linux-kernel - Re: [PATCH] drivers:iio:dac make expression evaluation 64-bit

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Message-ID: <20220105133902.GD7674@kadam>
Date:   Wed, 5 Jan 2022 16:39:02 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Mihail Chindris <mihail.chindris@...log.com>,
        "open list:IIO SUBSYSTEM AND DRIVERS" <linux-iio@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>, kernel@...labora.com,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drivers:iio:dac make expression evaluation 64-bit

On Wed, Dec 22, 2021 at 12:20:32AM +0500, Muhammad Usama Anjum wrote:
> Two 32-bit values are being evaluated using 32-bit arithmetic and then
> passed to s64 type. It is wrong. Expression should be evaluated using
> 64-bit arithmetic and then passed.
> 
> Fixes: 8f2b54824b ("drivers:iio:dac: Add AD3552R driver support")
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
>  drivers/iio/dac/ad3552r.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/dac/ad3552r.c b/drivers/iio/dac/ad3552r.c
> index 97f13c0b9631..b03d3c7cd4c4 100644
> --- a/drivers/iio/dac/ad3552r.c
> +++ b/drivers/iio/dac/ad3552r.c
> @@ -770,7 +770,7 @@ static void ad3552r_calc_gain_and_offset(struct ad3552r_desc *dac, s32 ch)
>  	dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem * 1000000,
>  							65536);
>  
> -	dac->ch_data[ch].offset_int = div_s64_rem(v_min * 65536, span, &rem);
> +	dac->ch_data[ch].offset_int = div_s64_rem(v_min * 65536L, span, &rem);

"v_min" is relatively close to zero on a number line so this can't
overflow.  There is no way that this change affects anything at runtime
(except making the code a tiny tiny bit slower).

And it should be 65536LL for 32 bit systems?

But I just don't see the point of this change.  Presumably it is to make
a static analyzer happy?

regards,
dan carpenter