linux-kernel - Re: [PATCH] media: qcom: use div64_u64() instead of do

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Date:   Thu, 10 Feb 2022 13:16:46 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Qing Wang <wangqing@...o.com>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: qcom: use div64_u64() instead of do_div()

On 09/02/2022 11:38, Qing Wang wrote:
> From: Wang Qing <wangqing@...o.com>
> 
> do_div() does a 64-by-32 division.
> When the divisor is u64, do_div() truncates it to 32 bits, this means it
> can test non-zero and be truncated to zero for division.
> 
> fix do_div.cocci warning:
> do_div() does a 64-by-32 division, please consider using div64_u64 instead.


NAK

The div64_u64 is not equivalent to do_div. It returns the quotient
rather than modifying the first arg. Moreover it is unoptimal on 32-bit
arches.

> 
> Signed-off-by: Wang Qing <wangqing@...o.com>
> ---
>   drivers/media/platform/qcom/venus/vdec.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/qcom/venus/vdec.c b/drivers/media/platform/qcom/venus/vdec.c
> index 91da3f5..941a904
> --- a/drivers/media/platform/qcom/venus/vdec.c
> +++ b/drivers/media/platform/qcom/venus/vdec.c
> @@ -435,7 +435,7 @@ static int vdec_s_parm(struct file *file, void *fh, struct v4l2_streamparm *a)
>   		return -EINVAL;
>   
>   	fps = (u64)USEC_PER_SEC;
> -	do_div(fps, us_per_frame);
> +	div64_u64(fps, us_per_frame);
>   
>   	inst->fps = fps;
>   	inst->timeperframe = *timeperframe;


-- 
With best wishes
Dmitry