linux-kernel - Re: [PATCH] hwmon: (ntc_thermistor) Add min and max temperature attributes

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Message-ID:
 <PA4PR04MB7630FFDEE9AF8E1458DBA6B0C5C82@PA4PR04MB7630.eurprd04.prod.outlook.com>
Date: Tue, 4 Mar 2025 11:33:23 +0000
From: Maud Spierings | GOcontroll <maudspierings@...ontroll.com>
To: Guenter Roeck <linux@...ck-us.net>, Jean Delvare <jdelvare@...e.com>
CC: "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hwmon: (ntc_thermistor) Add min and max temperature
 attributes

From: Guenter Roeck <groeck7@...il.com> on behalf of Guenter Roeck <linux@...ck-us.net>
Sent: Tuesday, March 4, 2025 12:12 PM

>On 3/4/25 00:24, Maud Spierings via B4 Relay wrote:
>> From: Maud Spierings <maudspierings@...ontroll.com>
>>
>> Add the min and max temperature attributes as it is trivial for this
>> driver.
>>
>> This can help with detecting implausible readings and indicates to users
>> which range they can actually measure, so they will not set a trip point
>> at a temperature higher than max or lower than min.
>>
>Unless I misunderstand the driver code, readings outside the table values
>are never reported. Also, min/max are supposed to be alarm temperatures.
>The reported values for min/max would be between -55 and +155 degrees C,
>which does not make sense and has zero value for trip point usage.

Regarding the driver not reporting values outside the table values:

That does seem to be true and is good in my opinion, however currently
125 can mean 125 or something higher, with an indication of a max
measurable temperature it can be determined that this is a max value and
thus might have extra considerations.

Regarding the meaning of attribues:

It is difficult that the attributes do not have descriptions in
include/linux/hwmon.h

Is there an attribute that should be used to indicate this maximum 
measurable value to userspace? HWMON_T_HIGHEST/LOWEST?
HWMON_T_RATED_MIN/MAX?

Some extra ramblings:

I want to have some indication of what the lowest and highest
temperatures that the sensor can measure are. Imagine I set my trip point
at 140 degrees, but the sensor can only measure up to 125, I would like
there to be some feedback that this trip point can never be measured.

Some kind of plausibility check may also be interesting. For example I
have an ntc in an lvds display, if this display is disconnected it shuts
down because the ADC reads zero, which means temp==temp_max.

>NACK.
>
>Guenter

Kind regards,
Maud