[PATCH v11 2/8] Documentation: iio: add Open Sensor Fusion driver overview

[Jinseob Kim <kimjinseob88@gmail.com>]

Document the supported OSF receiver profile, runtime sensor discovery,
IIO channel units, scale validation and session lifetime.

Link the project-maintained fixed wire specification instead of
duplicating its layouts and compatibility policy. Explain host IIO
timestamps and the explicit rebind needed after a changed inventory.

Assisted-by: LLM
Signed-off-by: Jinseob Kim <kimjinseob88@gmail.com>
---
 Documentation/iio/index.rst              |  1 +
 Documentation/iio/open-sensor-fusion.rst | 81 ++++++++++++++++++++++++
 MAINTAINERS                              |  1 +
 3 files changed, 83 insertions(+)
 create mode 100644 Documentation/iio/open-sensor-fusion.rst

diff --git a/Documentation/iio/index.rst b/Documentation/iio/index.rst
index d0fb32f4b80f079d837515c5f2451dd76d8f93c6..7f3654202ac457c2a8b3ade96d678426867c3362 100644
--- a/Documentation/iio/index.rst
+++ b/Documentation/iio/index.rst
@@ -42,4 +42,5 @@ Industrial I/O Kernel Drivers
    adxl345
    bno055
    ep93xx_adc
+   open-sensor-fusion
    opt4060
diff --git a/Documentation/iio/open-sensor-fusion.rst b/Documentation/iio/open-sensor-fusion.rst
new file mode 100644
index 0000000000000000000000000000000000000000..5d94a247ed8f351d92bfa2e1727d2004720015b6
--- /dev/null
+++ b/Documentation/iio/open-sensor-fusion.rst
@@ -0,0 +1,81 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+Open Sensor Fusion
+==================
+
+Open Sensor Fusion is a sensor aggregation hub interface. The Linux IIO driver
+receives device-to-host frames over UART and discovers sensor channels from
+capability reports. Device Tree describes the hub using the
+``opensensorfusion,osf-green`` compatible with an ``opensensorfusion,osf``
+fallback; individual sensors are discovered at runtime. See the binding in
+``Documentation/devicetree/bindings/iio/opensensorfusion,osf.yaml``.
+
+The OSF Device-to-Host 0.0 specification is maintained by the Open Sensor Fusion
+project. The canonical specification_ defines the wire layout, compatibility
+rules, reserved fields and physical units. This document describes the Linux
+receiver profile and its mapping to IIO.
+
+.. _specification: https://github.com/opensensorfusion/opensensorfusion-protocol/blob/ca9cdea1ae550c2b4d6f29f87877adae99470744/spec/osf-d2h-0.0.md
+
+Supported receive profile
+-------------------------
+
+The driver supports the fixed OSF0 frame layout with protocol major version 0.
+Compatible minor versions use the same known message layouts. The decoder
+requires exact lengths for known messages, validates CRC and bounds before
+interpreting payloads, and tolerates reserved padding. Unsupported major
+versions and unknown message types are ignored. Unsupported capability entries are skipped
+individually. A CRC-valid application rejection consumes the complete frame;
+an invalid frame candidate is resynchronized without trusting its payload length.
+
+The receive path handles sensor samples, complete capability reports and
+device status. Its limits are 4096 bytes per frame and 16 supported sensor
+descriptors. These are implementation limits, not protocol-wide maxima.
+
+IIO mapping
+-----------
+
+The supported sensor classes map to:
+
+* accelerometer: ``IIO_ACCEL`` X/Y/Z, in m/s^2;
+* gyroscope: ``IIO_ANGL_VEL`` X/Y/Z, in rad/s;
+* magnetometer: ``IIO_MAGN`` X/Y/Z, in gauss;
+* temperature: ``IIO_TEMP``, in millidegrees Celsius.
+
+Each registered channel exposes signed ``RAW`` data and descriptor-derived
+``SCALE``. Multiplying RAW by SCALE gives a value in the channel's IIO unit.
+Buffered samples use signed 32-bit values with native CPU endianness. Active
+channels are packed into the scan, with initialized padding before an optional
+64-bit timestamp.
+
+Discovery and session lifetime
+------------------------------
+
+Each capability report is a complete inventory snapshot, not an incremental
+fragment. The driver validates all supported descriptors before registering
+IIO devices. An initial report containing supported descriptors establishes
+the supported inventory. An empty or entirely unsupported initial report
+establishes no supported inventory and leaves discovery open for a later
+supported complete report. Supported descriptors must advertise a nonzero scale.
+A sample's channel count, format and scale must match its registered descriptor
+before it can update the latest RAW cache or be published to a buffer. A
+rejected sample preserves previously accepted data.
+
+After inventory establishment, later complete reports continue to be processed.
+Equivalent supported inventories are idempotent. Reports are compared by sensor
+type and index, independently of entry order. Reserved padding, advisory flags and unsupported additions do not change
+the supported inventory. A supported key appearing or disappearing, or a
+change in its channel count, format or scale, faults the bound session.
+
+After a session fault, new cache updates and buffer publication stop, and
+direct RAW reads fail. Existing IIO devices remain until driver teardown.
+An equivalent report does not clear the fault; explicit unbind/rebind starts
+fresh discovery. No fault-specific userspace ABI is exposed.
+
+Timestamps
+----------
+
+Buffered samples use the selected IIO host clock when they are pushed into
+the buffer. The device timestamp is not correlated with that clock and is not
+used as the IIO buffer timestamp. Device timestamp or sequence discontinuities
+alone do not start a new host session.
diff --git a/MAINTAINERS b/MAINTAINERS
index edae5bf594fc3c72fb39e14b797c7b2eccc11471..86a3e5de2f01f7a1e9791724caa6e19df4e61d70 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20541,6 +20541,7 @@ OPEN SENSOR FUSION
 M:	Jinseob Kim <kimjinseob88@gmail.com>
 S:	Maintained
 F:	Documentation/devicetree/bindings/iio/opensensorfusion,osf.yaml
+F:	Documentation/iio/open-sensor-fusion.rst
 K:	opensensorfusion
 
 OPENCOMPUTE PTP CLOCK DRIVER