mmal/vout: Configure latency target

The mmal video_render element allows to configure a latency target. Although
the parameter is named AUDIO_LATENCY_TARGET it is not actually related to
audio but synchronizes frame draws with hdmi vsync. It tries to match the hdmi
vsync rate to the picture render rate by slightly varying the hdmi pixel
clock. This makes only sense if the actual display refresh rate matches the
video frame rate, so that only minimal drift compensation is required. Thus
enable it only when mmal-adjust-refreshrate is activated.
Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/hw/mmal/vout.c

@@ -140,6 +140,7 @@ static int query_resolution(vout_display_t *vd, unsigned *width, unsigned *heigh
 static void tvservice_cb(void *callback_data, uint32_t reason, uint32_t param1,
                 uint32_t param2);
 static void adjust_refresh_rate(vout_display_t *vd);
+static int set_latency_target(vout_display_t *vd, bool enable);
 
 /* DispManX */
 static void display_subpicture(vout_display_t *vd, subpicture_t *subpicture);
@@ -402,8 +403,10 @@ static int configure_display(vout_display_t *vd, const vout_display_cfg_t *cfg,
     }
 
     show_background(vd, cfg->is_fullscreen);
-    if (var_InheritBool(vd, MMAL_ADJUST_REFRESHRATE_NAME))
+    if (var_InheritBool(vd, MMAL_ADJUST_REFRESHRATE_NAME)) {
         adjust_refresh_rate(vd);
+        set_latency_target(vd, true);
+    }
 
     if (fmt != &vd->fmt)
         memcpy(&vd->fmt, fmt, sizeof(video_format_t));
@@ -699,6 +702,32 @@ static void tvservice_cb(void *callback_data, uint32_t reason, uint32_t param1,
     vlc_mutex_unlock(&sys->manage_mutex);
 }
 
+static int set_latency_target(vout_display_t *vd, bool enable)
+{
+    vout_display_sys_t *sys = vd->sys;
+    MMAL_STATUS_T status;
+
+    MMAL_PARAMETER_AUDIO_LATENCY_TARGET_T latency_target = {
+        .hdr = { MMAL_PARAMETER_AUDIO_LATENCY_TARGET, sizeof(latency_target) },
+        .enable = enable ? MMAL_TRUE : MMAL_FALSE,
+        .filter = 2,
+        .target = 4000,
+        .shift = 3,
+        .speed_factor = -135,
+        .inter_factor = 500,
+        .adj_cap = 20
+    };
+
+    status = mmal_port_parameter_set(sys->input, &latency_target.hdr);
+    if (status != MMAL_SUCCESS) {
+        msg_Err(vd, "Failed to configure latency target on input port %s (status=%"PRIx32" %s)",
+                        sys->input->name, status, mmal_status_to_string(status));
+        return VLC_EGENERIC;
+    }
+
+    return VLC_SUCCESS;
+}
+
 static void adjust_refresh_rate(vout_display_t *vd)
 {
     TV_DISPLAY_STATE_T display_state;