linux-kernel - Re: [PATCH v2 3/8] Documentation: iio: ad4030: Add double PWM SPI offload doc

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Message-ID: <62989891-e166-419b-b6ab-cf1eca781b32@baylibre.com>
Date: Thu, 18 Sep 2025 14:50:45 -0500
From: David Lechner <dlechner@...libre.com>
To: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: jic23@...nel.org, michael.hennerich@...log.com, nuno.sa@...log.com,
 eblanc@...libre.com, andy@...nel.org, robh@...nel.org, krzk+dt@...nel.org,
 conor+dt@...nel.org, corbet@....net, marcelo.schmitt1@...il.com
Subject: Re: [PATCH v2 3/8] Documentation: iio: ad4030: Add double PWM SPI
 offload doc

Could use docs: instead of Documentation: in the subject to
make it a bit shorter. Seems common enough.

On 9/18/25 12:38 PM, Marcelo Schmitt wrote:
> Document double PWM setup SPI offload wiring schema.
> 
> Signed-off-by: Marcelo Schmitt <marcelo.schmitt@...log.com>
> ---
> Change log v1 -> v2
> - Swapped PWM numbering.
> - Expanded double PWM description and capture zone description.
> 
>  Documentation/iio/ad4030.rst | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/Documentation/iio/ad4030.rst b/Documentation/iio/ad4030.rst
> index b57424b650a8..9501d3fee9bb 100644
> --- a/Documentation/iio/ad4030.rst
> +++ b/Documentation/iio/ad4030.rst
> @@ -92,6 +92,41 @@ Interleaved mode
>  In this mode, both channels conversion results are bit interleaved one SDO line.
>  As such the wiring is the same as `One lane mode`_.
>  
> +SPI offload wiring
> +^^^^^^^^^^^^^^^^^^
> +
> +.. code-block::
> +
> +    +-------------+         +-------------+
> +    |         CNV |<-----+--| GPIO        |
> +    |             |      +--| PWM0        |
> +    |             |         |             |
> +    |             |      +--| PWM1        |
> +    |             |      |  +-------------+
> +    |             |      +->| TRIGGER     |
> +    |          CS |<--------| CS          |
> +    |             |         |             |
> +    |     ADC     |         |     SPI     |
> +    |             |         |             |
> +    |         SDI |<--------| SDO         |
> +    |         SDO |-------->| SDI         |
> +    |        SCLK |<--------| SCLK        |
> +    +-------------+         +-------------+
> +
> +In this mode, both the ``cnv-gpios`` and a ``pwms`` properties are required.
> +The ``pwms`` property specifies the PWM that is connected to the ADC CNV pin.
> +The SPI offload will have a ``trigger-sources`` property to indicate the SPI
> +offload (PWM) trigger source. For AD4030 and similar ADCs, there are two
> +possible data transfer zones for sample N. One of them (zone 1) starts after the
> +data conversion for sample N is complete while the other one (zone 2) starts 9.8
> +nanoseconds after the rising edge of CNV for sample N + 1.
> +
> +The configuration depicted in the above ASCII art is intended to perform data

Could say "diagram" instead of "ASCII art" if you want to be more formal.

> +transfer in zone 2. To achieve high sample rates while meeting ADC timing
> +requirements, an offset is added between the rising edges of PWM0 and PWM1 to
> +delay the SPI transfer until 9.8 nanoseconds after CNV rising edge. This
> +requires a specialized PWM controller that can provide such an offset.
> +

Could add a link to the HDL project as an example of such hardware.

>  SPI Clock mode
>  --------------
>  

Good enough as it is.

Reviewed-by: David Lechner <dlechner@...libre.com>