[4/6] time: Expose current clocksource in use by timekeeping framework

Commit Message

[e-mail] March 16, 2017, 6:20 a.m. UTC

From: Sourab Gupta <[e-mail]>

For the drivers to be able to use the cross timestamp framework,
they need the information of current clocksource being used by the
kernel timekeeping. This is needed since the callback given by driver
into the get_device_system_crosststamp(), in order to synchronously read
the device time and system counter value, requires the knowledge of
the clocksource being used to read system counter value (as a part of
struct system_counterval_t).

Cc: John Stultz <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Sourab Gupta <[e-mail]>
---
 include/linux/timekeeping.h |  5 +++++
 kernel/time/timekeeping.c   | 12 ++++++++++++
 2 files changed, 17 insertions(+)

Comments

Thomas Gleixner March 16, 2017, 8:10 a.m. UTC | #1

On Thu, 16 Mar 2017, [e-mail] wrote:

Please Cc LKML for patches which touch subsystems managed on LKML. Also
CC'ing people on the 0/N letter would be helpful, so we can see what this
is about.
 
> For the drivers to be able to use the cross timestamp framework,
> they need the information of current clocksource being used by the
> kernel timekeeping.

No.

> This is needed since the callback given by driver into the
> get_device_system_crosststamp(), in order to synchronously read the
> device time and system counter value, requires the knowledge of the
> clocksource being used to read system counter value (as a part of struct
> system_counterval_t).

Again you are saying WHAT you want but not WHY. The device callback does
not need to know anything about the timekeeping clocksource.

get_device_system_crosststamp() is for timestamps taken via a clock which
is directly correlated with the timekeeper clocksource.

ART and TSC are correlated via:

    TSC = (ART * scale) + offset

get_device_system_crosststamp() invokes the device function which reads
ART, which is converted to CLOCK_MONOTONIC_RAW by the conversion above, and
then uses interpolation to map the CLOCK_MONOTONIC_RAW value to
CLOCK_MONOTONIC.

The device function does not know anything about TSC. All it knows about is
ART.

I have no idea what you are trying to do, but given the explanation above
it's wrong.

Thanks,

	tglx

Patch

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index d2e804e..d62bb0e 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -337,6 +337,11 @@  extern int get_device_system_crosststamp(
 			struct system_device_crosststamp *xtstamp);
 
 /*
+ * Get current clocksource used by system timekeeping framework
+ */
+struct clocksource *get_current_clocksource(void);
+
+/*
  * Simultaneously snapshot realtime and monotonic raw clocks
  */
 extern void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 95b258d..bd85d14 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1167,6 +1167,18 @@  int get_device_system_crosststamp(int (*get_time_fn)
 EXPORT_SYMBOL_GPL(get_device_system_crosststamp);
 
 /**
+ * get_current_clocksource - Returns the current clocksource in used by tk_core
+ *
+ */
+struct clocksource *get_current_clocksource(void)
+{
+	struct timekeeper *tk = &tk_core.timekeeper;
+
+	return tk->tkr_mono.clock;
+}
+EXPORT_SYMBOL_GPL(get_current_clocksource);
+
+/**
  * do_gettimeofday - Returns the time of day in a timeval
  * @tv:		pointer to the timeval to be set
  *