hwclock: add support for tracking hardware clocks

Add a general support for tracking independent hardware clocks like PTP
hardware clocks (PHC) or real-time clocks (RTC).

hwclock.h

@@ -0,0 +1,48 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2016
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header for tracking of hardware clocks */
+
+#ifndef GOT_HWCLOCK_H
+#define GOT_HWCLOCK_H
+
+typedef struct HCL_Instance_Record *HCL_Instance;
+
+/* Create a new HW clock instance */
+extern HCL_Instance HCL_CreateInstance(void);
+
+/* Destroy a HW clock instance */
+extern void HCL_DestroyInstance(HCL_Instance clock);
+
+/* Check if a new sample should be accumulated at this time */
+extern int HCL_NeedsNewSample(HCL_Instance clock, struct timespec *now);
+
+/* Accumulate a new sample */
+extern void HCL_AccumulateSample(HCL_Instance clock, struct timespec *hw_ts,
+                                 struct timespec *local_ts, double err);
+
+/* Convert raw hardware time to cooked local time */
+extern int HCL_CookTime(HCL_Instance clock, struct timespec *raw, struct timespec *cooked,
+                        double *err);
+
+#endif