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

[Stefan Agner]

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 at agner.ch>
---
This rejects reconfiguration of framebuffer like
fbset -rgba 5,6,5 -depth 16 (when in 24 bit mode by default)
fbset -xres 123

I think all users of drm_fb_helper_check_var use also the generic
drm_fb_helper_set_par (or do otherwise not support changing size/
depth). Hence, afaict, the change should be the right thing to do
for all driver...

 drivers/gpu/drm/drm_fb_helper.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 03414bd..596c056 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1211,11 +1211,14 @@ 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,
-- 
2.10.0