[PATCH 1/2] drm: Fix off-by-one in vblank hardware counter wraparound handling

[Daniel Vetter]

On Tue, May 26, 2015 at 05:53:38PM +0900, Michel Dänzer wrote:
> From: Michel Dänzer <michel.daenzer at amd.com>
> 
> dev->max_vblank_count contains the largest value that can be represented
> by the hardware counter. When the hardware counter wraps around, we have
> to add that value + 1 to get the same value as if the hardware counter
> didn't wrap around.
> 
> Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>

Well there's two users of this really, one wants the max (this one here)
and the other a mask. And all the drivers use it as a mask. Maybe rename
it to vblank_counter_mask or similar while at it to prevent further
confusion?
-Daniel

> ---
>  drivers/gpu/drm/drm_irq.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index c8a3447..f9634da 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -100,7 +100,7 @@ static void drm_update_vblank_count(struct drm_device *dev, int crtc)
>  	/*
>  	 * Interrupts were disabled prior to this call, so deal with counter
>  	 * wrap if needed.
> -	 * NOTE!  It's possible we lost a full dev->max_vblank_count events
> +	 * NOTE!  It's possible we lost a full dev->max_vblank_count + 1 events
>  	 * here if the register is small or we had vblank interrupts off for
>  	 * a long time.
>  	 *
> @@ -117,7 +117,7 @@ static void drm_update_vblank_count(struct drm_device *dev, int crtc)
>  	/* Deal with counter wrap */
>  	diff = cur_vblank - vblank->last;
>  	if (cur_vblank < vblank->last) {
> -		diff += dev->max_vblank_count;
> +		diff += dev->max_vblank_count + 1;
>  
>  		DRM_DEBUG("last_vblank[%d]=0x%x, cur_vblank=0x%x => diff=0x%x\n",
>  			  crtc, vblank->last, cur_vblank, diff);
> -- 
> 2.1.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch