linux-kernel - Re: [PATCH 3/4] docs: iio: new docs for ad4052 driver

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Message-ID: <05b83988-b7aa-453a-bef7-8e7eda77f53a@baylibre.com>
Date: Fri, 14 Mar 2025 13:56:32 -0500
From: David Lechner <dlechner@...libre.com>
To: Jorge Marques <gastmaier@...il.com>, Jonathan Cameron <jic23@...nel.org>
Cc: Jorge Marques <jorge.marques@...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>, Jonathan Corbet <corbet@....net>,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 3/4] docs: iio: new docs for ad4052 driver

On 3/14/25 1:13 PM, Jorge Marques wrote:
> On Mon, Mar 10, 2025 at 07:54:16PM +0000, Jonathan Cameron wrote:
>> On Sun, 9 Mar 2025 21:49:24 +0100
>> Jorge Marques <gastmaier@...il.com> wrote:
>>
>>>>> +.. list-table:: Driver attributes
>>>>> +   :header-rows: 1
>>>>> +
>>>>> +   * - Attribute
>>>>> +     - Description
>>>>> +   * - ``in_voltage0_raw``
>>>>> +     - Raw ADC voltage value
>>>>> +   * - ``in_voltage0_oversampling_ratio``
>>>>> +     - Enable the device's burst averaging mode to over sample using
>>>>> +       the internal sample rate.
>>>>> +   * - ``in_voltage0_oversampling_ratio_available``
>>>>> +     - List of available oversampling values. Value 0 disable the burst
>>>>> +       averaging mode.
>>>>> +   * - ``sample_rate``
>>>>> +     - Device internal sample rate used in the burst averaging mode.
>>>>> +   * - ``sample_rate_available``
>>>>> +     - List of available sample rates.  
>>>>
>>>> Why not using the standard sampling_frequency[_available] attributes?  
>>> Because sampling_frequency is the sampling frequency for the pwm trigger
>>> during buffer readings.
>>> sample_rate is the internal device clock used during monitor and burst
>>> averaging modes.
>>
>> For an ABI that is very vague and the two use cases seem to be logically
>> quite different.
>>
>> Seems that for each trigger we have an oversampling ratio controlled number
>> of samples at this rate. It is unusual to be able to control oversampling
>> rate separately from the trigger clock, hence the lack of ABI.  If
>> we add something new for this it should something relating to oversampling.
>> oversampling_frequency perhaps.
>>
>> For monitor mode, it is tied to the sampling frequency for most devices.
>> But there are exceptions.  E.g. the max1363. Trick is to make it an event
>> ABI property and hence under events/ rather than in the root directory.
>>
>> In this case you'll have to store two values and write the appropriate
>> one into the register to suit a given operating mode.
>>
> 
> If doing buffer captures with oversampling enabled, both sampling
> frequencies have an impact:
> 
> e.g.,
> oversampling: 4
> sample_rate: 2MHz
> PWM sampling frequency: 500KHz
> 
> PWM trigger out (CNV)   |       |       |       |       |
> ADC conversion          ++++    ++++    ++++    ++++    ++++
> ADC data ready  (GP)       *       *       *       *       *
> 
> For monitor mode, it will constantly be doing conversion to check for
> threshold crossings, at the defined sample_rate.
> 
> I like the idea of having the device's sample_rate as
> conversion_frequency.

In addition to what makes sense for this chip, we should also consider what
makes sense other chips with similar features. For example, I am working on
ad7606c which has control for the oversampling burst frequency (frequency of
"+" in the diagram above). So it would make sense to have a standard attribute
that would work for both chips.

On ad4052, just because we have a single register that controls two different
functions doesn't mean we have to be limited to a single attribute that controls
that register.

So I would create the events/sampling_frequency{,_available} attributes like
Jonathan suggested for controlling the sampling frequency in monitor mode and
introduce new oversampling_burst_frequency{,_available} attributes for
controlling the conversion frequency when oversampling. When an attribute is
written, we can cache the requested value in the state struct instead of
writing it directly to the register on the ADC if we want the attributes to be
independent. Then only write the register when we enable monitor mode or when
we start reading samples with oversampling enabled.

Sure, it is more work to implement it in the driver this way, but that shouldn't
be an an excuse to do things in a way that isn't compatible with other ADCs.