gpiolib: cdev: allow edge event timestamps to be configured as REALTIME

Using CLOCK_REALTIME as the source for event timestamps is crucial for
some specific applications, particularly those requiring timetamps
relative to a PTP clock, so provide an option to switch the event
timestamp source from the default CLOCK_MONOTONIC to CLOCK_REALTIME.

Note that CLOCK_REALTIME was the default source clock for GPIO until
Linux 5.7 when it was changed to CLOCK_MONOTONIC due to issues with the
shifting of the realtime clock.
Providing this option maintains the CLOCK_MONOTONIC as the default,
while also providing a path forward for those dependent on the pre-5.7
behaviour.

Suggested-by: Jack Winch <[e-mail]>
Signed-off-by: Kent Gibson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpiolib-cdev.c

···
 	 GPIO_V2_LINE_DIRECTION_FLAGS | \
 	 GPIO_V2_LINE_DRIVE_FLAGS | \
 	 GPIO_V2_LINE_EDGE_FLAGS | \
+	 GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME | \
 	 GPIO_V2_LINE_BIAS_FLAGS)
 
 static void linereq_put_event(struct linereq *lr,
···
 		pr_debug_ratelimited("event FIFO is full - event dropped\n");
 }
 
+static u64 line_event_timestamp(struct line *line)
+{
+	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &line->desc->flags))
+		return ktime_get_real_ns();
+
+	return ktime_get_ns();
+}
+
 static irqreturn_t edge_irq_thread(int irq, void *p)
 {
 	struct line *line = p;
···
 		 * which case we didn't get the timestamp from
 		 * edge_irq_handler().
 		 */
-		le.timestamp_ns = ktime_get_ns();
+		le.timestamp_ns = line_event_timestamp(line);
 		if (lr->num_lines != 1)
 			line->req_seqno = atomic_inc_return(&lr->seqno);
 	}
···
 	 * Just store the timestamp in hardirq context so we get it as
 	 * close in time as possible to the actual event.
 	 */
-	line->timestamp_ns = ktime_get_ns();
+	line->timestamp_ns = line_event_timestamp(line);
 
 	if (lr->num_lines != 1)
 		line->req_seqno = atomic_inc_return(&lr->seqno);
···
 	memset(&le, 0, sizeof(le));
 
 	lr = line->req;
-	le.timestamp_ns = ktime_get_ns();
+	le.timestamp_ns = line_event_timestamp(line);
 	le.offset = gpio_chip_hwgpio(line->desc);
 	line->line_seqno++;
 	le.line_seqno = line->line_seqno;
···
 		   flags & GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN);
 	assign_bit(FLAG_BIAS_DISABLE, flagsp,
 		   flags & GPIO_V2_LINE_FLAG_BIAS_DISABLED);
+
+	assign_bit(FLAG_EVENT_CLOCK_REALTIME, flagsp,
+		   flags & GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME);
 }
 
 static long linereq_get_values(struct linereq *lr, void __user *ip)
···
 		info->flags |= GPIO_V2_LINE_FLAG_EDGE_RISING;
 	if (test_bit(FLAG_EDGE_FALLING, &desc->flags))
 		info->flags |= GPIO_V2_LINE_FLAG_EDGE_FALLING;
+
+	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &desc->flags))
+		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME;
 
 	debounce_period_us = READ_ONCE(desc->debounce_period_us);
 	if (debounce_period_us) {

drivers/gpio/gpiolib.h

···
 #define FLAG_BIAS_DISABLE    15	/* GPIO has pull disabled */
 #define FLAG_EDGE_RISING     16	/* GPIO CDEV detects rising edge events */
 #define FLAG_EDGE_FALLING    17	/* GPIO CDEV detects falling edge events */
+#define FLAG_EVENT_CLOCK_REALTIME	18 /* GPIO CDEV reports REALTIME timestamps in events */
 
 	/* Connection label */
 	const char		*label;

include/uapi/linux/gpio.h

···
  * @GPIO_V2_LINE_FLAG_BIAS_PULL_UP: line has pull-up bias enabled
  * @GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN: line has pull-down bias enabled
  * @GPIO_V2_LINE_FLAG_BIAS_DISABLED: line has bias disabled
+ * @GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME: line events contain REALTIME timestamps
  */
 enum gpio_v2_line_flag {
 	GPIO_V2_LINE_FLAG_USED			= _BITULL(0),
···
 	GPIO_V2_LINE_FLAG_BIAS_PULL_UP		= _BITULL(8),
 	GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN	= _BITULL(9),
 	GPIO_V2_LINE_FLAG_BIAS_DISABLED		= _BITULL(10),
+	GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME	= _BITULL(11),
 };
 
 /**
···
 /**
  * struct gpio_v2_line_event - The actual event being pushed to userspace
  * @timestamp_ns: best estimate of time of event occurrence, in nanoseconds.
- * The @timestamp_ns is read from %CLOCK_MONOTONIC and is intended to allow
- * the accurate measurement of the time between events. It does not provide
- * the wall-clock time.
  * @id: event identifier with value from &enum gpio_v2_line_event_id
  * @offset: the offset of the line that triggered the event
  * @seqno: the sequence number for this event in the sequence of events for
···
  * @line_seqno: the sequence number for this event in the sequence of
  * events on this particular line
  * @padding: reserved for future use
+ *
+ * By default the @timestamp_ns is read from %CLOCK_MONOTONIC and is
+ * intended to allow the accurate measurement of the time between events.
+ * It does not provide the wall-clock time.
+ *
+ * If the %GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME flag is set then the
+ * @timestamp_ns is read from %CLOCK_REALTIME.
  */
 struct gpio_v2_line_event {
 	__aligned_u64 timestamp_ns;