SDL: kmsdrm: Restore atomic support.

From 1fe926769c02b934b044e42c4fd43bec0fc6662c Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Wed, 20 Nov 2024 20:05:54 -0500
Subject: [PATCH] kmsdrm: Restore atomic support.

---
 include/SDL3/SDL_hints.h              |  22 +
 src/video/kmsdrm/SDL_kmsdrmmouse.c    | 137 ++++--
 src/video/kmsdrm/SDL_kmsdrmopengles.c | 330 +++++++++++++-
 src/video/kmsdrm/SDL_kmsdrmsym.h      |  10 +
 src/video/kmsdrm/SDL_kmsdrmvideo.c    | 615 ++++++++++++++++++++++++--
 src/video/kmsdrm/SDL_kmsdrmvideo.h    |  69 ++-
 src/video/kmsdrm/SDL_kmsdrmvulkan.c   |   8 +-
 7 files changed, 1109 insertions(+), 82 deletions(-)

diff --git a/include/SDL3/SDL_hints.h b/include/SDL3/SDL_hints.h
index c4ffae77a4c60..fa477bbc2780e 100644
--- a/include/SDL3/SDL_hints.h
+++ b/include/SDL3/SDL_hints.h
@@ -2473,6 +2473,28 @@ extern "C" {
  */
 #define SDL_HINT_KMSDRM_REQUIRE_DRM_MASTER "SDL_KMSDRM_REQUIRE_DRM_MASTER"
 
+/**
+ * A variable that controls whether KMSDRM will use "atomic" functionality.
+ *
+ * The KMSDRM backend can use atomic commits, if both DRM_CLIENT_CAP_ATOMIC
+ * and DRM_CLIENT_CAP_UNIVERSAL_PLANES is supported by the system. As of
+ * SDL 3.4.0, it will favor this functionality, but in case this doesn't
+ * work well on a given system or other surprises, this hint can be used
+ * to disable it.
+ *
+ * This hint can not enable the functionality if it isn't available.
+ *
+ * The variable can be set to the following values:
+ *
+ * - "0": SDL will not use the KMSDRM "atomic" functionality.
+ * - "1": SDL will allow usage of the KMSDRM "atomic" functionality. (default)
+ *
+ * This hint should be set before SDL is initialized.
+ *
+ * \since This hint is available since SDL 3.4.0.
+ */
+#define SDL_HINT_KMSDRM_ATOMIC "SDL_KMSDRM_ATOMIC"
+
 /**
  * A variable controlling the default SDL log levels.
  *
diff --git a/src/video/kmsdrm/SDL_kmsdrmmouse.c b/src/video/kmsdrm/SDL_kmsdrmmouse.c
index 383d7d43a7fec..f72bd3ae6a2c7 100644
--- a/src/video/kmsdrm/SDL_kmsdrmmouse.c
+++ b/src/video/kmsdrm/SDL_kmsdrmmouse.c
@@ -67,6 +67,23 @@ void KMSDRM_DestroyCursorBO(SDL_VideoDevice *_this, SDL_VideoDisplay *display)
 
     // Destroy the curso GBM BO.
     if (dispdata->cursor_bo) {
+        SDL_VideoData *viddata = (SDL_VideoData *) _this->internal;
+        if (viddata->is_atomic) {
+            if (dispdata->cursor_plane) {
+                // Unset the the cursor BO from the cursor plane.
+                KMSDRM_PlaneInfo info;
+                SDL_zero(info);
+                info.plane = dispdata->cursor_plane;
+                drm_atomic_set_plane_props(dispdata, &info);
+                // Wait until the cursor is unset from the cursor plane before destroying it's BO.
+                if (drm_atomic_commit(_this, dispdata, true, false)) {
+                    SDL_SetError("Failed atomic commit in KMSDRM_DenitMouse.");
+                }
+                // Free the cursor plane, on which the cursor was being shown.
+                free_plane(&dispdata->cursor_plane);
+            }
+        }
+
         KMSDRM_gbm_bo_destroy(dispdata->cursor_bo);
         dispdata->cursor_bo = NULL;
         dispdata->cursor_bo_drm_fd = -1;
@@ -78,11 +95,14 @@ void KMSDRM_DestroyCursorBO(SDL_VideoDevice *_this, SDL_VideoDisplay *display)
    build a window and assign a display to it. */
 bool KMSDRM_CreateCursorBO(SDL_VideoDisplay *display)
 {
-
     SDL_VideoDevice *dev = SDL_GetVideoDevice();
     SDL_VideoData *viddata = dev->internal;
     SDL_DisplayData *dispdata = display->internal;
 
