[KMS/DRM] Fix for bug #5518: only do async pageflips when hardware supports them.

src/video/kmsdrm/SDL_kmsdrmopengles.c

@@ -168,7 +168,7 @@ KMSDRM_GLES_SwapWindow(_THIS, SDL_Window * window) {
        to do so, so even if we don't block on EGL, the flip will have completed
        when we get here again. */
 
-    if (_this->egl_data->egl_swapinterval == 0) {
+    if (_this->egl_data->egl_swapinterval == 0 && viddata->async_pageflip_support) {
         flip_flags |= DRM_MODE_PAGE_FLIP_ASYNC;
     }
 

src/video/kmsdrm/SDL_kmsdrmvideo.c

@@ -653,6 +653,7 @@ int KMSDRM_InitDisplays (_THIS) {
     SDL_VideoData *viddata = ((SDL_VideoData *)_this->driverdata);
     drmModeRes *resources = NULL;
 
+    uint64_t async_pageflip = 0;
     int ret = 0;
     int i;
 
@@ -705,6 +706,13 @@ int KMSDRM_InitDisplays (_THIS) {
         goto cleanup;
     }
 
+    /* Determine if video hardware supports async pageflips. */
+    ret = KMSDRM_drmGetCap(viddata->drm_fd, DRM_CAP_ASYNC_PAGE_FLIP, &async_pageflip);
+    if (ret) {
+        SDL_LogError(SDL_LOG_CATEGORY_VIDEO, "Could not determine async page flip capability.");
+    }
+    viddata->async_pageflip_support = async_pageflip ? SDL_TRUE : SDL_FALSE;
+
     /***********************************/
     /* Block for Vulkan compatibility. */
     /***********************************/

src/video/kmsdrm/SDL_kmsdrmvideo.h

@@ -41,8 +41,9 @@ typedef struct SDL_VideoData
 
     struct gbm_device *gbm_dev;
 
-    SDL_bool video_init;        /* Has VideoInit succeeded? */
-    SDL_bool vulkan_mode;       /* Are we in Vulkan mode? One VK window is enough to be. */
+    SDL_bool video_init;             /* Has VideoInit succeeded? */
+    SDL_bool vulkan_mode;            /* Are we in Vulkan mode? One VK window is enough to be. */
+    SDL_bool async_pageflip_support; /* Does the hardware support async. pageflips? */ 
 
     SDL_Window **windows;
     int max_windows;