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

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Message-ID: <16f84d86-4135-8a19-c5e1-3fbe8db78947@metafoo.de>
Date:   Thu, 23 Dec 2021 17:38:19 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     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 12/23/21 5:34 PM, Muhammad Usama Anjum wrote:
> 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.

What Andy means is that this needs to be DIV_S64_ROUND_CLOSEST() to work 
on 32-bit platforms. But it is clearly unrelated to your change and 
should be in its own patch.