display/gtk: get proper refreshrate

Because some VMs in QEMU can get GPU virtualization (using technologies
such as iGVT-g, as mentioned previously), they could produce a video
output that had a higher display refresh rate than of what the GTK
display was displaying. (fxp. Playing a video game inside of a Windows
VM at 60 Hz, while the output stood locked at 33 Hz because of defaults
set in include/ui/console.h)

Since QEMU does indeed have internal systems for determining frame
times as defined in ui/console.c.
The code checks for a variable called update_interval that it later
uses for time calculation. This variable, however, isn't defined
anywhere in ui/gtk.c and instead ui/console.c just sets it to
GUI_REFRESH_INTERVAL_DEFAULT which is 30

update_interval represents the number of milliseconds per display
refresh, and by doing some math we get that 1000/30 = 33.33... Hz

This creates the mentioned problem and what this patch does is that it
checks for the display refresh rate reported by GTK itself (we can take
this as a safe value) and just converts it back to a number of
milliseconds per display refresh.

Signed-off-by: Nikola Pavlica <pavlica.nikola@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200108121342.29597-1-pavlica.nikola@gmail.com

[ kraxel: style tweak: add blank line between vars and code ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

include/ui/gtk.h

@@ -28,6 +28,8 @@
 #include "ui/egl-context.h"
 #endif
 
+#define MILLISEC_PER_SEC 1000000
+
 typedef struct GtkDisplayState GtkDisplayState;
 
 typedef struct VirtualGfxConsole {

ui/gtk.c

@@ -1966,6 +1966,11 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
                               GSList *group, GtkWidget *view_menu)
 {
     bool zoom_to_fit = false;
+    int refresh_rate_millihz;
+
+    GdkDisplay *dpy = gtk_widget_get_display(s->window);
+    GdkWindow *win = gtk_widget_get_window(s->window);
+    GdkMonitor *monitor = gdk_display_get_monitor_at_window(dpy, win);
 
     vc->label = qemu_console_get_label(con);
     vc->s = s;
@@ -2026,6 +2031,12 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
 
     vc->gfx.kbd = qkbd_state_init(con);
     vc->gfx.dcl.con = con;
+
+    refresh_rate_millihz = gdk_monitor_get_refresh_rate(monitor);
+    if (refresh_rate_millihz) {
+        vc->gfx.dcl.update_interval = MILLISEC_PER_SEC / refresh_rate_millihz;
+    }
+
     register_displaychangelistener(&vc->gfx.dcl);
 
     gd_connect_vc_gfx_signals(vc);