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

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Message-ID: <7fe18625-3a25-40c8-bfb7-a7a22a3eccff@baylibre.com>
Date: Wed, 30 Apr 2025 10:04:47 -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 9:56 AM, David Lechner wrote:
> 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.

Maybe calling it "conversion delay" would make more sense? Since the phase part
of it is really referring to the application rather than to what we are actually
adjusting.