drm/fb-helper: reject any changes to the fbdev

The current fbdev emulation does not allow to push back changes in
width, height or depth to KMS, hence reject any changes with an
error. This makes sure that fbdev ioctl's fail properly and user
space does not assume that changes succeeded.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161011231504.17688-1-stefan@agner.ch

drivers/gpu/drm/drm_fb_helper.c

@@ -1209,11 +1209,14 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
 	if (var->pixclock != 0 || in_dbg_master())
 		return -EINVAL;
 
-	/* Need to resize the fb object !!! */
-	if (var->bits_per_pixel > fb->bits_per_pixel ||
-	    var->xres > fb->width || var->yres > fb->height ||
-	    var->xres_virtual > fb->width || var->yres_virtual > fb->height) {
-		DRM_DEBUG("fb userspace requested width/height/bpp is greater than current fb "
+	/*
+	 * Changes struct fb_var_screeninfo are currently not pushed back
+	 * to KMS, hence fail if different settings are requested.
+	 */
+	if (var->bits_per_pixel != fb->bits_per_pixel ||
+	    var->xres != fb->width || var->yres != fb->height ||
+	    var->xres_virtual != fb->width || var->yres_virtual != fb->height) {
+		DRM_DEBUG("fb userspace requested width/height/bpp different than current fb "
 			  "request %dx%d-%d (virtual %dx%d) > %dx%d-%d\n",
 			  var->xres, var->yres, var->bits_per_pixel,
 			  var->xres_virtual, var->yres_virtual,