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From:   Robin Getz <rgetz@mathworks.com>
To:     Jonathan Cameron <jic23@kernel.org>,
        "marius.cristea@microchip.com" <marius.cristea@microchip.com>
CC:     "lars@metafoo.de" <lars@metafoo.de>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "krzysztof.kozlowski+dt@linaro.org" 
        <krzysztof.kozlowski+dt@linaro.org>,
        "conor+dt@kernel.org" <conor+dt@kernel.org>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2 2/2] iio: adc: adding support for pac193x
Thread-Topic: [PATCH v2 2/2] iio: adc: adding support for pac193x
Thread-Index: AQHaB0mtGa1Cq1s1TU+Mw+PCEyAktLBdwweAgAlbMjA=
Date:   Thu, 2 Nov 2023 14:28:36 +0000
Message-ID: <BYAPR05MB4424FBC5CFF90F55198C4351A9A6A@BYAPR05MB4424.namprd05.prod.outlook.com>
References: <20231025134404.131485-1-marius.cristea@microchip.com>
        <20231025134404.131485-3-marius.cristea@microchip.com>
 <20231027161813.7bff8866@jic23-huawei>
In-Reply-To: <20231027161813.7bff8866@jic23-huawei>
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On Friday, October 27, 2023 11:18 AM=20
Jonathan Cameron <jic23@kernel.org> wrote:

>On Wed, 25 Oct 2023 16:44:04 +0300
><mailto:marius.cristea@microchip.com> wrote:
>
>> From: Marius Cristea <mailto:marius.cristea@microchip.com>
>>=20
>> This is the iio driver for Microchip
>> PAC193X series of Power Monitor with Accumulator chip family.
>> diff --git a/Documentation/ABI/testing/sysfs-bus-iio-adc-pac1934 b/Docum=
entation/ABI/testing/sysfs-bus-iio-adc-pac1934
>> new file mode 100644
>> index 000000000000..ea43df292b9c
>> --- /dev/null
>> +++ b/Documentation/ABI/testing/sysfs-bus-iio-adc-pac1934
>> @@ -0,0 +1,15 @@
>> +What: /sys/bus/iio/devices/iio:deviceX/in_shunt_resistor_X
>> +KernelVersion: 6.6
>> +Contact: mailto:linux-iio@vger.kernel.org
>> +Description:
>> + The value of the shunt resistor may be known only at runtime and set
>> + by a client application.

What? End users (people with access to userspace) don't whip out their sold=
ering iron to add/change shunt resistors.

OEMs do this during PCB design. This is fixed per board in the wild (apart =
from device evaluation boards)
and is typically managed via device tree. (Allowing it to be changed by OEM=
s, but not end users).

>> This attribute allows to set its value
>> + in micro-ohms. X is the IIO index of the device. The value is
>> + used to calculate current, power and accumulated energy.
>
>How common is it that this isn't known?

I would say zero.

The ADM1177 (which is different, but also requires a shunt resistor),=20
hwmon driver:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/dri=
vers/hwmon/adm1177.c
iio driver:
https://github.com/analogdevicesinc/linux/blob/master/drivers/iio/adc/adm11=
77.c

uses device tree to manage the value of the shunt resistor.

> I'm not sure we've found it necessary to
> support userspace control of this for any other device and there are quit=
e a few
> where this could in theory be known only at runtime...

Not run time, but at PCB manufacturing time, when the device tree is compil=
ed by the OEM.=20
The assumption is - who ever controls the BOM - controls the device tree. T=
his has been pretty true in my history.

It allows OEMs (who manage the hardware) to change it as they want, but kee=
ps the complexity away from end users.

- Robin