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Date:   Thu, 27 Jul 2023 17:28:42 +0200
From:   Carsten =?UTF-8?B?U3BpZcOf?= <mail@carsten-spiess.de>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Jean Delvare <jdelvare@suse.com>, Jonathan Corbet <corbet@lwn.net>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Magnus Damm <magnus.damm@gmail.com>,
        linux-hwmon@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-renesas-soc@vger.kernel.org
Subject: Re: [PATCH 1/2] hwmon: (isl28022) new driver for ISL28022 power
 monitor
Message-ID: <20230727172842.3fe20026.mail@carsten-spiess.de>
In-Reply-To: <bbf1aba4-48ce-289d-aaa9-bc861effaffd@roeck-us.net>
References: <20230726152235.249569-1-mail@carsten-spiess.de>
 <20230726152235.249569-2-mail@carsten-spiess.de>
 <bbf1aba4-48ce-289d-aaa9-bc861effaffd@roeck-us.net>
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> On 7/26/23 18:19, Guenter Roeck wrote:
> I strongly suspect that the conversions will result in overflows and that=
 they are not always correct.
> I'll want to write unit test code before accepting the driver.
I changed the conversion for current to=20
				*val =3D ((long)regval * 1250L * (long)data->gain) /
					(long)data->shunt;

The term (1250 * gain) will be 10000 max, fits to 14 bit.
So no risk for 32bit overflow when multiply with 16bit value.

And changed the conversion for power to=20
				*val =3D ((51200000L * ((long)data->gain)) /
					(long)data->shunt) * (long)regval;

The first term (51200000 * gain / shunt) will be larger than 16bit when=20
(shunt/gain) is less than ~800. So min values for shunt are=20
- 6400 =C2=B5Ohm for 320 mV range
- 3200 =C2=B5Ohm for 160 mV range
- 1600 =C2=B5Ohm for 80 mV range
- 800 =C2=B5Ohm for 40 mV range
Can i set this conditionally in the .yaml file?

Regards Carsten

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