[PATCHv4 net-next] ptp/ioctl: support MONOTONIC_RAW timestamps for PTP_SYS_OFFSET_EXTENDED

[Mahesh Bandewar <maheshb@google.com>]

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 <maheshb@google.com>
---
v1 -> v2
   * Code-style fixes.
v2 -> v3
   * Reword commit log
   * Fix the compilation issue by using __kernel_clockid instead of clockid_t
     which has kernel only scope.
v3 -> v4
   * Typo/comment fixes.

 drivers/ptp/ptp_chardev.c        |  7 +++++--
 include/linux/ptp_clock_kernel.h | 30 ++++++++++++++++++++++++++----
 include/uapi/linux/ptp_clock.h   | 27 +++++++++++++++++++++------
 3 files changed, 52 insertions(+), 12 deletions(-)

diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
index 7513018c9f9a..c109109c9e8e 100644
--- a/drivers/ptp/ptp_chardev.c
+++ b/drivers/ptp/ptp_chardev.c
@@ -358,11 +358,14 @@ long ptp_ioctl(struct posix_clock_context *pccontext, unsigned int cmd,
 			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_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)
diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index 6e4b8206c7d0..74ded5f95d95 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -47,10 +47,12 @@ struct system_device_crosststamp;
  * 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;
 };
 
 /**
@@ -457,14 +459,34 @@ static inline ktime_t ptp_convert_timestamp(const ktime_t *hwtstamp,
 
 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_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_RAW:
+			ktime_get_raw_ts64(&sts->post_ts);
+			break;
+		default:
+			break;
+		}
+	}
 }
 
 #endif
diff --git a/include/uapi/linux/ptp_clock.h b/include/uapi/linux/ptp_clock.h
index 053b40d642de..5e3d70fbc499 100644
--- a/include/uapi/linux/ptp_clock.h
+++ b/include/uapi/linux/ptp_clock.h
@@ -155,13 +155,28 @@ struct ptp_sys_offset {
 	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.
+ *
+ * History:
+ * v1: Initial implementation.
+ *
+ * v2: Use the first word of the reserved-field for @clockid. That's
+ *     backward compatible since v1 expects all three reserved words
+ *     (@rsv[3]) to be 0 while the clockid (first word in v2) 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];
 };
 
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog