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SubjectRe: [PATCH v2 8/8] platform: vivid-cec: use 64-bit arithmetic instead of 32-bit
On Mon 2018-02-05 22:29:41, Hans Verkuil wrote:
> 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.
> */

Or maybe it would be better to add comment about Coverity having
false-positive and not to modify the code?

Hmm. Could we do something like BUG_ON(len > 16) to make Coverity
understand the ranges?

Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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