[PATCH] drm: Don't pass negative delta to ktime_sub_ns()

[Imre Deak]

On Wed, 2013-06-12 at 11:58 +0200, Michel Dänzer wrote:
> From: Michel Dänzer <michel.daenzer at amd.com>
> 
> It takes an unsigned value. This happens not to blow up on 64-bit
> architectures, but it does on 32-bit, causing
> drm_calc_vbltimestamp_from_scanoutpos() to calculate totally bogus
> timestamps for vblank events. Which in turn causes e.g. gnome-shell to
> hang after a DPMS off cycle with current xf86-video-ati Git.
> 
> Cc: stable at vger.kernel.org
> Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>

Yes, I introduced this regression in:

drm: use monotonic time in drm_calc_vbltimestamp_from_scanoutpos

The fix seems to be ok, perhaps you should also mention the regressing
commit in the commit message.

Reviewed-by: Imre Deak <imre.deak at intel.com>

> ---
>  drivers/gpu/drm/drm_irq.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index a6a8643..39665f8 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -708,7 +708,10 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
>  	/* Subtract time delta from raw timestamp to get final
>  	 * vblank_time timestamp for end of vblank.
>  	 */
> -	etime = ktime_sub_ns(etime, delta_ns);
> +	if (delta_ns < 0)
> +		etime = ktime_add_ns(etime, -delta_ns);
> +	else
> +		etime = ktime_sub_ns(etime, delta_ns);
>  	*vblank_time = ktime_to_timeval(etime);
>  
>  	DRM_DEBUG("crtc %d : v %d p(%d,%d)@ %ld.%ld -> %ld.%ld [e %d us, %d rep]\n",