aux/util: Provide a conversion function from CLOCK_MONOTONIC nanoseconds to adjusted time.

src/xrt/auxiliary/util/u_time.cpp

@@ -1,4 +1,4 @@
-// Copyright 2019, Collabora, Ltd.
+// Copyright 2019-2020, Collabora, Ltd.
 // SPDX-License-Identifier: BSL-1.0
 /*!
  * @file
@@ -8,20 +8,37 @@
  */
 
 #include "u_time.h"
+#include "xrt/xrt_config.h"
+#include "xrt/xrt_compiler.h"
 
 #include <chrono>
 #include <new>
 #include <assert.h>
 #include <stdlib.h>
+#include <time.h>
+
+#ifdef XRT_OS_LINUX
+#include <sys/time.h>
+#endif
 
 using namespace std::chrono;
 
 struct MatchingTimePoints
 {
+	MatchingTimePoints()
+	{
+#ifdef XRT_OS_LINUX
+		clock_gettime(CLOCK_MONOTONIC, &clock_monotonic);
+#endif
+	}
 	system_clock::time_point sys = system_clock::now();
 	steady_clock::time_point steady = steady_clock::now();
 	// high_resolution_clock::time_point highRes;
 
+#ifdef XRT_OS_LINUX
+	struct timespec clock_monotonic;
+#endif
+
 	timepoint_ns
 	getTimestamp(time_state const &prevState);
 };
@@ -134,3 +151,19 @@ time_state_from_timespec(struct time_state const *state,
 	return state->lastTime +
 	       duration_cast<nanoseconds>(sinceLastUpdate).count();
 }
+
+
+timepoint_ns
+time_state_from_monotonic_ns(struct time_state const *state,
+                             uint64_t monotonic_ns)
+{
+	assert(state != NULL);
+	auto sinceLastUpdate =
+	    seconds{state->lastTimePoints.clock_monotonic.tv_sec} +
+	    nanoseconds{state->lastTimePoints.clock_monotonic.tv_nsec} -
+	    nanoseconds{monotonic_ns};
+
+	// Offset the last timestamp by that duration.
+	return state->lastTime +
+	       duration_cast<nanoseconds>(sinceLastUpdate).count();
+}

src/xrt/auxiliary/util/u_time.h

@@ -1,9 +1,14 @@
-// Copyright 2019, Collabora, Ltd.
+// Copyright 2019-2020, Collabora, Ltd.
 // SPDX-License-Identifier: BSL-1.0
 /*!
  * @file
  * @brief  Time-keeping: a clock that is steady, convertible to system time, and
  * ideally high-resolution.
+ *
+ * Designed to suit the needs of OpenXR: you can and should use something
+ * simpler (like @ref aux_os_time) for most purposes that aren't in OpenXR
+ * interface code.
+ *
  * @author Ryan Pavlik <ryan.pavlik@collabora.com>
  * @ingroup aux_util
  *
@@ -146,6 +151,21 @@ timepoint_ns
 time_state_from_timespec(struct time_state const *state,
                          const struct timespec *timespecTime);
 
+/*!
+ * Convert a monotonic system time (such as from @ref aux_os_time) to an
+ * adjusted integer timestamp.
+ *
+ * Adjustments may need to be applied to achieve the other guarantees that e.g.
+ * CLOCK_MONOTONIC does not provide: this function performs those adjustments.
+ *
+ * Should not be called simultaneously with time_state_get_now_and_update.
+ *
+ * @public @memberof time_state
+ * @ingroup aux_util
+ */
+timepoint_ns
+time_state_from_monotonic_ns(struct time_state const *state,
+                             uint64_t monotonic_ns);
 #ifdef __cplusplus
 }
 #endif