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

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Message-ID: <0c7e4efc-7c2a-46ac-b970-4134c386bc71@baylibre.com>
Date: Thu, 1 May 2025 09:44:51 -0500
From: David Lechner <dlechner@...libre.com>
To: Angelo Dureghello <adureghello@...libre.com>
Cc: Andy Shevchenko <andy@...nel.org>, Nuno Sá
 <noname.nuno@...il.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 5/1/25 7:33 AM, Angelo Dureghello wrote:
> On 30.04.2025 10:04, David Lechner wrote:
>> 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.
> 
> Are there examples of a phase calibration in iio ? Becouse apply a radians 
> calibration seems complicated and maybe non approrpiate for non-periodic 
> signals as often used in real world applications.
> 
> So another viable idea could be to use a IIO_CHAN_INFO_CALIBDELAY instead.
> 
> Regards,
> angelo

I was looking at the datasheet on another ADC that popped up on the mailing list
today. https://www.ti.com/product/ADS1262

It has a "conversion delay" register that does basically the same thing. So I'm
liking that name even more now. Just calling it "delay" seems a bit too vague.
We could make it IIO_CHAN_INFO_CALIBCONV_DELAY to try to keep it shorter.