[PATCH 12/13] drm/i2c/tda998x: Fix signed overflow issue

[Jean-Francois Moine]

On Sat Apr 5 02:45:01 PDT 2014,
Daniel Vetter <daniel.vetter at ffwll.ch> 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.
        [snip]
>  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++;

This function may be simplified by:

--- tda998x_drv.c~
+++ tda998x_drv.c
@@ -568,11 +568,11 @@

 static uint8_t tda998x_cksum(uint8_t *buf, size_t bytes)
 {
-       uint8_t sum = 0;
+       int sum = 0;

        while (bytes--)
-               sum += *buf++;
-       return (255 - sum) + 1;
+               sum -= *buf++;
+       return sum;
 }

 #define HB(x) (x)


and the same may be done in hdmi.c:

diff --git a/drivers/video/hdmi.c b/drivers/video/hdmi.c
index 9e758a8..b6c9030 100644
--- a/drivers/video/hdmi.c
+++ b/drivers/video/hdmi.c
@@ -31,14 +31,14 @@
 static void hdmi_infoframe_checksum(void *buffer, size_t size)
 {
        u8 *ptr = buffer;
-       u8 csum = 0;
+       int csum = 0;
        size_t i;

        /* compute checksum */
        for (i = 0; i < size; i++)
-               csum += ptr[i];
+               csum -= ptr[i];

-       ptr[3] = 256 - csum;
+       ptr[3] = csum;
 }

 /**


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