From 851d05230af70b4a2f1f5754607d13f84d371887 Mon Sep 17 00:00:00 2001
From: Eric Anholt <eric@anholt.net>
Date: Mon, 11 Nov 2013 18:31:02 -0800
Subject: [PATCH] Do something sensible for timestamp wrapping.

Before we had this result in our frame timings (with the after/before
counters printed):

0x0000000fffa65090 0x0000000ffea7f9a0
frame: 99 16.6684 16.4952 0.0195919 0
0x0000000000a540a0 0x0000000fffa65090
frame: 100 1.84467e+13 1.84467e+13 1.7174e-14 0
0x0000000001a2fd30 0x0000000000a540a0
frame: 101 16.6289 16.4461 0.0216595 0

And now we get:

0x0000000fff3ab690 0x0000000ffe3c4f10
frame: 2387 16.6726 16.609 0.0173506 0
0x000000000038b420 0x0000000fff3ab690
frame: 2388 16.6455 16.5858 0.0169653 0
0x000000000137b4c0 0x000000000038b420
frame: 2389 16.7118 16.6509 0.0168314 0
---
 metrics.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/metrics.c b/metrics.c
index 36ef90b..c52bf2e 100644
--- a/metrics.c
+++ b/metrics.c
@@ -113,6 +113,28 @@ struct metrics
 	struct timespec cpu_time_ts;
 };
 
+/* Return (a - b) while accounting for the fact that between a and b,
+ * the counter may have wrapped around (with a maximum number of bits
+ * in the counter of GL_QUERY_COUNTER_BITS). */
+static uint64_t
+subtract_timestamp (uint64_t a, uint64_t b)
+{
+	GLint timestamp_bits;
+
+	glGetQueryiv (GL_TIMESTAMP, GL_QUERY_COUNTER_BITS, &timestamp_bits);
+
+	/* Note: "<<" isn't defined for the number of bits in the
+	 * type, and the compiler will punish you.
+	 */
+	if (timestamp_bits == 64)
+		return a - b;
+
+	if (b > a)
+		return (1ull << timestamp_bits) + a - b;
+	else
+		return a - b;
+}
+
 metrics_t *
 metrics_create (metrics_info_t *info)
 {
@@ -836,9 +858,11 @@ metrics_end_frame_pre_swap (metrics_t *metrics)
 		printf ("# frame: Frame_Number Frame_Time_milliseconds Frame_latency_milliseconds CPU_load GPU_load\n");
 	} else {
 		/* Subtract previous frame's times to get frame times. */
-		frame_time_ns = swap_end_timestamp - metrics->previous_swap_end_timestamp;
+		frame_time_ns = subtract_timestamp (swap_end_timestamp,
+						    metrics->previous_swap_end_timestamp);
 
-		latency_ns = swap_end_timestamp - metrics->swap_begin_timestamp;
+		latency_ns = subtract_timestamp (swap_end_timestamp,
+						 metrics->swap_begin_timestamp);
 
 		cpu_time = TIMESPEC_DIFF (metrics->previous_cpu_time_ts, metrics->cpu_time_ts);
 
-- 
2.43.0