linux-kernel - Re: [PATCH 1/5] Documentation: ABI: IIO: add calibphase

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Message-ID: <896023ae-c279-4201-a7a8-dfd9b33fe0e5@baylibre.com>
Date: Wed, 30 Apr 2025 09:56:42 -0500
From: David Lechner <dlechner@...libre.com>
To: Andy Shevchenko <andy@...nel.org>
Cc: Nuno Sá <noname.nuno@...il.com>,
 Angelo Dureghello <adureghello@...libre.com>,
 Jonathan Cameron <jic23@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>, Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 1/5] Documentation: ABI: IIO: add calibphase_delay
 documentation

On 4/30/25 9:45 AM, Andy Shevchenko wrote:
> On Wed, Apr 30, 2025 at 09:21:28AM -0500, David Lechner wrote:
>> On 4/30/25 12:40 AM, Nuno Sá wrote:
>>> On Tue, 2025-04-29 at 15:06 +0200, Angelo Dureghello wrote:
>>>> From: Angelo Dureghello <adureghello@...libre.com>
>>>>
>>>> Add new IIO calibphase_delay documentation.
>>>>
>>>> The delay suffix is added to specify that the phase, generally in
>>>> radiants, is for this case (needed from ad7606) in nanoseconds.
> 
> ...
> 
>>>> +What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_calibphase_delay
>>>
>>> Not sure if I'm too convinced on the _delay suffix
>>>
>> Phase is measured in radians, not seconds, so it seems wrong to use it here.
>>
>> https://en.wikipedia.org/wiki/Phase_(waves)
>>
>> And the delay here is with respect to individual samples in a simultaneous
>> conversion without regard for a sampling frequency, so I don't see how we could
>> convert the time to radians in any meaningful way.
> 
> And how this delay is aplicable to the phase in the hardware? Sounds to me that
> HW has some meaningful way of such a conversion?
> 

It is a calibration to account for a phase difference between two input signals.
This is a simultaneous sampling ADC, so all channels normally sample at exactly
the same time. This phase delay calibration factor can introduce a small delay
on an individual channel so that it starts it's conversion some microseconds
after the others.

There is a nice diagram here:

https://www.analog.com/media/en/technical-documentation/data-sheets/ad7606c-18.pdf#%5B%7B%22num%22%3A113%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C34%2C594%2C0%5D

To convert the phase delay to a phase angle and back would require also knowing
the frequency of the input voltage signals.