timespec: Add timespec subtraction helpers

Add helpers to subtract two timespecs, then return the difference in
either milliseconds or nanoseconds. These will be used to compare
timestamps during the repaint cycle.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

shared/timespec-util.h

@@ -93,6 +93,20 @@ timespec_to_nsec(const struct timespec *a)
 	return (int64_t)a->tv_sec * NSEC_PER_SEC + a->tv_nsec;
 }
 
+/* Subtract timespecs and return result in nanoseconds
+ *
+ * \param a[in] operand
+ * \param b[in] operand
+ * \return to_nanoseconds(a - b)
+ */
+static inline int64_t
+timespec_sub_to_nsec(const struct timespec *a, const struct timespec *b)
+{
+	struct timespec r;
+	timespec_sub(&r, a, b);
+	return timespec_to_nsec(&r);
+}
+
 /* Convert timespec to milliseconds
  *
  * \param a timespec
@@ -106,6 +120,18 @@ timespec_to_msec(const struct timespec *a)
 	return (int64_t)a->tv_sec * 1000 + a->tv_nsec / 1000000;
 }
 
+/* Subtract timespecs and return result in milliseconds
+ *
+ * \param a[in] operand
+ * \param b[in] operand
+ * \return to_milliseconds(a - b)
+ */
+static inline int64_t
+timespec_sub_to_msec(const struct timespec *a, const struct timespec *b)
+{
+	return timespec_sub_to_nsec(a, b) / 1000000;
+}
+
 /* Convert milli-Hertz to nanoseconds
  *
  * \param mhz frequency in mHz, not zero

tests/timespec-test.c

@@ -142,3 +142,25 @@ ZUC_TEST(timespec_test, timespec_add_msec)
 	ZUC_ASSERT_EQ(1002, r.tv_sec);
 	ZUC_ASSERT_EQ(2000001, r.tv_nsec);
 }
+
+ZUC_TEST(timespec_test, timespec_sub_to_nsec)
+{
+	struct timespec a, b;
+
+	a.tv_sec = 1000;
+	a.tv_nsec = 1;
+	b.tv_sec = 1;
+	b.tv_nsec = 2;
+	ZUC_ASSERT_EQ((999L * NSEC_PER_SEC) - 1, timespec_sub_to_nsec(&a, &b));
+}
+
+ZUC_TEST(timespec_test, timespec_sub_to_msec)
+{
+	struct timespec a, b;
+
+	a.tv_sec = 1000;
+	a.tv_nsec = 2000000L;
+	b.tv_sec = 2;
+	b.tv_nsec = 1000000L;
+	ZUC_ASSERT_EQ((998 * 1000) + 1, timespec_sub_to_msec(&a, &b));
+}