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

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Message-ID: <eb59f405-24c6-69b8-beb1-465bfb783f61@collabora.com>
Date:   Thu, 23 Dec 2021 21:34:30 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Andy Shevchenko <andy.shevchenko@...il.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>,
        Collabora Kernel ML <kernel@...labora.com>,
        kernel-janitors <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] drivers:iio:dac make expression evaluation 64-bit

On 12/22/21 11:49 PM, Andy Shevchenko wrote:
> On Wed, Dec 22, 2021 at 5:59 PM Muhammad Usama Anjum
> <usama.anjum@...labora.com> 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.
> 
> ...
> 
>>         dac->ch_data[ch].scale_dec = DIV_ROUND_CLOSEST((s64)rem * 1000000,
>>                                                         65536);
> 
> Shouldn't the above be fixed as well? Has anybody tried to compile on
> 32-bit arch this?
No, it correct already. In this case, rem is being typecasted to s64 and
then multiplied with a 32-bit number, 1000000. Thus 64-bit arithmetic is
being performed here.