+    if (viddata->is_atomic) {
+        setup_plane(dev, dispdata, &dispdata->cursor_plane, DRM_PLANE_TYPE_CURSOR);
+    }
+
     if (!KMSDRM_gbm_device_is_format_supported(viddata->gbm_dev,
                                                GBM_FORMAT_ARGB8888,
                                                GBM_BO_USE_CURSOR | GBM_BO_USE_WRITE)) {
@@ -121,15 +141,29 @@ static bool KMSDRM_RemoveCursorFromBO(SDL_VideoDisplay *display)
     SDL_VideoDevice *video_device = SDL_GetVideoDevice();
     SDL_VideoData *viddata = video_device->internal;
 
-    const int rc = KMSDRM_drmModeSetCursor(viddata->drm_fd, dispdata->crtc->crtc_id, 0, 0, 0);
-    if (rc < 0) {
-        result = SDL_SetError("drmModeSetCursor() failed: %s", strerror(-rc));
+    if (viddata->is_atomic) {
+        if (dispdata->cursor_plane) {
+            KMSDRM_PlaneInfo info;
+            SDL_zero(info);
+            info.plane = dispdata->cursor_plane;
+            // The rest of the members are zeroed, so this takes away the cursor from the cursor plane.
+            drm_atomic_set_plane_props(dispdata, &info);
+            if (drm_atomic_commit(video_device, dispdata, true, false)) {
+                result = SDL_SetError("Failed atomic commit in KMSDRM_ShowCursor.");
+            }
+        }
+    } else {
+        const int rc = KMSDRM_drmModeSetCursor(viddata->drm_fd, dispdata->crtc.crtc->crtc_id, 0, 0, 0);
+        if (rc < 0) {
+            result = SDL_SetError("drmModeSetCursor() failed: %s", strerror(-rc));
+        }
     }
+
     return result;
 }
 
 // Dump a cursor buffer to a display's DRM cursor BO.
-static bool KMSDRM_DumpCursorToBO(SDL_VideoDisplay *display, SDL_Cursor *cursor)
+static bool KMSDRM_DumpCursorToBO(SDL_VideoDisplay *display, SDL_Mouse *mouse, SDL_Cursor *cursor)
 {
     SDL_DisplayData *dispdata = display->internal;
     SDL_CursorData *curdata = cursor->internal;
@@ -173,22 +207,42 @@ static bool KMSDRM_DumpCursorToBO(SDL_VideoDisplay *display, SDL_Cursor *cursor)
         goto cleanup;
     }
 
-    // Put the GBM BO buffer on screen using the DRM interface.
-    bo_handle = KMSDRM_gbm_bo_get_handle(dispdata->cursor_bo).u32;
-    if (curdata->hot_x == 0 && curdata->hot_y == 0) {
-        rc = KMSDRM_drmModeSetCursor(viddata->drm_fd, dispdata->crtc->crtc_id,
-                                      bo_handle, dispdata->cursor_w, dispdata->cursor_h);
+    if (viddata->is_atomic) {
+        // Get the fb_id for the GBM BO so we can show it on the cursor plane.
+        KMSDRM_FBInfo *fb = KMSDRM_FBFromBO(video_device, dispdata->cursor_bo);
+        KMSDRM_PlaneInfo info;
+
+        // Show the GBM BO buffer on the cursor plane.
+        SDL_zero(info);
+        info.plane = dispdata->cursor_plane;
+        info.crtc_id = dispdata->crtc.crtc->crtc_id;
+        info.fb_id = fb->fb_id;
+        info.src_w = dispdata->cursor_w;
+        info.src_h = dispdata->cursor_h;
+        info.crtc_x = ((int32_t) SDL_roundf(mouse->x)) - curdata->hot_x;
+        info.crtc_y = ((int32_t) SDL_roundf(mouse->y)) - curdata->hot_y;
+        info.crtc_w = curdata->w;
+        info.crtc_h = curdata->h;
+        drm_atomic_set_plane_props(dispdata, &info);
+        if (drm_atomic_commit(video_device, dispdata, true, false)) {
+            result = SDL_SetError("Failed atomic commit in KMSDRM_ShowCursor.");
+            goto cleanup;
+        }
     } else {
-        rc = KMSDRM_drmModeSetCursor2(viddata->drm_fd, dispdata->crtc->crtc_id,
-                                       bo_handle, dispdata->cursor_w, dispdata->cursor_h, curdata->hot_x, curdata->hot_y);
-    }
-    if (rc < 0) {
-        result = SDL_SetError("Failed to set DRM cursor: %s", strerror(-rc));
-        goto cleanup;
+        // Put the GBM BO buffer on screen using the DRM interface.
+        bo_handle = KMSDRM_gbm_bo_get_handle(dispdata->cursor_bo).u32;
+        if (curdata->hot_x == 0 && curdata->hot_y == 0) {
+            rc = KMSDRM_drmModeSetCursor(viddata->drm_fd, dispdata->crtc.crtc->crtc_id, bo_handle, dispdata->cursor_w, dispdata->cursor_h);
+        } else {
+            rc = KMSDRM_drmModeSetCursor2(viddata->drm_fd, dispdata->crtc.crtc->crtc_id, bo_handle, dispdata->cursor_w, dispdata->cursor_h, curdata->hot_x, curdata->hot_y);
+        }
+        if (rc < 0) {
+            result = SDL_SetError("Failed to set DRM cursor: %s", strerror(-rc));
+            goto cleanup;
+        }
     }
 
