[U-Boot] [PATCH] timer: Fix possible underflow in udelay

[Troy Kisky]

On 8/25/2016 11:40 AM, Andrew F. Davis wrote:
> When the inputed usec is too large we process it in chunks of
> CONFIG_WD_PERIOD size. Subtracting this from usec until usec is
> zero. If usec is not an integer multiple of CONFIG_WD_PERIOD it
> will underflow and the condition will not become false when it
> should. Fix this logic.
> 
> Signed-off-by: Andrew F. Davis <afd at ti.com>
> ---
>  lib/time.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/time.c b/lib/time.c
> index f37150f..4ec3cb9 100644
> --- a/lib/time.c
> +++ b/lib/time.c
> @@ -145,14 +145,14 @@ void __weak __udelay(unsigned long usec)
>  
>  void udelay(unsigned long usec)
>  {
> -	ulong kv;
> +	ulong kv = usec > CONFIG_WD_PERIOD ? CONFIG_WD_PERIOD : usec;
> +	ulong elapsed = 0;
>  
>  	do {
>  		WATCHDOG_RESET();
> -		kv = usec > CONFIG_WD_PERIOD ? CONFIG_WD_PERIOD : usec;
> -		__udelay (kv);
> -		usec -= kv;
> -	} while(usec);
> +		__udelay(kv);
> +		elapsed += kv;
> +	} while (elapsed < usec);
>  }
>  
>  void mdelay(unsigned long msec)
> 

The original code looks fine to me. Can you give an example of failure ?
ie.
If udelay is passed value of CONFIG_WD_PERIOD+1, the udelay sequence will be

udelay(CONFIG_WD_PERIOD)
udelay(1)

whereas the need code does
udelay(CONFIG_WD_PERIOD)
udelay(CONFIG_WD_PERIOD)