linux-kernel - Re: [PATCH v2 8/8] platform: vivid-cec: use 64-bit arithmetic instead of 32-bit

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Message-ID: <dc931d9d-8cbd-bbd2-0199-b1846e41f274@xs4all.nl>
Date:   Mon, 5 Feb 2018 22:29:41 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: Re: [PATCH v2 8/8] platform: vivid-cec: use 64-bit arithmetic instead
 of 32-bit

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@...eddedor.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);
>