drm/fb-helper: Instanciate shadow FB if configured in device's mode_config

Generic framebuffer emulation uses a shadow buffer for framebuffers with
dirty() function. If drivers want to use the shadow FB without such a
function, they can now set prefer_shadow or prefer_shadow_fbdev in their
mode_config structures. The former flag is exported to userspace, the
latter flag is fbdev-only.

v3:
	* only schedule dirty worker if fbdev uses shadow fb
	* test shadow fb settings with boolean operators
	* use bool for struct drm_mode_config.prefer_shadow_fbdev
	* fix documentation comments
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/315834/

drivers/gpu/drm/drm_fb_helper.c

@@ -421,7 +421,9 @@ static void drm_fb_helper_dirty_work(struct work_struct *work)
 				return;
 			drm_fb_helper_dirty_blit_real(helper, &clip_copy);
 		}
-		helper->fb->funcs->dirty(helper->fb, NULL, 0, 0, &clip_copy, 1);
+		if (helper->fb->funcs->dirty)
+			helper->fb->funcs->dirty(helper->fb, NULL, 0, 0,
+						 &clip_copy, 1);
 
 		if (helper->buffer)
 			drm_client_buffer_vunmap(helper->buffer);
@@ -613,6 +615,16 @@ void drm_fb_helper_unlink_fbi(struct drm_fb_helper *fb_helper)
 }
 EXPORT_SYMBOL(drm_fb_helper_unlink_fbi);
 
+static bool drm_fbdev_use_shadow_fb(struct drm_fb_helper *fb_helper)
+{
+	struct drm_device *dev = fb_helper->dev;
+	struct drm_framebuffer *fb = fb_helper->fb;
+
+	return dev->mode_config.prefer_shadow_fbdev ||
+	       dev->mode_config.prefer_shadow ||
+	       fb->funcs->dirty;
+}
+
 static void drm_fb_helper_dirty(struct fb_info *info, u32 x, u32 y,
 				u32 width, u32 height)
 {
@@ -620,7 +632,7 @@ static void drm_fb_helper_dirty(struct fb_info *info, u32 x, u32 y,
 	struct drm_clip_rect *clip = &helper->dirty_clip;
 	unsigned long flags;
 
-	if (!helper->fb->funcs->dirty)
+	if (!drm_fbdev_use_shadow_fb(helper))
 		return;
 
 	spin_lock_irqsave(&helper->dirty_lock, flags);
@@ -2213,7 +2225,7 @@ int drm_fb_helper_generic_probe(struct drm_fb_helper *fb_helper,
 
 	drm_fb_helper_fill_info(fbi, fb_helper, sizes);
 
-	if (fb->funcs->dirty) {
+	if (drm_fbdev_use_shadow_fb(fb_helper)) {
 		struct fb_ops *fbops;
 		void *shadow;
 

include/drm/drm_mode_config.h

@@ -852,6 +852,13 @@ struct drm_mode_config {
 	/* dumb ioctl parameters */
 	uint32_t preferred_depth, prefer_shadow;
 
+	/**
+	 * @prefer_shadow_fbdev:
+	 *
+	 * Hint to framebuffer emulation to prefer shadow-fb rendering.
+	 */
+	bool prefer_shadow_fbdev;
+
 	/**
 	 * @quirk_addfb_prefer_xbgr_30bpp:
 	 *