 cleanup:
-
     if (ready_buffer) {
         SDL_free(ready_buffer);
     }
@@ -316,7 +370,7 @@ static bool KMSDRM_ShowCursor(SDL_Cursor *cursor)
             if (cursor) {
                 /* Dump the cursor to the display DRM cursor BO so it becomes visible
                    on that display. */
-                result = KMSDRM_DumpCursorToBO(display, cursor);
+                result = KMSDRM_DumpCursorToBO(display, mouse, cursor);
             } else {
                 // Hide the cursor on that display.
                 result = KMSDRM_RemoveCursorFromBO(display);
@@ -327,6 +381,18 @@ static bool KMSDRM_ShowCursor(SDL_Cursor *cursor)
     return result;
 }
 
+static void drm_atomic_movecursor(SDL_DisplayData *dispdata, const SDL_CursorData *curdata, uint16_t x, uint16_t y)
+{
+    if (dispdata->cursor_plane) {  // We can't move a non-existing cursor, but that's ok.
+        // Do we have a set of changes already in the making? If not, allocate a new one.
+        if (!dispdata->atomic_req) {
+            dispdata->atomic_req = KMSDRM_drmModeAtomicAlloc();
+        }
+        add_plane_property(dispdata->atomic_req, dispdata->cursor_plane, "CRTC_X", x - curdata->hot_x);
+        add_plane_property(dispdata->atomic_req, dispdata->cursor_plane, "CRTC_Y", y - curdata->hot_y);
+    }
+}
+
 static bool KMSDRM_WarpMouseGlobal(float x, float y)
 {
     SDL_Mouse *mouse = SDL_GetMouse();
@@ -340,17 +406,25 @@ static bool KMSDRM_WarpMouseGlobal(float x, float y)
 
         // And now update the cursor graphic position on screen.
         if (dispdata->cursor_bo) {
-            const int rc = KMSDRM_drmModeMoveCursor(dispdata->cursor_bo_drm_fd, dispdata->crtc->crtc_id, (int)x, (int)y);
-            if (rc < 0) {
-                return SDL_SetError("drmModeMoveCursor() failed: %s", strerror(-rc));
+            SDL_VideoDevice *dev = SDL_GetVideoDevice();
+            SDL_VideoData *viddata = dev->internal;
+            if (viddata->is_atomic) {
+                const SDL_CursorData *curdata = (const SDL_CursorData *) mouse->cur_cursor->internal;
+                drm_atomic_movecursor(dispdata, curdata, (uint16_t) (int) x, (uint16_t) (int) y);
+            } else {
+                const int rc = KMSDRM_drmModeMoveCursor(dispdata->cursor_bo_drm_fd, dispdata->crtc.crtc->crtc_id, (int)x, (int)y);
+                if (rc < 0) {
+                    return SDL_SetError("drmModeMoveCursor() failed: %s", strerror(-rc));
+                }
             }
-            return true;
         } else {
             return SDL_SetError("Cursor not initialized properly.");
         }
     } else {
         return SDL_SetError("No mouse or current cursor.");
     }
+
+    return true;
 }
 
 static bool KMSDRM_WarpMouse(SDL_Window *window, float x, float y)
@@ -394,14 +468,27 @@ static bool KMSDRM_MoveCursor(SDL_Cursor *cursor)
     if (mouse && mouse->cur_cursor && mouse->focus) {
         SDL_Window *window = mouse->focus;
         SDL_DisplayData *dispdata = SDL_GetDisplayDriverDataForWindow(window);
+        SDL_VideoDevice *dev = SDL_GetVideoDevice();
+        SDL_VideoData *viddata = dev->internal;
 
         if (!dispdata->cursor_bo) {
             return SDL_SetError("Cursor not initialized properly.");
         }
 
