output: allow tearing with atomic mode setting

Additionally, track errors and abandon the tearing allowance when it
cannot be set for two-seconds' worth of consecutive frames.

docs/labwc-config.5.scd

@@ -186,10 +186,13 @@ this is for compatibility with Openbox.
 	mode.
 
 *<core><allowTearing>* [yes|no]
-	Allow tearing to reduce input lag. Default is no.
+	Allow tearing, if requested by the active window, to reduce input lag.
-	This option requires setting the environment variable
+	Default is no.
-	WLR_DRM_NO_ATOMIC=1.
+
-	*yes* allow tearing if requested by the active window.
+	Note: Enabling this option with atomic mode setting is experimental. If
+	you experience undesirable side effects when tearing is allowed,
+	consider setting the environment variable WLR_DRM_NO_ATOMIC=1 when
+	launching labwc.
 
 *<core><reuseOutputMode>* [yes|no]
 	Try to re-use the existing output mode (resolution / refresh rate).

include/labwc.h

@@ -389,6 +389,8 @@ struct output {
 
 	bool leased;
 	bool gamma_lut_changed;
+
+	uint32_t nr_tearing_failures;
 };
 
 #undef LAB_NR_LAYERS

src/config/rcxml.c

@@ -885,13 +885,6 @@ entry(xmlNode *node, char *nodename, char *content)
 		set_adaptive_sync_mode(content, &rc.adaptive_sync);
 	} else if (!strcasecmp(nodename, "allowTearing.core")) {
 		set_bool(content, &rc.allow_tearing);
-		if (rc.allow_tearing) {
-			char *no_atomic_env = getenv("WLR_DRM_NO_ATOMIC");
-			if (!no_atomic_env || strcmp(no_atomic_env, "1") != 0) {
-				rc.allow_tearing = false;
-				wlr_log(WLR_ERROR, "tearing requires WLR_DRM_NO_ATOMIC=1");
-			}
-		}
 	} else if (!strcasecmp(nodename, "reuseOutputMode.core")) {
 		set_bool(content, &rc.reuse_output_mode);
 	} else if (!strcmp(nodename, "policy.placement")) {

src/output.c

@@ -30,6 +30,14 @@
 #include "view.h"
 #include "xwayland.h"
 
+static unsigned int
+get_tearing_retry_count(struct output *output)
+{
+	/* Two seconds worth of frames, guessing 60Hz if refresh is invalid */
+	int refresh = output->wlr_output->refresh;
+	return refresh > 0 ? refresh / 500 : 120;
+}
+
 static bool
 get_tearing_preference(struct output *output)
 {
@@ -45,6 +53,11 @@ get_tearing_preference(struct output *output)
 		return false;
 	}
 
+	/* Tearing should not have failed too many times */
+	if (output->nr_tearing_failures >= get_tearing_retry_count(output)) {
+		return false;
+	}
+
 	/* If the active view requests tearing, or it is toggled on with action, allow it */
 	return server->active_view->tearing_hint;
 }
@@ -112,11 +125,27 @@ output_frame_notify(struct wl_listener *listener, void *data)
 		 */
 		output_apply_gamma(output);
 	} else {
-		output->pending.tearing_page_flip =
+		struct wlr_scene_output *scene_output = output->scene_output;
-			get_tearing_preference(output);
+		struct wlr_output_state *pending = &output->pending;
 
-		lab_wlr_scene_output_commit(output->scene_output,
+		pending->tearing_page_flip = get_tearing_preference(output);
-			&output->pending);
+
+		bool committed =
+			lab_wlr_scene_output_commit(scene_output, pending);
+
+		if (pending->tearing_page_flip) {
+			if (committed) {
+				output->nr_tearing_failures = 0;
+			} else {
+				if (++output->nr_tearing_failures >=
+						get_tearing_retry_count(output)) {
+					wlr_log(WLR_INFO, "setting tearing allowance failed "
+						"for two consecutive seconds, disabling");
+				}
+				pending->tearing_page_flip = false;
+				lab_wlr_scene_output_commit(scene_output, pending);
+			}
+		}
 	}
 
 	struct timespec now = { 0 };