timekeeping: Provide time setter for auxiliary clocks

Add clock_settime(2) support for auxiliary clocks. The function affects the
AUX offset which is added to the "monotonic" clock readout of these clocks.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20250625183757.995688714@linutronix.de

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 10c6e37dc0dca3fb657f362ad0e93090dea5e8c2..b6ac7847bc0ae7024a0b0df37cafc879cd72fd93 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2765,9 +2765,53 @@ static int aux_get_timespec(clockid_t id, struct timespec64 *tp)
        return ktime_get_aux_ts64(id, tp) ? 0 : -ENODEV;
 }
 
+static int aux_clock_set(const clockid_t id, const struct timespec64 *tnew)
+{
+       struct tk_data *aux_tkd = aux_get_tk_data(id);
+       struct timekeeper *aux_tks;
+       ktime_t tnow, nsecs;
+
+       if (!timespec64_valid_settod(tnew))
+               return -EINVAL;
+       if (!aux_tkd)
+               return -ENODEV;
+
+       aux_tks = &aux_tkd->shadow_timekeeper;
+
+       guard(raw_spinlock_irq)(&aux_tkd->lock);
+       if (!aux_tks->clock_valid)
+               return -ENODEV;
+
+       /* Forward the timekeeper base time */
+       timekeeping_forward_now(aux_tks);
+       /*
+        * Get the updated base time. tkr_mono.base has not been
+        * updated yet, so do that first. That makes the update
+        * in timekeeping_update_from_shadow() redundant, but
+        * that's harmless. After that @tnow can be calculated
+        * by using tkr_mono::cycle_last, which has been set
+        * by timekeeping_forward_now().
+        */
+       tk_update_ktime_data(aux_tks);
+       nsecs = timekeeping_cycles_to_ns(&aux_tks->tkr_mono, aux_tks->tkr_mono.cycle_last);
+       tnow = ktime_add(aux_tks->tkr_mono.base, nsecs);
+
+       /*
+        * Calculate the new AUX offset as delta to @tnow ("monotonic").
+        * That avoids all the tk::xtime back and forth conversions as
+        * xtime ("realtime") is not applicable for auxiliary clocks and
+        * kept in sync with "monotonic".
+        */
+       aux_tks->offs_aux = ktime_sub(timespec64_to_ktime(*tnew), tnow);
+
+       timekeeping_update_from_shadow(aux_tkd, TK_UPDATE_ALL);
+       return 0;
+}
+
 const struct k_clock clock_aux = {
        .clock_getres           = aux_get_res,
        .clock_get_timespec     = aux_get_timespec,
+       .clock_set              = aux_clock_set,
 };
 
 static __init void tk_aux_setup(void)