From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Romanick <idr@freedesktop.org>
Subject: Re: [PATCH 12/13] drm/i2c/tda998x: Fix signed overflow issue
Date: Mon, 07 Apr 2014 08:31:06 -0700
Message-ID: <5342C4BA.3020606@freedesktop.org>
References: <1396691102-22904-1-git-send-email-daniel.vetter@ffwll.ch>
 <1396691102-22904-14-git-send-email-daniel.vetter@ffwll.ch>
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To: Daniel Vetter <daniel.vetter@ffwll.ch>, DRI Development <dri-devel@lists.freedesktop.org>
Cc: Dave Jones <davej@redhat.com>, Russell King <rmk+kernel@arm.linux.org.uk>
List-Id: dri-devel@lists.freedesktop.org

On 04/05/2014 02:45 AM, Daniel Vetter wrote:
> This is C standard hair-splitting, but afaict
> - sum will be promoted to signed int in computation since
>   uint8_t fits
> - signed overflow is undefined.
> 
> No we need to add up an awful lot of bytes to actually make it
  ^^
Now

> overflow. But I guess the real risk is gcc spotting this and going
> bananas. Fix this by simply using unsigned in to force all computations
> to use the well-defined unsigned behaviour.

Seems reasonable... it also seems impossible (ha!) to break anything.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

> Spotted by coverity.
> 
> Cc: Russell King <rmk+kernel@arm.linux.org.uk>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: Jean-Francois Moine <moinejf@free.fr>
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/i2c/tda998x_drv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i2c/tda998x_drv.c b/drivers/gpu/drm/i2c/tda998x_drv.c
> index 48af5cac1902..ae2754760d77 100644
> --- a/drivers/gpu/drm/i2c/tda998x_drv.c
> +++ b/drivers/gpu/drm/i2c/tda998x_drv.c
> @@ -568,7 +568,7 @@ static irqreturn_t tda998x_irq_thread(int irq, void *data)
>  
>  static uint8_t tda998x_cksum(uint8_t *buf, size_t bytes)
>  {
> -	uint8_t sum = 0;
> +	unsigned sum = 0;
>  
>  	while (bytes--)
>  		sum += *buf++;
>