backend: turn max_buffer_age into a function

We want to change the backend interface as little as possible once we
release it as a public interface, so while we still can, we should try
to give it maximum flexibility.

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

include/picom/backend.h

@@ -450,7 +450,7 @@ struct backend_operations {
 	bool (*last_render_time)(backend_t *backend_data, struct timespec *ts);
 
 	/// The maximum number buffer_age might return.
-	int max_buffer_age;
+	int (*max_buffer_age)(backend_t *backend_data);
 
 	// ===========    Post-processing   ============
 	/// Create a blur context that can be used to call `blur` for images with a

src/backend/dummy/dummy.c

@@ -204,6 +204,10 @@ bool dummy_is_format_supported(struct backend_base *base attr_unused,
 	return true;
 }
 
+static int dummy_max_buffer_age(struct backend_base *base attr_unused) {
+	return 5;
+}
+
 struct backend_operations dummy_ops = {
     .apply_alpha = dummy_apply_alpha,
     .back_buffer = dummy_back_buffer,
@@ -222,7 +226,7 @@ struct backend_operations dummy_ops = {
     .init = dummy_init,
     .deinit = dummy_deinit,
     .buffer_age = dummy_buffer_age,
-    .max_buffer_age = 5,
+    .max_buffer_age = dummy_max_buffer_age,
 
     .create_blur_context = dummy_create_blur_context,
     .destroy_blur_context = dummy_destroy_blur_context,

src/backend/gl/egl.c

@@ -339,6 +339,14 @@ static void egl_diagnostics(backend_t *base) {
 	}
 }
 
+static int egl_max_buffer_age(backend_t *base attr_unused) {
+	if (!eglext.has_EGL_EXT_buffer_age) {
+		return 0;
+	}
+
+	return 5;        // Why?
+}
+
 struct backend_operations egl_ops = {
     .apply_alpha = gl_apply_alpha,
     .back_buffer = gl_back_buffer,
@@ -369,7 +377,7 @@ struct backend_operations egl_ops = {
     .create_shader = gl_create_window_shader,
     .destroy_shader = gl_destroy_window_shader,
     .get_shader_attributes = gl_get_shader_attributes,
-    .max_buffer_age = 5,        // Why?
+    .max_buffer_age = egl_max_buffer_age,
 };
 
 struct eglext_info eglext = {0};

src/backend/gl/glx.c

@@ -517,6 +517,10 @@ static void glx_diagnostics(backend_t *base) {
 	}
 }
 
+static int glx_max_buffer_age(struct backend_base *base attr_unused) {
+	return 5;        // Why?
+}
+
 struct backend_operations glx_ops = {
     .apply_alpha = gl_apply_alpha,
     .back_buffer = gl_back_buffer,
@@ -547,7 +551,7 @@ struct backend_operations glx_ops = {
     .create_shader = gl_create_window_shader,
     .destroy_shader = gl_destroy_window_shader,
     .get_shader_attributes = gl_get_shader_attributes,
-    .max_buffer_age = 5,        // Why?
+    .max_buffer_age = glx_max_buffer_age,
 };
 
 struct glxext_info glxext = {0};

src/backend/xrender/xrender.c

@@ -1020,6 +1020,10 @@ uint32_t xrender_quirks(struct backend_base *base) {
 	return ((struct xrender_data *)base)->quirks;
 }
 
+static int xrender_max_buffer_age(struct backend_base *base) {
+	return ((struct xrender_data *)base)->vsync ? 2 : 1;
+}
+
 struct backend_operations xrender_ops = {
     .apply_alpha = xrender_apply_alpha,
     .back_buffer = xrender_back_buffer,
@@ -1042,7 +1046,7 @@ struct backend_operations xrender_ops = {
     //             `render_shadow`, and `backend_compat_shadow_from_mask` for
     //             `shadow_from_mask`
     .buffer_age = xrender_buffer_age,
-    .max_buffer_age = 2,
+    .max_buffer_age = xrender_max_buffer_age,
     .create_blur_context = xrender_create_blur_context,
     .destroy_blur_context = xrender_destroy_blur_context,
     .get_blur_size = xrender_get_blur_size

src/picom.c

@@ -437,7 +437,7 @@ void add_damage(session_t *ps, const region_t *damage) {
 		return;
 	}
 
-	if (!damage) {
+	if (!damage || ps->damage_ring.count <= 0) {
 		return;
 	}
 	log_trace("Adding damage: ");
@@ -1418,9 +1418,9 @@ static bool redirect_start(session_t *ps) {
 
 	if (!ps->o.use_legacy_backends) {
 		assert(ps->backend_data);
-		ps->damage_ring.count = ps->backend_data->ops->max_buffer_age;
-		ps->layout_manager =
-		    layout_manager_new((unsigned)ps->backend_data->ops->max_buffer_age);
+		ps->damage_ring.count =
+		    ps->backend_data->ops->max_buffer_age(ps->backend_data);
+		ps->layout_manager = layout_manager_new((unsigned)ps->damage_ring.count);
 	} else {
 		ps->damage_ring.count = maximum_buffer_age(ps);
 	}

src/renderer/renderer.c

@@ -154,7 +154,7 @@ renderer_init(struct renderer *renderer, struct backend_base *backend,
 		}
 		sum_kernel_preprocess(renderer->shadow_kernel);
 	}
-	renderer->max_buffer_age = backend->ops->max_buffer_age + 1;
+	renderer->max_buffer_age = backend->ops->max_buffer_age(backend) + 1;
 	return true;
 }