Introduce:
- ops for getting and setting the HW timestamp latch point,
- enumerate possible HW timestamp latch point,
- new sysfs ptp device attribute as user knob.

HW support of PTP/timesync solutions in network PHY chips can be
achieved with two different approaches, the timestamp maybe latched
either in the beginning or after the Start of Frame Delimiter (SFD) [1].

Allow ptp device drivers to provide user with control over the timestamp
latch point.

[1] https://www.ieee802.org/3/cx/public/april20/tse_3cx_01_0420.pdf

Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
---
 drivers/ptp/ptp_sysfs.c          | 44 ++++++++++++++++++++++++++++++++
 include/linux/ptp_clock_kernel.h | 26 +++++++++++++++++++
 2 files changed, 70 insertions(+)

diff --git a/drivers/ptp/ptp_sysfs.c b/drivers/ptp/ptp_sysfs.c
index 6b1b8f57cd95..7e9f6ef368b6 100644
--- a/drivers/ptp/ptp_sysfs.c
+++ b/drivers/ptp/ptp_sysfs.c
@@ -28,6 +28,46 @@ static ssize_t max_phase_adjustment_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(max_phase_adjustment);
 
+static ssize_t ts_point_show(struct device *dev, struct device_attribute *attr,
+			     char *page)
+{
+	struct ptp_clock *ptp = dev_get_drvdata(dev);
+	enum ptp_ts_point point;
+	int err;
+
+	if (!ptp->info->get_ts_point)
+		return -EOPNOTSUPP;
+	err = ptp->info->get_ts_point(ptp->info, &point);
+	if (err)
+		return err;
+
+	return sysfs_emit(page, "%d\n", point);
+}
+
+static ssize_t ts_point_store(struct device *dev, struct device_attribute *attr,
+			      const char *buf, size_t count)
+{
+	struct ptp_clock *ptp = dev_get_drvdata(dev);
+	enum ptp_ts_point point;
+	int err;
+	u8 val;
+
+	if (!ptp->info->set_ts_point)
+		return -EOPNOTSUPP;
+	if (kstrtou8(buf, 0, &val))
+		return -EINVAL;
+	if (val > PTP_TS_POINT_MAX)
+		return -EINVAL;
+	point = val;
+
+	err = ptp->info->set_ts_point(ptp->info, point);
+	if (err)
+		return err;
+
+	return count;
+}
+static DEVICE_ATTR_RW(ts_point);
+
 #define PTP_SHOW_INT(name, var)						\
 static ssize_t var##_show(struct device *dev,				\
 			   struct device_attribute *attr, char *page)	\
@@ -335,6 +375,7 @@ static struct attribute *ptp_attrs[] = {
 	&dev_attr_pps_enable.attr,
 	&dev_attr_n_vclocks.attr,
 	&dev_attr_max_vclocks.attr,
+	&dev_attr_ts_point.attr,
 	NULL
 };
 
@@ -363,6 +404,9 @@ static umode_t ptp_is_attribute_visible(struct kobject *kobj,
 	} else if (attr == &dev_attr_max_phase_adjustment.attr) {
 		if (!info->adjphase || !info->getmaxphase)
 			mode = 0;
+	} else if (attr == &dev_attr_ts_point.attr) {
+		if (!info->get_ts_point && !info->set_ts_point)
+			mode = 0;
 	}
 
 	return mode;
diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index c892d22ce0a7..ee4b625bca2c 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -55,6 +55,20 @@ struct ptp_system_timestamp {
 	clockid_t clockid;
 };
 
+/**
+ * enum ptp_ts_point - possible timestamp latch points (IEEE 802.3cx)
+ * PTP_TS_POINT_SFD: timestamp latched at the beginning of sending Start
+ *	of Frame Delimiter (SFD)
+ * PTP_TS_POINT_POST_SFD: timestamp latched at the end of sending Start
+ *	of Frame Delimiter (SFD)
+ */
+enum ptp_ts_point {
+	PTP_TS_POINT_SFD,
+	PTP_TS_POINT_POST_SFD,
+	__PTP_TS_POINT_MAX
+};
+#define PTP_TS_POINT_MAX (__PTP_TS_POINT_MAX - 1)
+
 /**
  * struct ptp_clock_info - describes a PTP hardware clock
  *
@@ -159,6 +173,14 @@ struct ptp_system_timestamp {
  *                scheduling time (>=0) or negative value in case further
  *                scheduling is not required.
  *
+ * @set_ts_point: Request change of timestamp latch point, as the timestamp
+ *                could be latched at the beginning or at the end of start
+ *                frame delimiter (SFD), as described in IEEE 802.3cx
+ *                specification.
+ *
+ * @get_ts_point: Obtain the timestamp measurement latch point, counterpart of
+ *                .set_ts_point() for getting currently configured value.
+ *
  * Drivers should embed their ptp_clock_info within a private
  * structure, obtaining a reference to it using container_of().
  *
@@ -195,6 +217,10 @@ struct ptp_clock_info {
 	int (*verify)(struct ptp_clock_info *ptp, unsigned int pin,
 		      enum ptp_pin_function func, unsigned int chan);
 	long (*do_aux_work)(struct ptp_clock_info *ptp);
+	int (*set_ts_point)(struct ptp_clock_info *ptp,
+			    enum ptp_ts_point point);
+	int (*get_ts_point)(struct ptp_clock_info *ptp,
+			    enum ptp_ts_point *point);
 };
 
 struct ptp_clock;
-- 
2.38.1