config: add debug options to enable timing based pacing

Disable timing estimation based pacing by default, as it might not work
well across drivers, and might have subtle bugs.

You can try setting `PICOM_DEBUG=smart_frame_pacing` if you want to try
it out.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/config.c

@@ -8,6 +8,7 @@
 #include <limits.h>
 #include <math.h>
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -623,7 +624,9 @@ struct debug_options_entry {
 };
 
 static const struct debug_options_entry debug_options_entries[] = {
-
+    "smart_frame_pacing",
+    NULL,
+    offsetof(struct debug_options, smart_frame_pacing),
 };
 
 void parse_debug_option_single(char *setting, struct debug_options *debug_options) {

src/config.h

@@ -85,7 +85,9 @@ enum vblank_scheduler_type {
 
 /// Internal, private options for debugging and development use.
 struct debug_options {
-
+	/// Try to reduce frame latency by using vblank interval and render time
+	/// estimates. Right now it's not working well across drivers.
+	int smart_frame_pacing;
 };
 
 /// Structure representing all options.

src/picom.c

@@ -152,6 +152,12 @@ collect_vblank_interval_statistics(struct vblank_event *e, void *ud) {
 	assert(ps->frame_pacing);
 	assert(ps->vblank_scheduler);
 
+	if (!ps->o.debug_options.smart_frame_pacing) {
+		// We don't need to collect statistics if we are not doing smart frame
+		// pacing.
+		return VBLANK_CALLBACK_DONE;
+	}
+
 	// TODO(yshui): this naive method of estimating vblank interval does not handle
 	//              the variable refresh rate case very well. This includes the case
 	//              of a VRR enabled monitor; or a monitor that's turned off, in which
@@ -220,13 +226,15 @@ enum vblank_callback_action schedule_render_at_vblank(struct vblank_event *e, vo
 	}
 
 	// The frame has been finished and presented, record its render time.
-	int render_time_us =
-	    (int)(render_time.tv_sec * 1000000L + render_time.tv_nsec / 1000L);
-	render_statistics_add_render_time_sample(
-	    &ps->render_stats, render_time_us + (int)ps->last_schedule_delay);
-	log_verbose("Last render call took: %d (gpu) + %d (cpu) us, "
-	            "last_msc: %" PRIu64,
-	            render_time_us, (int)ps->last_schedule_delay, ps->last_msc);
+	if (ps->o.debug_options.smart_frame_pacing) {
+		int render_time_us =
+		    (int)(render_time.tv_sec * 1000000L + render_time.tv_nsec / 1000L);
+		render_statistics_add_render_time_sample(
+		    &ps->render_stats, render_time_us + (int)ps->last_schedule_delay);
+		log_verbose("Last render call took: %d (gpu) + %d (cpu) us, "
+		            "last_msc: %" PRIu64,
+		            render_time_us, (int)ps->last_schedule_delay, ps->last_msc);
+	}
 	ps->last_schedule_delay = 0;
 	ps->backend_busy = false;
 
@@ -319,6 +327,9 @@ void schedule_render(session_t *ps, bool triggered_by_vblank attr_unused) {
 		return;
 	}
 
+	// if ps->o.debug_options.smart_frame_pacing is false, we won't have any render
+	// time or vblank interval estimates, so we would naturally fallback to schedule
+	// render immediately.
 	auto render_budget = render_statistics_get_budget(&ps->render_stats, &divisor);
 	auto frame_time = render_statistics_get_vblank_time(&ps->render_stats);
 	if (frame_time == 0) {