[PATCH RFC] counter: Expand API with a function for an exact timestamp

[Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>]

Some hardware units capture a timestamp for the counted event. To
increase precision add a variant of counter_push_event() that allows
passing this timestamp.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
Hello,

the difficulty is that the captured timer value is in a unit different
from the one provided by ktime_get_ns(). So maybe some helper functions
will be needed to convert the local timer value to a ktime timestamp?

So usage would be something like:

	ktime_now = ktime_get_ns();
	local_now = readl(CNT);
	local_event = readl(...);

	ktime_event = ktime_now - (local_now - local_event) * somefactor >> someshift;

	counter_push_event_ts(count, event, channel, ktime_event);

This improves the precision because irq latency doesn't influence
the resulting timestamp. The precision then only depends on the timer
resolution and the delay between ktime_get_ns() and readl(CNT);

I don't have a driver (yet) that makes use of this, the hardware where
this will matter will be stm32mp1.

Best regards
Uwe

 drivers/counter/counter-chrdev.c | 25 +++++++++++++++++++++----
 include/linux/counter.h          |  2 ++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
index b7c62f957a6a..6381f7246d59 100644
--- a/drivers/counter/counter-chrdev.c
+++ b/drivers/counter/counter-chrdev.c
@@ -521,16 +521,17 @@ static int counter_get_data(struct counter_device *const counter,
 }
 
 /**
- * counter_push_event - queue event for userspace reading
+ * counter_push_event_ts - queue event with a timestamp for userspace reading
  * @counter:	pointer to Counter structure
  * @event:	triggered event
  * @channel:	event channel
+ * @timestamp:	the ktime when the event occurred
  *
  * Note: If no one is watching for the respective event, it is silently
  * discarded.
  */
-void counter_push_event(struct counter_device *const counter, const u8 event,
-			const u8 channel)
+void counter_push_event_ts(struct counter_device *const counter, const u8 event,
+			   const u8 channel, u64 timestamp)
 {
 	struct counter_event ev;
 	unsigned int copied = 0;
@@ -538,7 +539,7 @@ void counter_push_event(struct counter_device *const counter, const u8 event,
 	struct counter_event_node *event_node;
 	struct counter_comp_node *comp_node;
 
-	ev.timestamp = ktime_get_ns();
+	ev.timestamp = timestamp;
 	ev.watch.event = event;
 	ev.watch.channel = channel;
 
@@ -570,4 +571,20 @@ void counter_push_event(struct counter_device *const counter, const u8 event,
 	if (copied)
 		wake_up_poll(&counter->events_wait, EPOLLIN);
 }
+EXPORT_SYMBOL_GPL(counter_push_event_ts);
+
+/**
+ * counter_push_event - queue event for userspace reading
+ * @counter:	pointer to Counter structure
+ * @event:	triggered event
+ * @channel:	event channel
+ *
+ * Note: If no one is watching for the respective event, it is silently
+ * discarded.
+ */
+void counter_push_event(struct counter_device *const counter, const u8 event,
+			const u8 channel)
+{
+	counter_push_event_ts(counter, event, channel, ktime_get_ns());
+}
 EXPORT_SYMBOL_GPL(counter_push_event);
diff --git a/include/linux/counter.h b/include/linux/counter.h
index b7d0a00a61cf..596e7e58e463 100644
--- a/include/linux/counter.h
+++ b/include/linux/counter.h
@@ -333,6 +333,8 @@ int counter_register(struct counter_device *const counter);
 void counter_unregister(struct counter_device *const counter);
 int devm_counter_register(struct device *dev,
 			  struct counter_device *const counter);
+void counter_push_event_ts(struct counter_device *const counter, const u8 event,
+			   const u8 channel, u64 timestamp)
 void counter_push_event(struct counter_device *const counter, const u8 event,
 			const u8 channel);
 
-- 
2.30.2