-        const int rc = KMSDRM_drmModeMoveCursor(dispdata->cursor_bo_drm_fd, dispdata->crtc->crtc_id, (int)mouse->x, (int)mouse->y);
-        if (rc < 0) {
-            return SDL_SetError("drmModeMoveCursor() failed: %s", strerror(-rc));
+        if (viddata->is_atomic) {
+            /* !!! FIXME: Some programs expect cursor movement even while they don't do SwapWindow() calls,
+               and since we ride on the atomic_commit() in SwapWindow() for cursor movement,
+               cursor won't move in these situations. We could do an atomic_commit() here
+               for each cursor movement request, but it cripples the movement to 30FPS,
+               so a future solution is needed. SDLPoP "QUIT?" menu is an example of this
+               situation. */
+            const SDL_CursorData *curdata = (const SDL_CursorData *) mouse->cur_cursor->internal;
+            drm_atomic_movecursor(dispdata, curdata, (uint16_t) (int) mouse->x, (uint16_t) (int) mouse->y);
+        } else {
+            const int rc = KMSDRM_drmModeMoveCursor(dispdata->cursor_bo_drm_fd, dispdata->crtc.crtc->crtc_id, (int)mouse->x, (int)mouse->y);
+            if (rc < 0) {
+                return SDL_SetError("drmModeMoveCursor() failed: %s", strerror(-rc));
+            }
         }
     }
     return true;
diff --git a/src/video/kmsdrm/SDL_kmsdrmopengles.c b/src/video/kmsdrm/SDL_kmsdrmopengles.c
index cc93cde9c0359..f098cf5b1d559 100644
--- a/src/video/kmsdrm/SDL_kmsdrmopengles.c
+++ b/src/video/kmsdrm/SDL_kmsdrmopengles.c
@@ -28,10 +28,25 @@
 #include "SDL_kmsdrmdyn.h"
 #include <errno.h>
 
+#define VOID2U64(x) ((uint64_t)(size_t)(x))
+
 #ifndef EGL_PLATFORM_GBM_MESA
 #define EGL_PLATFORM_GBM_MESA 0x31D7
 #endif
 
+#ifndef EGL_SYNC_NATIVE_FENCE_ANDROID
+#define EGL_SYNC_NATIVE_FENCE_ANDROID     0x3144
+#endif
+
+#ifndef EGL_SYNC_NATIVE_FENCE_FD_ANDROID
+#define EGL_SYNC_NATIVE_FENCE_FD_ANDROID  0x3145
+#endif
+
+#ifndef EGL_NO_NATIVE_FENCE_FD_ANDROID
+#define EGL_NO_NATIVE_FENCE_FD_ANDROID    -1
+#endif
+
+
 // EGL implementation of SDL OpenGL support
 
 void KMSDRM_GLES_DefaultProfileConfig(SDL_VideoDevice *_this, int *mask, int *major, int *minor)
@@ -83,7 +98,291 @@ bool KMSDRM_GLES_SetSwapInterval(SDL_VideoDevice *_this, int interval)
     return true;
 }
 
