *tkr)
{
	u64 delta, cycles = tk_clock_read(tkr);

	delta = clocksource_delta(cycles, tkr->cycle_last, tkr->mask);
	return timekeeping_delta_to_ns(tkr, delta);
}

static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
{
	struct tk_read_base *tkr;
	unsigned int seq;
	u64 now;

	do {