ui/gtk: limit virtual console max update interval

Limit the virtual console maximum update interval to
GUI_REFRESH_INTERVAL_DEFAULT. This papers over a integer
overflow bug in gtk3 on Windows where the reported monitor
refresh frequency can be much smaller than the real refresh
frequency.

The gtk bug report can be found here:
https://gitlab.gnome.org/GNOME/gtk/-/issues/3394

On my Windows 10 system gtk reports a monitor refresh rate of
1.511Hz instead of 60.031Hz and slows down the screen update
rate in qemu to a crawl. Provided you are affected by the gtk
bug on Windows, these are the steps to reproduce the issue:

Start qemu with -display gtk and activate all qemu virtual
consoles and notice the reduced qemu refresh rate. Activating
all virtual consoles is necessary, because gui_update() in
ui/console.c uses the minimum of all display change listeners
update interval and not yet activated virtual consoles report
the default update interval (30ms).

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20201213165724.13418-3-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

1 files changed, 11 insertions, 11 deletions

diff --git a/ui/gtk.c b/ui/gtk.c
index 439c1e949f..d2004a4dc1 100644
--- a/ui/gtk.c
+++ b/ui/gtk.c
@@ -749,10 +749,10 @@ static void gd_resize_event(GtkGLArea *area,
 #endif
 
 /*
- * If available, return the refresh rate of the display in milli-Hertz,
- * else return 0.
+ * If available, return the update interval of the monitor in ms,
+ * else return 0 (the default update interval).
  */
-static int gd_refresh_rate_millihz(GtkWidget *widget)
+static int gd_monitor_update_interval(GtkWidget *widget)
 {
 #ifdef GDK_VERSION_3_22
     GdkWindow *win = gtk_widget_get_window(widget);
@@ -760,8 +760,13 @@ static int gd_refresh_rate_millihz(GtkWidget *widget)
     if (win) {
         GdkDisplay *dpy = gtk_widget_get_display(widget);
         GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
+        int refresh_rate = gdk_monitor_get_refresh_rate(monitor); /* [mHz] */
 
-        return gdk_monitor_get_refresh_rate(monitor);
+        if (refresh_rate) {
+            /* T = 1 / f = 1 [s*Hz] / f = 1000*1000 [ms*mHz] / f */
+            return MIN(1000 * 1000 / refresh_rate,
+                       GUI_REFRESH_INTERVAL_DEFAULT);
+        }
     }
 #endif
     return 0;
@@ -774,7 +779,6 @@ static gboolean gd_draw_event(GtkWidget *widget, cairo_t *cr, void *opaque)
     int mx, my;
     int ww, wh;
     int fbw, fbh;
-    int refresh_rate_millihz;
 
 #if defined(CONFIG_OPENGL)
     if (vc->gfx.gls) {
@@ -795,12 +799,8 @@ static gboolean gd_draw_event(GtkWidget *widget, cairo_t *cr, void *opaque)
         return FALSE;
     }
 
-    refresh_rate_millihz = gd_refresh_rate_millihz(vc->window ?
-                                                   vc->window : s->window);
-    if (refresh_rate_millihz) {
-        /* T = 1 / f = 1 [s*Hz] / f = 1000*1000 [ms*mHz] / f */
-        vc->gfx.dcl.update_interval = 1000 * 1000 / refresh_rate_millihz;
-    }
+    vc->gfx.dcl.update_interval =
+        gd_monitor_update_interval(vc->window ? vc->window : s->window);
 
     fbw = surface_width(vc->gfx.ds);
     fbh = surface_height(vc->gfx.ds);