linux-kernel - [PATCH v2 5/5] Documentation: iio: ad4000: Describe offload support

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Message-ID: <888928d2224f5dad03a86b7108217f7b4b078979.1742394806.git.marcelo.schmitt@analog.com>
Date: Wed, 19 Mar 2025 11:58:14 -0300
From: Marcelo Schmitt <marcelo.schmitt@...log.com>
To: <linux-iio@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC: <jic23@...nel.org>, <lars@...afoo.de>, <Michael.Hennerich@...log.com>,
        <corbet@....net>, <marcelo.schmitt1@...il.com>
Subject: [PATCH v2 5/5] Documentation: iio: ad4000: Describe offload support

When SPI offloading is supported, the IIO device provides different sysfs
interfaces to allow using the adjusting the sample rate. Document SPI
offload support for AD4000 and similar devices.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@...log.com>
---
 Documentation/iio/ad4000.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/Documentation/iio/ad4000.rst b/Documentation/iio/ad4000.rst
index 468d30dc9214..e490f9604b94 100644
--- a/Documentation/iio/ad4000.rst
+++ b/Documentation/iio/ad4000.rst
@@ -191,3 +191,25 @@ Typical voltage channel attributes of a differential AD4000 series device:
 +-------------------------------------------+------------------------------------------+
 | ``in_voltage0-voltage1_scale_available``  | Toggles input span compression           |
 +-------------------------------------------+------------------------------------------+
+
+SPI offload support
+-------------------
+
+To be able to achieve the maximum sample rate, the driver can be used with the
+`AXI SPI Engine`_ to provide SPI offload support.
+
+.. _AXI SPI Engine: http://analogdevicesinc.github.io/hdl/projects/pulsar_adc/index.html
+
+When set for SPI offload support, the IIO device will provide different
+interfaces.
+
+* Either ``in_voltage0_sampling_frequency`` or
+  ``in_voltage0-voltage1_sampling_frequency`` file is provided to allow setting
+  the sample rate.
+* IIO trigger device is not provided (no ``trigger`` directory).
+* ``timestamp`` channel is not provided.
+
+Also, because the ADC output has a one sample latency (delay) when the device is
+wired in "3-wire" mode and only one transfer per sample is done when using SPI
+offloading, the first data sample in the buffer is not valid because it contains
+the output of an earlier conversion result.
-- 
2.47.2