conf: make delayed rendering timeouts configurable

This adds an undocumented 'tweak' section to footrc, with two new
options:

* delayed-render-lower
* delayed-render-upper

Both takes an integer value, representing the lower/upper timeout
values (in nano seconds) for delayed rendering.

config.c

@@ -584,6 +584,51 @@ parse_section_mouse_bindings(
     return false;
 }
 
+static bool
+parse_section_tweak(
+    const char *key, const char *value, struct config *conf,
+    const char *path, unsigned lineno)
+{
+    if (strcmp(key, "delayed-render-lower") == 0) {
+        unsigned long ns;
+        if (!str_to_ulong(value, 10, &ns)) {
+            LOG_ERR("%s:%d: expected an integer: %s", path, lineno, value);
+            return false;
+        }
+
+        if (ns > 16666666) {
+            LOG_ERR("%s:%d: timeout must not exceed 16ms", path, lineno);
+            return false;
+        }
+
+        conf->tweak.delayed_render_lower_ns = ns;
+        LOG_WARN("tweak: delayed-render-lower=%lu", ns);
+    }
+
+    else if (strcmp(key, "delayed-render-upper") == 0) {
+        unsigned long ns;
+        if (!str_to_ulong(value, 10, &ns)) {
+            LOG_ERR("%s:%d: expected an integer: %s", path, lineno, value);
+            return false;
+        }
+
+        if (ns > 16666666) {
+            LOG_ERR("%s:%d: timeout must not exceed 16ms", path, lineno);
+            return false;
+        }
+
+        conf->tweak.delayed_render_upper_ns = ns;
+        LOG_WARN("tweak: delayed-render-upper=%lu", ns);
+    }
+
+    else {
+        LOG_ERR("%s:%u: invalid key: %s", path, lineno, key);
+        return false;
+    }
+
+    return true;
+}
+
 static bool
 parse_config_file(FILE *f, struct config *conf, const char *path)
 {
@@ -594,6 +639,7 @@ parse_config_file(FILE *f, struct config *conf, const char *path)
         SECTION_CSD,
         SECTION_KEY_BINDINGS,
         SECTION_MOUSE_BINDINGS,
+        SECTION_TWEAK,
         SECTION_COUNT,
     } section = SECTION_MAIN;
 
@@ -612,6 +658,7 @@ parse_config_file(FILE *f, struct config *conf, const char *path)
         [SECTION_CSD] =            {&parse_section_csd, "csd"},
         [SECTION_KEY_BINDINGS] =   {&parse_section_key_bindings, "key-bindings"},
         [SECTION_MOUSE_BINDINGS] = {&parse_section_mouse_bindings, "mouse-bindings"},
+        [SECTION_TWEAK] =          {&parse_section_tweak, "tweak"},
     };
 
     static_assert(ALEN(section_info) == SECTION_COUNT, "section info array size mismatch");
@@ -823,6 +870,11 @@ config_load(struct config *conf, const char *conf_path)
         .server_socket_path = get_server_socket_path(),
         .presentation_timings = false,
         .hold_at_exit = false,
+
+        .tweak = {
+            .delayed_render_lower_ns = 500000,         /* 0.5ms */
+            .delayed_render_upper_ns = 16666666 / 2,   /* half a frame period (60Hz) */
+        },
     };
 
     char *default_path = NULL;

config.h

@@ -73,6 +73,11 @@ struct config {
     char *server_socket_path;
     bool presentation_timings;
     bool hold_at_exit;
+
+    struct {
+        uint64_t delayed_render_lower_ns;
+        uint64_t delayed_render_upper_ns;
+    } tweak;
 };
 
 bool config_load(struct config *conf, const char *path);

terminal.c

@@ -145,11 +145,6 @@ fdm_ptmx(struct fdm *fdm, int fd, int events, void *data)
             return false;
     }
 
-#if 0
-    if (!pollin)
-        return true;
-#endif
-
     /* Prevent blinking while typing */
     term_cursor_blink_restart(term);
 
@@ -220,9 +215,15 @@ fdm_ptmx(struct fdm *fdm, int fd, int events, void *data)
             last = now;
 #endif
 
+            uint64_t lower_ns = term->conf->tweak.delayed_render_lower_ns;
+            uint64_t upper_ns = term->conf->tweak.delayed_render_upper_ns;
+            assert(lower_ns < 1000000000);
+            assert(upper_ns < 1000000000);
+            assert(upper_ns > lower_ns);
+
             timerfd_settime(
                 term->delayed_render_timer.lower_fd, 0,
-                &(struct itimerspec){.it_value = {.tv_nsec = 500000}},
+                &(struct itimerspec){.it_value = {.tv_nsec = lower_ns}},
                 NULL);
 
             /* Second timeout - only reset when we render. Set to one
@@ -230,7 +231,7 @@ fdm_ptmx(struct fdm *fdm, int fd, int events, void *data)
             if (!term->delayed_render_timer.is_armed) {
                 timerfd_settime(
                     term->delayed_render_timer.upper_fd, 0,
-                    &(struct itimerspec){.it_value = {.tv_nsec = 16666666 / 2}},
+                    &(struct itimerspec){.it_value = {.tv_nsec = upper_ns}},
                     NULL);
                 term->delayed_render_timer.is_armed = true;
             }