linux-kernel - Re: [PATCH] dt-bindings: iio: afe: voltage-divider: Add io-channel-cells

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Date: Sat, 27 Jan 2024 11:03:14 +0000
From: Conor Dooley <conor@...nel.org>
To: Peter Rosin <peda@...ntia.se>
Cc: Naresh Solanki <naresh.solanki@...ements.com>,
	Jonathan Cameron <jic23@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>, mazziesaccount@...il.com,
	linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dt-bindings: iio: afe: voltage-divider: Add
 io-channel-cells

On Sat, Jan 27, 2024 at 10:40:31AM +0100, Peter Rosin wrote:
> 
> 
> 2024-01-26 at 23:14, Conor Dooley wrote:
> > On Fri, Jan 26, 2024 at 11:10:36PM +0530, Naresh Solanki wrote:
> 
> > I did look at what you have there and I think your dts is wrong.
> > 
> > The iio-hwmon binding says:
> > | description: >
> > |   Bindings for hardware monitoring devices connected to ADC controllers
> > |   supporting the Industrial I/O bindings.
> > | 
> > |   io-channels:
> > |     minItems: 1
> > |     maxItems: 51 # Should be enough
> > |     description: >
> > |       List of phandles to ADC channels to read the monitoring values
> > 
> > And then you have:
> > |	iio-hwmon {
> > |		compatible = "iio-hwmon";
> > |		// Voltage sensors top to down
> > |		io-channels = <&p12v_vd 0>, <&p5v_aux_vd 0>, <&p5v_bmc_aux_vd 0>, <&p3v3_aux_vd 0>,
> > |			<&p3v3_bmc_aux_vd 0>, <&p1v8_bmc_aux_vd 0>, <&adc1 4>, <&adc0 2>, <&adc1 0>,
> > |			<&p2V5_aux_vd 0>, <&p3v3_rtc_vd 0>;
> > |	};
> > |
> > |	p12v_vd: voltage_divider1 {
> > |		compatible = "voltage-divider";
> > |		io-channels = <&adc1 3>;
> > |		#io-channel-cells = <1>;
> > |
> > |		/* Scale the system voltage by 1127/127 to fit the ADC range.
> > |		 * Use small nominator to prevent integer overflow.
> > |		 */
> > |		output-ohms = <15>;
> > |		full-ohms = <133>;
> > |	};
> > 
> > A voltage divider is _not_ an ADC channel, so I don't know why you are
> > treating it as one in the iio-hwmon entry. Can you explain this please?
> 
> This is the exact intent of the voltage divider (and the other bindings
> handled by the iio-rescaler). The raw ADC reports the voltage at its input,
> which is fine, but if there is an analog frontend in front of the ADC
> such as a voltage divider the voltage at the ADC is not the interesting
> property. You are likely to want the "real" voltage before the voltage
> divider to better understand the value.
> 
> In this case it's much more interesting to see values such as 12.050V
> which is presumably close to the nominal voltage (12V? guessing from
> the node name) rather than some unscaled raw ADC voltage (in this
> example it would be ~1.359V, which tells you rather little w/o rescaling
> it first).

Thanks for explaining it. Naresh, can you respin please with an
explanation of why the property belongs in the binding please?

> It's all in the description of the binding...

Obviously it was not sufficiently clear, it's not as if I didn't look at
it...

Cheers,
Conor.

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