wsi/x11: Rewrite logic for how we consider minImageCount.

MAILBOX reduced from 5 to 4 to be more inline with WL.

With xwayland_wait_ready = false now being the default,
add some extra consideration for non-FIFO present modes.
Add a lengthy rationale for why minImageCount = 3 is exposed on Xwl.

Signed-off-by: Hans-Kristian Arntzen <post@arntzen-software.no>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27074>

src/vulkan/wsi/wsi_common_x11.c

@@ -1542,6 +1542,53 @@ x11_needs_wait_for_fences(const struct wsi_device *wsi_device,
    }
 }
 
+/* This matches Wayland. */
+#define X11_SWAPCHAIN_MAILBOX_IMAGES 4
+
+static bool
+x11_requires_mailbox_image_count(const struct wsi_device *device,
+                                 struct wsi_x11_connection *wsi_conn,
+                                 VkPresentModeKHR present_mode)
+{
+   /* If we're resorting to wait for fences, we're assuming a MAILBOX-like model,
+    * and we should allocate accordingly.
+    *
+    * One potential concern here is IMMEDIATE mode on Wayland.
+    * This situation could arise:
+    * - Fullscreen FLIP mode
+    * - Compositor does not support tearing protocol (we cannot know this here)
+    *
+    * With 3 images, during the window between latch and flip, there is only one image left to app,
+    * so peak FPS may not be reached if the window between latch and flip is large,
+    * but tests on contemporary compositors suggest this effect is minor.
+    * Frame rate in the thousands can easily be reached.
+    *
+    * There are pragmatic reasons to expose 3 images for IMMEDIATE on Xwl.
+    * - minImageCount is not intended as a tool to tune performance, its intent is to signal forward progress.
+    *   Our X11 and WL implementations do so for pragmatic reasons due to sync acquire interacting poorly with 2 images.
+    *   A jump from 3 to 4 is at best a minor improvement which only affects applications
+    *   running at extremely high frame rates, way beyond the monitor refresh rate.
+    *   On the other hand, lowering minImageCount to 2 would break the fundamental idea of MAILBOX
+    *   (and IMMEDIATE without tear), since FPS > refresh rate would not be possible.
+    *
+    * - Several games developed for other platforms and other Linux WSI implementations
+    *   do not expect that image counts arbitrarily change when changing present mode,
+    *   and will crash when Mesa does so.
+    *   There are several games using the strict_image_count drirc to work around this,
+    *   and it would be good to be friendlier in the first place, so we don't have to work around more games.
+    *   IMMEDIATE is a common presentation mode on those platforms, but MAILBOX is more Wayland-centric in nature,
+    *   so increasing image count for that mode is more reasonable.
+    *
+    * - IMMEDIATE expects tearing, and when tearing, 3 images are more than enough.
+    *
+    * - With EXT_swapchain_maintenance1, toggling between FIFO / IMMEDIATE (used extensively by D3D layering)
+    *   would require application to allocate >3 images which is unfortunate for memory usage,
+    *   and potentially disastrous for latency unless KHR_present_wait is used.
+    */
+   return x11_needs_wait_for_fences(device, wsi_conn, present_mode) ||
+          present_mode == VK_PRESENT_MODE_MAILBOX_KHR;
+}
+
 /**
  * Send image to the X server for presentation at target_msc.
  */
@@ -2312,10 +2359,11 @@ x11_get_min_image_count_for_present_mode(struct wsi_device *wsi_device,
                                          struct wsi_x11_connection *wsi_conn,
                                          VkPresentModeKHR present_mode)
 {
-   if (x11_needs_wait_for_fences(wsi_device, wsi_conn, present_mode))
+   uint32_t min_image_count = x11_get_min_image_count(wsi_device, wsi_conn->is_xwayland);
-      return 5;
+   if (x11_requires_mailbox_image_count(wsi_device, wsi_conn, present_mode))
+      return MAX2(min_image_count, X11_SWAPCHAIN_MAILBOX_IMAGES);
    else
-      return x11_get_min_image_count(wsi_device, wsi_conn->is_xwayland);
+      return min_image_count;
 }
 
 /**
@@ -2354,12 +2402,14 @@ x11_surface_create_swapchain(VkIcdSurfaceBase *icd_surface,
     * - presentation mode.
     */
    unsigned num_images = pCreateInfo->minImageCount;
-   if (wsi_device->x11.strict_imageCount)
+   if (!wsi_device->x11.strict_imageCount) {
-      num_images = pCreateInfo->minImageCount;
+      if (x11_requires_mailbox_image_count(wsi_device, wsi_conn, present_mode) ||
-   else if (x11_needs_wait_for_fences(wsi_device, wsi_conn, present_mode))
+          wsi_device->x11.ensure_minImageCount) {
-      num_images = MAX2(num_images, 5);
+         unsigned present_mode_images = x11_get_min_image_count_for_present_mode(
-   else if (wsi_device->x11.ensure_minImageCount)
+               wsi_device, wsi_conn, pCreateInfo->presentMode);
-      num_images = MAX2(num_images, x11_get_min_image_count(wsi_device, wsi_conn->is_xwayland));
+         num_images = MAX2(num_images, present_mode_images);
+      }
+   }
 
    /* Check that we have a window up-front. It is an error to not have one. */
    xcb_window_t window = x11_surface_get_window(icd_surface);