[PATCH v6 3/9] Add correlated clocksource relating aliased auxiliary and system clocks

["Christopher S. Hall" <christopher.s.hall@intel.com>]

ACKNOWLEDGMENT: The original correlated clock source and cross
timestamp code was developed by Thomas Gleixner
<tglx@linutronix.de>. It has changed considerably and any mistakes are
mine.

The timekeeping and clocksource code don't currently comprehend two
clocks that are aliases of one another. That is clocks that are
*exactly* related. The correlated_cs struct encapsulated this
relationship between a clocksource and its alias clock. Modern Intel
hardware provides an Always Running Timer (ART) which is exactly
related to TSC through a known frequency ratio. The ART is an example
of a correlated clocksource.

Signed-off-by: Christopher S. Hall <christopher.s.hall@intel.com>
---
 include/linux/clocksource.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 4542293..726ca68 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -267,4 +267,18 @@ struct system_counterval_t {
 	struct clocksource	*cs;
 };
 
+/*
+ * struct correlated_cs - Descriptor for a clocksource correlated to another
+ *	clocksource
+ * @related_cs:		Pointer to the related timekeeping clocksource
+ * @convert:		Conversion function to convert a timestamp from
+ *			the correlated clocksource to cycles of the related
+ *			timekeeping clocksource
+ */
+struct correlated_cs {
+	struct clocksource	*related_cs;
+	cycle_t			(*convert)(struct correlated_cs *cs,
+					   cycle_t cycles);
+};
+
 #endif /* _LINUX_CLOCKSOURCE_H */
-- 
2.1.4