Re: [PATCH] drm/fb-helper: Only reject FB changes if FB_MISC_USER_EVENT is set

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: "Michel Dänzer" <michel@daenzer.net>
Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm/fb-helper: Only reject FB changes if FB_MISC_USER_EVENT is set
Date: Thu, 16 Mar 2017 12:09:00 +0200	[thread overview]
Message-ID: <20170316100900.GC31595@intel.com> (raw)
In-Reply-To: <20170316095553.1586-1-michel@daenzer.net>

On Thu, Mar 16, 2017 at 06:55:53PM +0900, Michel Dänzer wrote:
> From: Michel Dänzer <michel.daenzer@amd.com>
> 
> Otherwise this can also prevent modesets e.g. for switching VTs.
> 
> FB_MISC_USER_EVENT is set when the request originates from userspace,
> which is what we're interested in here according to the DRM_DEBUG
> output.

So why is the kernel allowed to violate this?

The checks look somewhat bogus to me anyway. The 'virtual size == fb size'
check makes some kind of sense. Although I don't see why the virtual
resolution couldn't be smaller than the fb size. But requiring that the
visible resolutionn matches the fb size as well definitely seems wrong.

Maybe Cc whoever broke this?

> 
> Bugzilla: https://bugs.freedesktop.org/99841
> Fixes: 865afb11949e ("drm/fb-helper: reject any changes to the fbdev")
> Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
> ---
> 
> I'm not entirely sure why the values can not match for a VT switch. If
> anybody thinks this just papers over the real issue, please speak up.
> 
>  drivers/gpu/drm/drm_fb_helper.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index f6d4d9700734..9663f3b8f287 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1259,9 +1259,10 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
>  	 * 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->format->cpp[0] * 8 ||
> -	    var->xres != fb->width || var->yres != fb->height ||
> -	    var->xres_virtual != fb->width || var->yres_virtual != fb->height) {
> +	if ((info->flags & FBINFO_MISC_USEREVENT) &&
> +	    (var->bits_per_pixel != fb->format->cpp[0] * 8 ||
> +	     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,
> -- 
> 2.11.0
> 
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-- 
Ville Syrjälä
Intel OTC