linux-kernel - Re: [PATCH v3 3/3] hwmon: Driver for Texas Instruments INA238

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Message-ID: <43c17bba-d4bd-1f9d-5034-1f5a9279d751@roeck-us.net>
Date:   Sun, 31 Oct 2021 20:48:07 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Nathan Rossi <nathan@...hanrossi.com>
Cc:     linux-hwmon@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Nathan Rossi <nathan.rossi@...i.com>,
        Jean Delvare <jdelvare@...e.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v3 3/3] hwmon: Driver for Texas Instruments INA238

On 10/31/21 7:20 PM, Nathan Rossi wrote:
[ ... ]
>>> +
>>> +     if (attr != hwmon_in_max && attr != hwmon_in_min)
>>> +             return -EOPNOTSUPP;
>>> +
>>> +     /* convert decimal to register value */
>>> +     switch (channel) {
>>> +     case 0:
>>> +             /* signed value, clamp to max range +/-163 mV */
>>> +             regval = clamp_val(val, -163, 163);
>>> +             regval = (regval * 1000L * (4 - (int)data->gain + 1)) /
>>
>> nit: The typecast "(int)" is not needed here.
> 
> Due to the unsigned type of gain, it causes promotion of regval (and
> the rest of the numerator) to unsigned long which causes issues with
> negative numbers on the divide. It makes more sense for gain to be an
> int to begin with, I will change it to int to avoid the need for type
> casting.
> 

Are you sure ? I initially thought that as well and wrote a little test
program with that expression, but it didn't do the promotion to unsigned.

Thanks,
Guenter