-bool KMSDRM_GLES_SwapWindow(SDL_VideoDevice *_this, SDL_Window *window)
+static EGLSyncKHR create_fence(SDL_VideoDevice *_this, int fd)
+{
+    EGLint attrib_list[] = {
+        EGL_SYNC_NATIVE_FENCE_FD_ANDROID, fd,
+        EGL_NONE,
+    };
+
+    EGLSyncKHR fence = _this->egl_data->eglCreateSyncKHR(_this->egl_data->egl_display, EGL_SYNC_NATIVE_FENCE_ANDROID, attrib_list);
+
+    SDL_assert(fence);
+    return fence;
+}
+
+/***********************************************************************************/
+/* Comments about buffer access protection mechanism (=fences) are the ones boxed. */
+/* Also, DON'T remove the asserts: if a fence-related call fails, it's better that */
+/* program exits immediately, or we could leave KMS waiting for a failed/missing   */
+/* fence forever.                                                                  */
+/***********************************************************************************/
+static bool KMSDRM_GLES_SwapWindowFenced(SDL_VideoDevice *_this, SDL_Window * window)
+{
+    SDL_WindowData *windata = ((SDL_WindowData *) window->internal);
+    SDL_DisplayData *dispdata = SDL_GetDisplayDriverDataForWindow(window);
+    KMSDRM_FBInfo *fb;
+    KMSDRM_PlaneInfo info;
+    bool modesetting = false;
+
+    SDL_zero(info);
+
+    /******************************************************************/
+    /* Create the GPU-side FENCE OBJECT. It will be inserted into the */
+    /* GL CMDSTREAM exactly at the end of the gl commands that form a */
+    /* frame.(KMS will have to wait on it before doing a pageflip.)   */
+    /******************************************************************/
+    dispdata->gpu_fence = create_fence(_this, EGL_NO_NATIVE_FENCE_FD_ANDROID);
+    SDL_assert(dispdata->gpu_fence);
+
+    /******************************************************************/
+    /* eglSwapBuffers flushes the fence down the GL CMDSTREAM, so we  */
+    /* know for sure it's there now.                                  */
+    /* Also it marks, at EGL level, the buffer that we want to become */
+    /* the new front buffer. (Remember that won't really happen until */
+    /* we request a pageflip at the KMS level and it completes.       */
+    /******************************************************************/
+    if (! _this->egl_data->eglSwapBuffers(_this->egl_data->egl_display, windata->egl_surface)) {
+        return SDL_EGL_SetError("Failed to swap EGL buffers", "eglSwapBuffers");
+    }
+
+    /******************************************************************/
+    /* EXPORT the GPU-side FENCE OBJECT to the fence INPUT FD, so we  */
+    /* can pass it into the kernel. Atomic ioctl will pass the        */
+    /* in-fence fd into the kernel, thus telling KMS that it has to   */
+    /* wait for GPU to finish rendering the frame (remember where we  */
+    /* put the fence in the GL CMDSTREAM) before doing the changes    */
+    /* requested in the atomic ioct (the pageflip in this case).      */
+    /* (We export the GPU-side FENCE OJECT to the fence INPUT FD now, */
+    /* not sooner, because now we are sure that the GPU-side fence is */
+    /* in the CMDSTREAM to be lifted when the CMDSTREAM to this point */
+    /* is completed).                                                 */
+    /******************************************************************/
+    dispdata->kms_in_fence_fd = _this->egl_data->eglDupNativeFenceFDANDROID (_this->egl_data->egl_display, dispdata->gpu_fence);
+
+    _this->egl_data->eglDestroySyncKHR(_this->egl_data->egl_display, dispdata->gpu_fence);
+    SDL_assert(dispdata->kms_in_fence_fd != -1);
+
+    /* Lock the buffer that is marked by eglSwapBuffers() to become the
+       next front buffer (so it can not be chosen by EGL as back buffer
+       to draw on), and get a handle to it to request the pageflip on it.
+       REMEMBER that gbm_surface_lock_front_buffer() ALWAYS has to be
+       called after eglSwapBuffers(). */
+    windata->next_bo = KMSDRM_gbm_surface_lock_front_buffer(windata->gs);
+    if (!windata->next_bo) {
+        return SDL_SetError("Failed to lock frontbuffer");
+    }
+    fb = KMSDRM_FBFromBO(_this, windata->next_bo);
+    if (!fb) {
+        return SDL_SetError("Failed to get a new framebuffer from BO");
+    }
+
+    if (!windata->bo) {
+        /* On the first swap, immediately present the new front buffer. Before
+           drmModePageFlip can be used the CRTC has to be configured to use
+           the current connector and mode with drmModeSetCrtc */
+        SDL_VideoData *viddata = _this->internal;
+        const int ret = KMSDRM_drmModeSetCrtc(viddata->drm_fd,
+                                    dispdata->crtc.crtc->crtc_id, fb->fb_id, 0, 0,
+                                    &dispdata->connector.connector->connector_id, 1, &dispdata->mode);
+
+        if (ret) {
+            return SDL_SetError("Could not set videomode on CRTC.");
+        }
+    }
+
+    /* Add the pageflip to the request list. */
+    info.plane = dispdata->display_plane;
+    info.crtc_id = dispdata->crtc.crtc->crtc_id;
+    info.fb_id = fb->fb_id;
+    info.src_w = window->w;  // !!! FIXME: was windata->src_w in the original atomic patch
