STEAMOS: Dynamic swapchain override for gamescope limiter for DRI3 only

The original patch (from Bas) contained WSI VK support too but it's
been removed because the Gamescope WSI layer already handles that.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/gallium/frontends/dri/loader_dri3_helper.c

@@ -274,6 +274,30 @@ dri3_update_max_num_back(struct loader_dri3_drawable *draw)
    }
 }
 
+static unsigned
+gamescope_swapchain_override()
+{
+   const char *path = getenv("GAMESCOPE_LIMITER_FILE");
+   if (!path)
+      return 0;
+
+   static simple_mtx_t mtx = SIMPLE_MTX_INITIALIZER;
+   static int fd = -1;
+
+   simple_mtx_lock(&mtx);
+   if (fd < 0) {
+      fd = open(path, O_RDONLY);
+   }
+   simple_mtx_unlock(&mtx);
+
+   if (fd < 0)
+      return 0;
+
+   uint32_t override_value = 0;
+   pread(fd, &override_value, sizeof(override_value), 0);
+   return override_value;
+}
+
 void
 loader_dri3_set_swap_interval(struct loader_dri3_drawable *draw, int interval)
 {
@@ -288,10 +312,12 @@ loader_dri3_set_swap_interval(struct loader_dri3_drawable *draw, int interval)
     * PS. changing from value A to B and A < B won't cause swap out of order but
     * may still gets wrong target_msc value at the beginning.
     */
-   if (draw->swap_interval != interval)
+   if (draw->orig_swap_interval != interval)
       loader_dri3_swapbuffer_barrier(draw);
 
-   draw->swap_interval = interval;
+   draw->orig_swap_interval = interval;
+   if (gamescope_swapchain_override() != 1)
+      draw->swap_interval = interval;
 }
 
 static void
@@ -420,6 +446,12 @@ loader_dri3_drawable_init(xcb_connection_t *conn,
 
    draw->swap_interval = dri_get_initial_swap_interval(draw->dri_screen_render_gpu);
 
+   draw->orig_swap_interval = draw->swap_interval;
+
+   unsigned gamescope_override = gamescope_swapchain_override();
+   if (gamescope_override == 1)
+      draw->swap_interval = 1;
+
    dri3_update_max_num_back(draw);
 
    /* Create a new drawable */
@@ -1064,6 +1096,12 @@ loader_dri3_swap_buffers_msc(struct loader_dri3_drawable *draw,
    if (draw->type == LOADER_DRI3_DRAWABLE_WINDOW) {
       dri3_fence_reset(draw->conn, back);
 
+      unsigned gamescope_override = gamescope_swapchain_override();
+      if (gamescope_override == 1)
+         draw->swap_interval = 1;
+      else
+         draw->swap_interval = draw->orig_swap_interval;
+
       /* Compute when we want the frame shown by taking the last known
        * successful MSC and adding in a swap interval for each outstanding swap
        * request. target_msc=divisor=remainder=0 means "Use glXSwapBuffers()

src/gallium/frontends/dri/loader_dri3_helper.h

@@ -170,6 +170,7 @@ struct loader_dri3_drawable {
    bool block_on_depleted_buffers;
    bool queries_buffer_age;
    int swap_interval;
+   int orig_swap_interval;
 
    const struct loader_dri3_vtable *vtable;