linux-kernel - [PATCH v3 2/2] docs: iio: ad7380: add SPI offload support

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Message-Id: <20250304-wip-bl-spi-offload-ad7380-v3-2-2d830f863bd1@baylibre.com>
Date: Tue, 04 Mar 2025 16:25:45 +0100
From: Angelo Dureghello <adureghello@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>, 
 Lars-Peter Clausen <lars@...afoo.de>, 
 Michael Hennerich <Michael.Hennerich@...log.com>, 
 Nuno Sá <nuno.sa@...log.com>, 
 David Lechner <dlechner@...libre.com>, Jonathan Corbet <corbet@....net>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-doc@...r.kernel.org, Angelo Dureghello <adureghello@...libre.com>
Subject: [PATCH v3 2/2] docs: iio: ad7380: add SPI offload support

Document SPI offload support for the ad7380 driver.

Signed-off-by: Angelo Dureghello <adureghello@...libre.com>
---
 Documentation/iio/ad7380.rst | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/iio/ad7380.rst b/Documentation/iio/ad7380.rst
index cff688bcc2d9601a9faf42d5e9c217486639ca66..12709ce051110e68b6f0c9b9a0baefcc954f2bc0 100644
--- a/Documentation/iio/ad7380.rst
+++ b/Documentation/iio/ad7380.rst
@@ -169,6 +169,42 @@ gain is selectable from device tree using the ``adi,gain-milli`` property.
 Refer to the typical connection diagrams section of the datasheet for pin
 wiring.
 
+
+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/ad738x_fmc/index.html
+
+When SPI offload is being used, some attributes will be different.
+
+* ``in_voltage-voltage_sampling_frequency`` attribute is added for setting the
+  sample rate.
+* ``in_voltage-voltage_sampling_frequency_available`` attribute is added for
+  querying the max sample rate.
+* ``timestamp`` channel is removed.
+* Buffer data format may be different compared to when offload is not used,
+  e.g. the ``buffer0/in_voltage0-voltage1_type`` and the
+  ``buffer0/in_voltage2-voltage3_type`` attributes.
+
+Effective sample rate for buffered reads
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Using SPI offload, the PWM generator drives the starting of the conversion by
+executing the pre-recorded SPI transfer at each PWM cycle, asserting CS and
+reading the previous available sample values for all channels.
+Default sample rate is set to a quite low frequency, to allow oversampling x32,
+user is then responsible to adjust ``in_voltage-voltage_sampling_frequency`` for
+the specific case.
+
+For single-ended chips where 2 banks of simultaneous inputs are available, as
+ad7386, ad7387 and ad7388, if at least one channel from each bank is enabled in
+a buffered read, the effective sample rate will be 1/2 of what is set as
+``in_voltage-voltage_sampling_frequency``, this because a separate conversion
+needs to be done for each bank.
+
 Unimplemented features
 ----------------------
 

-- 
2.48.1