[PATCH v4 4/4] Documentation: iio: ad4000: Describe offload support

[Marcelo Schmitt]

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.

Reviewed-by: David Lechner <dlechner@xxxxxxxxxxxx>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>
---
 Documentation/iio/ad4000.rst | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/Documentation/iio/ad4000.rst b/Documentation/iio/ad4000.rst
index 468d30dc9214..c1d04d3436d2 100644
--- a/Documentation/iio/ad4000.rst
+++ b/Documentation/iio/ad4000.rst
@@ -191,3 +191,30 @@ 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 SPI
+offload engines such as the one usually present in `AXI SPI Engine`_, to provide
+SPI offload support.
+
+.. _AXI SPI Engine: http://analogdevicesinc.github.io/hdl/projects/pulsar_adc/index.html
+
+To keep up with SPI offloading transfer speeds, the ADC must be connected either
+in 3-wire turbo mode or in 3-wire without busy indicator mode and have SPI
+controller CS line connected to the CNV pin.
+
+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