ptp/ioctl: support MONOTONIC{,_RAW} timestamps for PTP_SYS_OFFSET_EXTENDED

The ability to read the PHC (Physical Hardware Clock) alongside
multiple system clocks is currently dependent on the specific
hardware architecture. This limitation restricts the use of
PTP_SYS_OFFSET_PRECISE to certain hardware configurations.

The generic soultion which would work across all architectures
is to read the PHC along with the latency to perform PHC-read as
offered by PTP_SYS_OFFSET_EXTENDED which provides pre and post
timestamps. However, these timestamps are currently limited
to the CLOCK_REALTIME timebase. Since CLOCK_REALTIME is affected
by NTP (or similar time synchronization services), it can
experience significant jumps forward or backward. This hinders
the precise latency measurements that PTP_SYS_OFFSET_EXTENDED
is designed to provide.

This problem could be addressed by supporting MONOTONIC_RAW
timestamps within PTP_SYS_OFFSET_EXTENDED. Unlike CLOCK_REALTIME
or CLOCK_MONOTONIC, the MONOTONIC_RAW timebase is unaffected
by NTP adjustments.

This enhancement can be implemented by utilizing one of the three
reserved words within the PTP_SYS_OFFSET_EXTENDED struct to pass
the clock-id for timestamps. The current behavior aligns with
clock-id for CLOCK_REALTIME timebase (value of 0), ensuring
backward compatibility of the UAPI.

Signed-off-by: Mahesh Bandewar <[e-mail]>
Signed-off-by: Vadim Fedorenko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/ptp/ptp_chardev.c

···
 			extoff = NULL;
 			break;
 		}
-		if (extoff->n_samples > PTP_MAX_SAMPLES
-		    || extoff->rsv[0] || extoff->rsv[1] || extoff->rsv[2]) {
+		if (extoff->n_samples > PTP_MAX_SAMPLES ||
+		    extoff->rsv[0] || extoff->rsv[1] ||
+		    (extoff->clockid != CLOCK_REALTIME &&
+		     extoff->clockid != CLOCK_MONOTONIC &&
+		     extoff->clockid != CLOCK_MONOTONIC_RAW)) {
 			err = -EINVAL;
 			break;
 		}
+		sts.clockid = extoff->clockid;
 		for (i = 0; i < extoff->n_samples; i++) {
 			err = ptp->info->gettimex64(ptp->info, &ts, &sts);
 			if (err)

include/linux/ptp_clock_kernel.h

···
  * struct ptp_system_timestamp - system time corresponding to a PHC timestamp
  * @pre_ts: system timestamp before capturing PHC
  * @post_ts: system timestamp after capturing PHC
+ * @clockid: clock-base used for capturing the system timestamps
  */
 struct ptp_system_timestamp {
 	struct timespec64 pre_ts;
 	struct timespec64 post_ts;
+	clockid_t clockid;
 };
 
 /**
···
 
 static inline void ptp_read_system_prets(struct ptp_system_timestamp *sts)
 {
-	if (sts)
-		ktime_get_real_ts64(&sts->pre_ts);
+	if (sts) {
+		switch (sts->clockid) {
+		case CLOCK_REALTIME:
+			ktime_get_real_ts64(&sts->pre_ts);
+			break;
+		case CLOCK_MONOTONIC:
+			ktime_get_ts64(&sts->pre_ts);
+			break;
+		case CLOCK_MONOTONIC_RAW:
+			ktime_get_raw_ts64(&sts->pre_ts);
+			break;
+		default:
+			break;
+		}
+	}
 }
 
 static inline void ptp_read_system_postts(struct ptp_system_timestamp *sts)
 {
-	if (sts)
-		ktime_get_real_ts64(&sts->post_ts);
+	if (sts) {
+		switch (sts->clockid) {
+		case CLOCK_REALTIME:
+			ktime_get_real_ts64(&sts->post_ts);
+			break;
+		case CLOCK_MONOTONIC:
+			ktime_get_ts64(&sts->post_ts);
+			break;
+		case CLOCK_MONOTONIC_RAW:
+			ktime_get_raw_ts64(&sts->post_ts);
+			break;
+		default:
+			break;
+		}
+	}
 }
 
 #endif

include/uapi/linux/ptp_clock.h

···
 	struct ptp_clock_time ts[2 * PTP_MAX_SAMPLES + 1];
 };
 
+/*
+ * ptp_sys_offset_extended - data structure for IOCTL operation
+ *			     PTP_SYS_OFFSET_EXTENDED
+ *
+ * @n_samples:	Desired number of measurements.
+ * @clockid:	clockid of a clock-base used for pre/post timestamps.
+ * @rsv:	Reserved for future use.
+ * @ts:		Array of samples in the form [pre-TS, PHC, post-TS]. The
+ *		kernel provides @n_samples.
+ *
+ * Starting from kernel 6.12 and onwards, the first word of the reserved-field
+ * is used for @clockid. That's backward compatible since previous kernel
+ * expect all three reserved words (@rsv[3]) to be 0 while the clockid (first
+ * word in the new structure) for CLOCK_REALTIME is '0'.
+ */
 struct ptp_sys_offset_extended {
-	unsigned int n_samples; /* Desired number of measurements. */
-	unsigned int rsv[3];    /* Reserved for future use. */
-	/*
-	 * Array of [system, phc, system] time stamps. The kernel will provide
-	 * 3*n_samples time stamps.
-	 */
+	unsigned int n_samples;
+	__kernel_clockid_t clockid;
+	unsigned int rsv[2];
 	struct ptp_clock_time ts[PTP_MAX_SAMPLES][3];
 };