lkml.org 
[lkml]   [2018]   [Feb]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v2 8/8] platform: vivid-cec: use 64-bit arithmetic instead of 32-bit
From
Date
On 02/05/2018 09:36 PM, Gustavo A. R. Silva wrote:
> Add suffix ULL to constant 10 in order to give the compiler complete
> information about the proper arithmetic to use. Notice that this
> constant is used in a context that expects an expression of type
> u64 (64 bits, unsigned).
>
> The expression len * 10 * CEC_TIM_DATA_BIT_TOTAL is currently being
> evaluated using 32-bit arithmetic.
>
> Also, remove unnecessary parentheses and add a code comment to make it
> clear what is the reason of the code change.
>
> Addresses-Coverity-ID: 1454996
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
> Changes in v2:
> - Update subject and changelog to better reflect the proposed code changes.
> - Add suffix ULL to constant instead of casting a variable.
> - Remove unncessary parentheses.

unncessary -> unnecessary

> - Add code comment.
>
> drivers/media/platform/vivid/vivid-cec.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/platform/vivid/vivid-cec.c b/drivers/media/platform/vivid/vivid-cec.c
> index b55d278..614787b 100644
> --- a/drivers/media/platform/vivid/vivid-cec.c
> +++ b/drivers/media/platform/vivid/vivid-cec.c
> @@ -82,8 +82,15 @@ static void vivid_cec_pin_adap_events(struct cec_adapter *adap, ktime_t ts,
>
> if (adap == NULL)
> return;
> - ts = ktime_sub_us(ts, (CEC_TIM_START_BIT_TOTAL +
> - len * 10 * CEC_TIM_DATA_BIT_TOTAL));
> +
> + /*
> + * Suffix ULL on constant 10 makes the expression
> + * CEC_TIM_START_BIT_TOTAL + 10ULL * len * CEC_TIM_DATA_BIT_TOTAL
> + * be evaluated using 64-bit unsigned arithmetic (u64), which
> + * is what ktime_sub_us expects as second argument.
> + */

That's not really the comment that I was looking for. It still doesn't
explain *why* this is needed at all. How about something like this:

/*
* Add the ULL suffix to the constant 10 to work around a false Coverity
* "Unintentional integer overflow" warning. Coverity isn't smart enough
* to understand that len is always <= 16, so there is no chance of an
* integer overflow.
*/

Regards,

Hans

> + ts = ktime_sub_us(ts, CEC_TIM_START_BIT_TOTAL +
> + 10ULL * len * CEC_TIM_DATA_BIT_TOTAL);
> cec_queue_pin_cec_event(adap, false, ts);
> ts = ktime_add_us(ts, CEC_TIM_START_BIT_LOW);
> cec_queue_pin_cec_event(adap, true, ts);
>

\
 
 \ /
  Last update: 2018-02-05 22:30    [W:0.059 / U:15.292 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site