ptp: Add a method for obtaining the device index.

This commit adds a method that MAC drivers may call in order to find out
the device number of their associated PTP Hardware Clock.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index dd2e44f..945704c 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -136,4 +136,12 @@
 extern void ptp_clock_event(struct ptp_clock *ptp,
 			    struct ptp_clock_event *event);
 
+/**
+ * ptp_clock_index() - obtain the device index of a PTP clock
+ *
+ * @ptp:    The clock obtained from ptp_clock_register().
+ */
+
+extern int ptp_clock_index(struct ptp_clock *ptp);
+
 #endif