+    info.src_h = window->h;  // !!! FIXME: was windata->src_h in the original atomic patch
+    info.crtc_w = dispdata->mode.hdisplay;  // !!! FIXME: was windata->output_w in the original atomic patch
+    info.crtc_h = dispdata->mode.vdisplay;  // !!! FIXME: was windata->output_h in the original atomic patch
+    info.crtc_x = 0;  // !!! FIXME: was windata->output_x in the original atomic patch
+
+    drm_atomic_set_plane_props(dispdata, &info);
+
+    /*****************************************************************/
+    /* Tell the display (KMS) that it will have to wait on the fence */
+    /* for the GPU-side FENCE.                                       */
+    /*                                                               */
+    /* Since KMS is a kernel thing, we have to pass an FD into       */
+    /* the kernel, and get another FD out of the kernel.             */
+    /*                                                               */
+    /* 1) To pass the GPU-side fence into the kernel, we set the     */
+    /* INPUT FD as the IN_FENCE_FD prop of the PRIMARY PLANE.        */
+    /* This FD tells KMS (the kernel) to wait for the GPU-side fence.*/
+    /*                                                               */
+    /* 2) To get the KMS-side fence out of the kernel, we set the    */
+    /* OUTPUT FD as the OUT_FEWNCE_FD prop of the CRTC.              */
+    /* This FD will be later imported as a FENCE OBJECT which will be*/
+    /* used to tell the GPU to wait for KMS to complete the changes  */
+    /* requested in atomic_commit (the pageflip in this case).       */
+    /*****************************************************************/
+    if (dispdata->kms_in_fence_fd != -1)
+    {
+        add_plane_property(dispdata->atomic_req, dispdata->display_plane,
+            "IN_FENCE_FD", dispdata->kms_in_fence_fd);
+        add_crtc_property(dispdata->atomic_req, &dispdata->crtc,
+            "OUT_FENCE_PTR", VOID2U64(&dispdata->kms_out_fence_fd));
+    }
+ 
+    /* Do we have a pending modesetting? If so, set the necessary
+       props so it's included in the incoming atomic commit. */
+    if (windata->egl_surface_dirty) {
+        // !!! FIXME: this CreateSurfaces call is what the legacy path does; it's not clear to me if the atomic paths need to do it too.
+        KMSDRM_CreateSurfaces(_this, window);
+
+        uint32_t blob_id;
+        SDL_VideoData *viddata = (SDL_VideoData *)_this->internal;
+
+        add_connector_property(dispdata->atomic_req, &dispdata->connector, "CRTC_ID", dispdata->crtc.crtc->crtc_id);
+        KMSDRM_drmModeCreatePropertyBlob(viddata->drm_fd, &dispdata->mode, sizeof(dispdata->mode), &blob_id);
+        add_crtc_property(dispdata->atomic_req, &dispdata->crtc, "MODE_ID", blob_id);
+        add_crtc_property(dispdata->atomic_req, &dispdata->crtc, "active", 1);
+        modesetting = true;
+    }
+
+    /*****************************************************************/
+    /* Issue a non-blocking atomic commit: for triple buffering,     */
+    /* this must not block so the game can start building another    */
+    /* frame, even if the just-requested pageflip hasnt't completed. */
+    /*****************************************************************/
+    if (drm_atomic_commit(_this, dispdata, false, modesetting)) {
+        return SDL_SetError("Failed to issue atomic commit on pageflip");
+    }
+
+    /* Release the previous front buffer so EGL can chose it as back buffer
+       and render on it again. */
+    if (windata->bo) {
+        KMSDRM_gbm_surface_release_buffer(windata->gs, windata->bo);
+    }
+    /* Take note of the buffer about to become front buffer, so next
+       time we come here we can free it like we just did with the previous
+       front buffer. */
+    windata->bo = windata->next_bo;
+
+    /****************************************************************/
+    /* Import the KMS-side FENCE OUTPUT FD from the kernel to the   */
+    /* KMS-side FENCE OBJECT so we can use use it to fence the GPU. */
+    /****************************************************************/
+    dispdata->kms_fence = create_fence(_this, dispdata->kms_out_fence_fd);
+    SDL_assert(dispdata->kms_fence);
+
+    /****************************************************************/
+    /* "Delete" the fence OUTPUT FD, because we already have the    */
+    /* KMS FENCE OBJECT, the fence itself is away from us, on the   */
+    /* kernel side.                                                 */
+    /****************************************************************/
+    dispdata->kms_out_fence_fd = -1;
+
+    /*****************************************************************/
+    /* Tell the GPU to wait on the fence for the KMS-side FENCE,     */
+    /* which means waiting until the requested pageflip is completed.*/
+    /*****************************************************************/
+    _this->egl_data->eglWaitSyncKHR(_this->egl_data->egl_display, dispdata->kms_fence, 0);
+
+    return true;
+}
+
+static bool KMSDRM_GLES_SwapWindowDoubleBuffered(SDL_VideoDevice *_this, SDL_Window * window)
+{
