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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Oct 11, 2016 at 04:15:04PM -0700, Stefan Agner wrote:
> 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>
> ---
> 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) {

This still looks somewhat incomplete. Sounds like we should just
reject changes to everything except xoffset/yoffset.

> +		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
> 
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-- 
Ville Syrjälä
Intel OTC
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