timekeeping: Provide ktime_get_clock_ts64()

PTP implements an inline switch case for taking timestamps from various
POSIX clock IDs, which already consumes quite some text space. Expanding it
for auxiliary clocks really becomes too big for inlining.

Provide a out of line version. 

The function invalidates the timestamp in case the clock is invalid. The
invalidation allows to implement a validation check without the need to
propagate a return value through deep existing call chains.

Due to merge logistics this temporarily defines CLOCK_AUX[_LAST] if
undefined, so that the plain branch, which does not contain any of the core
timekeeper changes, can be pulled into the networking tree as prerequisite
for the PTP side changes. These temporary defines are removed after that
branch is merged into the tip::timers/ptp branch. That way the result in
-next or upstream in the next merge window has zero dependencies.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20250701132628.357686408@linutronix.de

Author: KAGA-KOKO

include/linux/timekeeping.h

@@ -44,6 +44,7 @@ extern void ktime_get_ts64(struct timespec64 *ts);
 extern void ktime_get_real_ts64(struct timespec64 *tv);
 extern void ktime_get_coarse_ts64(struct timespec64 *ts);
 extern void ktime_get_coarse_real_ts64(struct timespec64 *ts);
+extern void ktime_get_clock_ts64(clockid_t id, struct timespec64 *ts);
 
 /* Multigrain timestamp interfaces */
 extern void ktime_get_coarse_real_ts64_mg(struct timespec64 *ts);
@@ -345,4 +346,13 @@ void read_persistent_wall_and_boot_offset(struct timespec64 *wall_clock,
 extern int update_persistent_clock64(struct timespec64 now);
 #endif
 
+/* Temporary workaround to avoid merge dependencies and cross tree messes */
+#ifndef CLOCK_AUX
+#define CLOCK_AUX			MAX_CLOCKS
+#define MAX_AUX_CLOCKS			8
+#define CLOCK_AUX_LAST			(CLOCK_AUX + MAX_AUX_CLOCKS - 1)
+
+static inline bool ktime_get_aux_ts64(clockid_t id, struct timespec64 *kt) { return false; }
+#endif
+
 #endif

kernel/time/timekeeping.c

@@ -1573,6 +1573,39 @@ void ktime_get_raw_ts64(struct timespec64 *ts)
 }
 EXPORT_SYMBOL(ktime_get_raw_ts64);
 
+/**
+ * ktime_get_clock_ts64 - Returns time of a clock in a timespec
+ * @id:		POSIX clock ID of the clock to read
+ * @ts:		Pointer to the timespec64 to be set
+ *
+ * The timestamp is invalidated (@ts->sec is set to -1) if the
+ * clock @id is not available.
+ */
+void ktime_get_clock_ts64(clockid_t id, struct timespec64 *ts)
+{
+	/* Invalidate time stamp */
+	ts->tv_sec = -1;
+	ts->tv_nsec = 0;
+
+	switch (id) {
+	case CLOCK_REALTIME:
+		ktime_get_real_ts64(ts);
+		return;
+	case CLOCK_MONOTONIC:
+		ktime_get_ts64(ts);
+		return;
+	case CLOCK_MONOTONIC_RAW:
+		ktime_get_raw_ts64(ts);
+		return;
+	case CLOCK_AUX ... CLOCK_AUX_LAST:
+		if (IS_ENABLED(CONFIG_POSIX_AUX_CLOCKS))
+			ktime_get_aux_ts64(id, ts);
+		return;
+	default:
+		WARN_ON_ONCE(1);
+	}
+}
+EXPORT_SYMBOL_GPL(ktime_get_clock_ts64);
 
 /**
  * timekeeping_valid_for_hres - Check if timekeeping is suitable for hres