+    SDL_WindowData *windata = ((SDL_WindowData *) window->internal);
+    SDL_DisplayData *dispdata = SDL_GetDisplayDriverDataForWindow(window);
+    KMSDRM_FBInfo *fb;
+    KMSDRM_PlaneInfo info;
+    bool modesetting = false;
+
+    SDL_zero(info);
+
+    /**********************************************************************************/
+    /* In double-buffer mode, atomic_commit will always be synchronous/blocking (ie:  */
+    /* won't return until the requested changes are really done).                     */
+    /* Also, there's no need to fence KMS or the GPU, because we won't be entering    */
+    /* game loop again (hence not building or executing a new cmdstring) until        */
+    /* pageflip is done, so we don't need to protect the KMS/GPU access to the buffer.*/
+    /**********************************************************************************/
+
+    /* Mark, at EGL level, the buffer that we want to become the new front buffer.
+       It won't really happen until we request a pageflip at the KMS level and it
+       completes. */
+    if (! _this->egl_data->eglSwapBuffers(_this->egl_data->egl_display, windata->egl_surface)) {
+        return SDL_EGL_SetError("Failed to swap EGL buffers", "eglSwapBuffers");
+    }
+    /* Lock the buffer that is marked by eglSwapBuffers() to become the next front buffer
+       (so it can not be chosen by EGL as back buffer to draw on), and get a handle to it,
+       to request the pageflip on it. */
+    windata->next_bo = KMSDRM_gbm_surface_lock_front_buffer(windata->gs);
+    if (!windata->next_bo) {
+        return SDL_SetError("Failed to lock frontbuffer");
+     }
+    fb = KMSDRM_FBFromBO(_this, windata->next_bo);
+    if (!fb) {
+        return SDL_SetError("Failed to get a new framebuffer BO");
+    }
+
+    if (!windata->bo) {
+        /* On the first swap, immediately present the new front buffer. Before
+           drmModePageFlip can be used the CRTC has to be configured to use
+           the current connector and mode with drmModeSetCrtc */
+        SDL_VideoData *viddata = _this->internal;
+        const int ret = KMSDRM_drmModeSetCrtc(viddata->drm_fd,
+                                    dispdata->crtc.crtc->crtc_id, fb->fb_id, 0, 0,
+                                    &dispdata->connector.connector->connector_id, 1, &dispdata->mode);
+
+        if (ret) {
+            return SDL_SetError("Could not set videomode on CRTC.");
+        }
+    }
+
+    /* Add the pageflip to the request list. */
+    info.plane = dispdata->display_plane;
+    info.crtc_id = dispdata->crtc.crtc->crtc_id;
+    info.fb_id = fb->fb_id;
+    info.src_w = window->w;  // !!! FIXME: was windata->src_w in the original atomic patch
+    info.src_h = window->h;  // !!! FIXME: was windata->src_h in the original atomic patch
+    info.crtc_w = dispdata->mode.hdisplay;  // !!! FIXME: was windata->output_w in the original atomic patch
+    info.crtc_h = dispdata->mode.vdisplay;  // !!! FIXME: was windata->output_h in the original atomic patch
+    info.crtc_x = 0;  // !!! FIXME: was windata->output_x in the original atomic patch
+
+    drm_atomic_set_plane_props(dispdata, &info);
+
+    /* Do we have a pending modesetting? If so, set the necessary
+       props so it's included in the incoming atomic commit. */
+    if (windata->egl_surface_dirty) {
+        // !!! FIXME: this CreateSurfaces call is what the legacy path does; it's not clear to me if the atomic paths need to do it too.
+        KMSDRM_CreateSurfaces(_this, window);
+
+        uint32_t blob_id;
+
+        SDL_VideoData *viddata = (SDL_VideoData *)_this->internal;
+
+        add_connector_property(dispdata->atomic_req, &dispdata->connector, "CRTC_ID", dispdata->crtc.crtc->crtc_id);
+        KMSDRM_drmModeCreatePropertyBlob(viddata->drm_fd, &dispdata->mode, sizeof(dispdata->mode), &blob_id);
+        add_crtc_property(dispdata->atomic_req, &dispdata->crtc, "MODE_ID", blob_id);
+        add_crtc_property(dispdata->atomic_req, &dispdata->crtc, "active", 1);
+        modesetting = true;
+    }
+ 
+    /* Issue the one and only atomic commit where all changes will be requested!
+       Blocking for double buffering: won't return until completed. */
+    if (drm_atomic_commit(_this, dispdata, true, modesetting)) {
+        return SDL_SetError("Failed to issue atomic commit on pageflip");
+    }
+
+    /* Release last front buffer so EGL can chose it as back buffer and render on it again. */
+    if (windata->bo) {
+        KMSDRM_gbm_surface_release_buffer(windata->gs, windata->bo);
+    }
+
+    /* Take note of current front buffer, so we can free it next time we come here. */
+    windata->bo = windata->next_bo;
+
+    return true;
+}
+
+static bool KMSDRM_GLES_SwapWindowLegacy(SDL_VideoDevice *_this, SDL_Window *window)
 {
     SDL_WindowData *windata = window->internal;
     SDL_DisplayData *dispdata = SDL_GetDisplayDriverDataForWindow(window);
@@ -116,13 +415,12 @@ bool KMSDRM_GLES_SwapWindow(SDL_VideoDevice *_this, SDL_Window *window)
     // Release the previous front buffer
     if (windata->bo) {
         KMSDRM_gbm_surface_release_buffer(windata->gs, windata->bo);
-        windata->bo = NULL;
     }
 
