qxl: alloc & use shadow for dumb buffers

This patch changes the way the primary surface is used for dumb
framebuffers.  Instead of configuring the bo itself as primary surface
a shadow bo is created and used instead.  Framebuffers can share the
shadow bo in case they have the same format and resolution.

On atomic plane updates we don't have to update the primary surface in
case we pageflip from one framebuffer to another framebuffer which
shares the same shadow.  This in turn avoids the flicker caused by the
primary-destroy + primary-create cycle, which is very annonying when
running wayland on qxl.

The qxl driver never actually writes to the shadow bo.  It sends qxl
blit commands which update it though, and the spice server might
actually execute them (and thereby write to the shadow) in case the
local rendering is kicked for some reason.  This happens for example in
case qemu is asked to write out a dump of the guest display (screendump
monitor command).
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20171019062150.28090-3-kraxel@redhat.com

drivers/gpu/drm/qxl/qxl_cmd.c

@@ -387,7 +387,11 @@ void qxl_io_create_primary(struct qxl_device *qdev,
 	create->width = bo->surf.width;
 	create->height = bo->surf.height;
 	create->stride = bo->surf.stride;
+	if (bo->shadow) {
+		create->mem = qxl_bo_physical_address(qdev, bo->shadow, offset);
+	} else {
 		create->mem = qxl_bo_physical_address(qdev, bo, offset);
+	}
 
 	DRM_DEBUG_DRIVER("mem = %llx, from %p\n", create->mem, bo->kptr);
 

drivers/gpu/drm/qxl/qxl_display.c

@@ -305,7 +305,9 @@ static const struct drm_crtc_funcs qxl_crtc_funcs = {
 void qxl_user_framebuffer_destroy(struct drm_framebuffer *fb)
 {
 	struct qxl_framebuffer *qxl_fb = to_qxl_framebuffer(fb);
+	struct qxl_bo *bo = gem_to_qxl_bo(qxl_fb->obj);
 
+	WARN_ON(bo->shadow);
 	drm_gem_object_unreference_unlocked(qxl_fb->obj);
 	drm_framebuffer_cleanup(fb);
 	kfree(qxl_fb);
@@ -508,6 +510,7 @@ static void qxl_primary_atomic_update(struct drm_plane *plane,
 	    .x2 = qfb->base.width,
 	    .y2 = qfb->base.height
 	};
+	bool same_shadow = false;
 
 	if (old_state->fb) {
 		qfb_old = to_qxl_framebuffer(old_state->fb);
@@ -519,15 +522,23 @@ static void qxl_primary_atomic_update(struct drm_plane *plane,
 	if (bo == bo_old)
 		return;
 
+	if (bo_old && bo_old->shadow && bo->shadow &&
+	    bo_old->shadow == bo->shadow) {
+		same_shadow = true;
+	}
+
 	if (bo_old && bo_old->is_primary) {
+		if (!same_shadow)
 			qxl_io_destroy_primary(qdev);
 		bo_old->is_primary = false;
 	}
 
 	if (!bo->is_primary) {
+		if (!same_shadow)
 			qxl_io_create_primary(qdev, 0, bo);
 		bo->is_primary = true;
 	}
+
 	qxl_draw_dirty_fb(qdev, qfb, bo, 0, 0, &norect, 1, 1);
 }
 
@@ -679,8 +690,9 @@ static void qxl_cursor_atomic_disable(struct drm_plane *plane,
 static int qxl_plane_prepare_fb(struct drm_plane *plane,
 				struct drm_plane_state *new_state)
 {
+	struct qxl_device *qdev = plane->dev->dev_private;
 	struct drm_gem_object *obj;
-	struct qxl_bo *user_bo;
+	struct qxl_bo *user_bo, *old_bo = NULL;
 	int ret;
 
 	if (!new_state->fb)
@@ -689,6 +701,32 @@ static int qxl_plane_prepare_fb(struct drm_plane *plane,
 	obj = to_qxl_framebuffer(new_state->fb)->obj;
 	user_bo = gem_to_qxl_bo(obj);
 
+	if (plane->type == DRM_PLANE_TYPE_PRIMARY &&
+	    user_bo->is_dumb && !user_bo->shadow) {
+		if (plane->state->fb) {
+			obj = to_qxl_framebuffer(plane->state->fb)->obj;
+			old_bo = gem_to_qxl_bo(obj);
+		}
+		if (old_bo && old_bo->shadow &&
+		    user_bo->gem_base.size == old_bo->gem_base.size &&
+		    plane->state->crtc     == new_state->crtc &&
+		    plane->state->crtc_w   == new_state->crtc_w &&
+		    plane->state->crtc_h   == new_state->crtc_h &&
+		    plane->state->src_x    == new_state->src_x &&
+		    plane->state->src_y    == new_state->src_y &&
+		    plane->state->src_w    == new_state->src_w &&
+		    plane->state->src_h    == new_state->src_h &&
+		    plane->state->rotation == new_state->rotation &&
+		    plane->state->zpos     == new_state->zpos) {
+			drm_gem_object_get(&old_bo->shadow->gem_base);
+			user_bo->shadow = old_bo->shadow;
+		} else {
+			qxl_bo_create(qdev, user_bo->gem_base.size,
+				      true, true, QXL_GEM_DOMAIN_VRAM, NULL,
+				      &user_bo->shadow);
+		}
+	}
+
 	ret = qxl_bo_pin(user_bo, QXL_GEM_DOMAIN_CPU, NULL);
 	if (ret)
 		return ret;
@@ -713,6 +751,11 @@ static void qxl_plane_cleanup_fb(struct drm_plane *plane,
 	obj = to_qxl_framebuffer(old_state->fb)->obj;
 	user_bo = gem_to_qxl_bo(obj);
 	qxl_bo_unpin(user_bo);
+
+	if (user_bo->shadow && !user_bo->is_primary) {
+		drm_gem_object_put_unlocked(&user_bo->shadow->gem_base);
+		user_bo->shadow = NULL;
+	}
 }
 
 static const uint32_t qxl_cursor_plane_formats[] = {

drivers/gpu/drm/qxl/qxl_drv.h

@@ -89,6 +89,8 @@ struct qxl_bo {
 	/* Constant after initialization */
 	struct drm_gem_object		gem_base;
 	bool is_primary; /* is this now a primary surface */
+	bool is_dumb;
+	struct qxl_bo *shadow;
 	bool hw_surf_alloc;
 	struct qxl_surface surf;
 	uint32_t surface_id;

drivers/gpu/drm/qxl/qxl_dumb.c

@@ -63,6 +63,7 @@ int qxl_mode_dumb_create(struct drm_file *file_priv,
 					      &handle);
 	if (r)
 		return r;
+	qobj->is_dumb = true;
 	args->pitch = pitch;
 	args->handle = handle;
 	return 0;