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

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Date:   Tue, 4 Jan 2022 10:28:40 +0000
From:   "Chindris, Mihail" <Mihail.Chindris@...log.com>
To:     Lars-Peter Clausen <lars@...afoo.de>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
CC:     "Hennerich, Michael" <Michael.Hennerich@...log.com>,
        Jonathan Cameron <jic23@...nel.org>,
        "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

> >>> 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.

Indeed, I didn't test it on 32 bits.
But both changes make sense to me.

Regards,
Mihail