loader/dri3: Enable adaptive_sync via _VARIABLE_REFRESH property

The DDX driver can be notified of adaptive sync suitability by
flagging the application's window with the _VARIABLE_REFRESH property.

This property is set on the first swap the application performs
when adaptive_sync is set to true in the drirc.

It's performed here instead of when the loader is initialized for
two reasons:

(1) The window's drawable can be missing during loader init.
    This can be observed during the Unigine Superposition benchmark.

(2) Adaptive sync will only be enabled closer to when the application
    actually begins rendering.

If adaptive_sync is false then the _VARIABLE_REFRESH property
is deleted on loader init.

The property is only managed on the glx DRI3 backend for now. This
should cover most common applications and games on modern hardware.

Vulkan support can be implemented in a similar manner but would likely
require splitting the function out into a common helper function.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>

src/loader/loader_dri3_helper.c

@@ -101,6 +101,32 @@ get_xcb_visualtype_for_depth(struct loader_dri3_drawable *draw, int depth)
    return NULL;
 }
 
+/* Sets the adaptive sync window property state. */
+static void
+set_adaptive_sync_property(xcb_connection_t *conn, xcb_drawable_t drawable,
+                           uint32_t state)
+{
+   static char const name[] = "_VARIABLE_REFRESH";
+   xcb_intern_atom_cookie_t cookie;
+   xcb_intern_atom_reply_t* reply;
+   xcb_void_cookie_t check;
+
+   cookie = xcb_intern_atom(conn, 0, sizeof(name), name);
+   reply = xcb_intern_atom_reply(conn, cookie, NULL);
+   if (reply == NULL)
+      return;
+
+   if (state)
+      check = xcb_change_property_checked(conn, XCB_PROP_MODE_REPLACE,
+                                          drawable, reply->atom,
+                                          XCB_ATOM_CARDINAL, 32, 1, &state);
+   else
+      check = xcb_delete_property_checked(conn, drawable, reply->atom);
+
+   xcb_discard_reply(conn, check.sequence);
+   free(reply);
+}
+
 /* Get red channel mask for given drawable at given depth. */
 static unsigned int
 dri3_get_red_mask_for_depth(struct loader_dri3_drawable *draw, int depth)
@@ -331,16 +357,30 @@ loader_dri3_drawable_init(xcb_connection_t *conn,
    draw->have_back = 0;
    draw->have_fake_front = 0;
    draw->first_init = true;
+   draw->adaptive_sync = false;
+   draw->adaptive_sync_active = false;
 
    draw->cur_blit_source = -1;
    draw->back_format = __DRI_IMAGE_FORMAT_NONE;
    mtx_init(&draw->mtx, mtx_plain);
    cnd_init(&draw->event_cnd);
 
-   if (draw->ext->config)
+   if (draw->ext->config) {
+      unsigned char adaptive_sync;
+
       draw->ext->config->configQueryi(draw->dri_screen,
                                       "vblank_mode", &vblank_mode);
 
+      draw->ext->config->configQueryb(draw->dri_screen,
+                                      "adaptive_sync",
+                                      &adaptive_sync);
+
+      draw->adaptive_sync = adaptive_sync;
+   }
+
+   if (!draw->adaptive_sync)
+      set_adaptive_sync_property(conn, draw->drawable, false);
+
    switch (vblank_mode) {
    case DRI_CONF_VBLANK_NEVER:
    case DRI_CONF_VBLANK_DEF_INTERVAL_0:
@@ -879,6 +919,12 @@ loader_dri3_swap_buffers_msc(struct loader_dri3_drawable *draw,
    back = dri3_find_back_alloc(draw);
 
    mtx_lock(&draw->mtx);
+
+   if (draw->adaptive_sync && !draw->adaptive_sync_active) {
+      set_adaptive_sync_property(draw->conn, draw->drawable, true);
+      draw->adaptive_sync_active = true;
+   }
+
    if (draw->is_different_gpu && back) {
       /* Update the linear buffer before presenting the pixmap */
       (void) loader_dri3_blit_image(draw,

src/loader/loader_dri3_helper.h

@@ -156,6 +156,8 @@ struct loader_dri3_drawable {
    xcb_special_event_t *special_event;
 
    bool first_init;
+   bool adaptive_sync;
+   bool adaptive_sync_active;
    int swap_interval;
 
    struct loader_dri3_extensions *ext;