     windata->bo = windata->next_bo;
 
     /* Mark a buffer to become the next front buffer.
-       This won't happen until pagelip completes. */
+       This won't happen until pageflip completes. */
     if (!(_this->egl_data->eglSwapBuffers(_this->egl_data->egl_display,
                                           windata->egl_surface))) {
         return SDL_SetError("eglSwapBuffers failed");
@@ -147,8 +445,8 @@ bool KMSDRM_GLES_SwapWindow(SDL_VideoDevice *_this, SDL_Window *window)
            drmModePageFlip can be used the CRTC has to be configured to use
            the current connector and mode with drmModeSetCrtc */
         ret = KMSDRM_drmModeSetCrtc(viddata->drm_fd,
-                                    dispdata->crtc->crtc_id, fb_info->fb_id, 0, 0,
-                                    &dispdata->connector->connector_id, 1, &dispdata->mode);
+                                    dispdata->crtc.crtc->crtc_id, fb_info->fb_id, 0, 0,
+                                    &dispdata->connector.connector->connector_id, 1, &dispdata->mode);
 
         if (ret) {
             return SDL_SetError("Could not set videomode on CRTC.");
@@ -170,7 +468,7 @@ bool KMSDRM_GLES_SwapWindow(SDL_VideoDevice *_this, SDL_Window *window)
             flip_flags |= DRM_MODE_PAGE_FLIP_ASYNC;
         }
 
-        ret = KMSDRM_drmModePageFlip(viddata->drm_fd, dispdata->crtc->crtc_id,
+        ret = KMSDRM_drmModePageFlip(viddata->drm_fd, dispdata->crtc.crtc->crtc_id,
                                      fb_info->fb_id, flip_flags, &windata->waiting_for_flip);
 
         if (ret == 0) {
@@ -198,6 +496,26 @@ bool KMSDRM_GLES_SwapWindow(SDL_VideoDevice *_this, SDL_Window *window)
     return true;
 }
 
+bool KMSDRM_GLES_SwapWindow(SDL_VideoDevice *_this, SDL_Window * window)
+{
+    SDL_WindowData *windata = (SDL_WindowData *) window->internal;
+
+    if (windata->swap_window == NULL) {
+        SDL_VideoData *viddata = _this->internal;
+        if (viddata->is_atomic) {
+            // We want the fenced version by default, but it needs extensions.
+            if ( (SDL_GetHintBoolean(SDL_HINT_VIDEO_DOUBLE_BUFFER, false)) || (!SDL_EGL_HasExtension(_this, SDL_EGL_DISPLAY_EXTENSION, "EGL_ANDROID_native_fence_sync")) ) {
+                windata->swap_window = KMSDRM_GLES_SwapWindowDoubleBuffered;
+            } else {
+                windata->swap_window = KMSDRM_GLES_SwapWindowFenced;
+            }
+        } else {
+            windata->swap_window = KMSDRM_GLES_SwapWindowLegacy;
+        }
+    }
+    return windata->swap_window(_this, window);
+}
+
 SDL_EGL_MakeCurrent_impl(KMSDRM)
 
 #en

(Patch may be truncated, please check the link at the top of this post.)