iio: add Open Sensor Fusion OSF0 UART driver

This RFC series adds an Industrial I/O driver for Open Sensor Fusion
OSF0 UART devices.

Open Sensor Fusion is an open hardware project for sensor aggregation
devices and Linux IIO host support. OSF0 is not a general standard; it is
the current wire format used by this project and by the RFC driver in this
series. The first concrete hardware target is OSF GREEN, an STM32F405-based
sensor aggregation board that streams sensor samples to a Linux host.

Project links:
https://www.opensensorfusion.org/
https://github.com/opensensorfusion
https://github.com/opensensorfusion/opensensorfusion-linux

The driver receives OSF0 frames over a serdev UART, validates the stream,
decodes capability and sample frames, and registers IIO devices for the
supported sensor types. The current RFC driver covers the device-to-host
path used for accelerometer, gyroscope, magnetometer, and temperature
samples.

Changes since v2:

* Reworked the binding around the concrete OSF GREEN hardware target.
* Changed the compatible from opensensorfusion,osf-uart to
  opensensorfusion,osf-green.
* Renamed the binding file to opensensorfusion,osf-green.yaml.
* Updated the example node name to generic sensor.
* Added serial-peripheral-props.yaml and unevaluatedProperties: false.
* Added public project links and clarified that OSF0 is not a general
  standard.
* Separated the OSF0 wire format from the subset currently supported by
  this RFC driver.
* Clarified SENSOR_SAMPLE as a 16-byte payload header followed by
  4 * channel_count bytes of s32 channel data.
* Clarified device-side timestamp limitations.
* Spelled out Attitude and Heading Reference System (AHRS).
* Added sensor_type, sample_format, channel_count, reserved-field, and
  payload length overflow validation.
* Changed reserved fields to validate-only handling.
* Fixed IIO_BUFFER / IIO_KFIFO_BUF dependency handling.
* Added channel_count checks before pushing samples to IIO buffers.
* Added locking for cached latest samples.
* Removed explicit linux-iio and devicetree list entries from
  MAINTAINERS.
* Folded MAINTAINERS updates into the patches that add the corresponding
  files.
* Addressed Sashiko feedback from v2.

The runtime smoke test used for the previous revision was performed with an
OSF GREEN prototype connected to a Raspberry Pi 4 over UART/serdev. This v3
series was also checked with dt_binding_check, checkpatch, and a W=1 target
build in the local full-tree environment.


Jinseob Kim (6):
  dt-bindings: iio: add OSF GREEN sensor aggregation device
  Documentation: iio: add Open Sensor Fusion protocol v0 reference
  iio: osf: add protocol v0 decoding
  iio: osf: add stream parser
  iio: osf: add UART serdev transport
  iio: osf: register IIO devices from capabilities

 .../iio/imu/opensensorfusion,osf-green.yaml   |  43 +++
 .../devicetree/bindings/vendor-prefixes.yaml  |   2 +
 .../iio/open-sensor-fusion-protocol-v0.rst    | 308 ++++++++++++++++++
 MAINTAINERS                                   |  13 +
 drivers/iio/Kconfig                           |   1 +
 drivers/iio/Makefile                          |   1 +
 drivers/iio/opensensorfusion/Kconfig          |  14 +
 drivers/iio/opensensorfusion/Makefile         |   6 +
 drivers/iio/opensensorfusion/osf_core.c       | 305 +++++++++++++++++
 drivers/iio/opensensorfusion/osf_core.h       |  70 ++++
 drivers/iio/opensensorfusion/osf_iio.c        | 285 ++++++++++++++++
 drivers/iio/opensensorfusion/osf_iio.h        |  22 ++
 drivers/iio/opensensorfusion/osf_protocol.c   | 247 ++++++++++++++
 drivers/iio/opensensorfusion/osf_protocol.h   |  95 ++++++
 drivers/iio/opensensorfusion/osf_serdev.c     | 111 +++++++
 drivers/iio/opensensorfusion/osf_stream.c     | 207 ++++++++++++
 drivers/iio/opensensorfusion/osf_stream.h     |  31 ++
 17 files changed, 1761 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/imu/opensensorfusion,osf-green.yaml
 create mode 100644 Documentation/iio/open-sensor-fusion-protocol-v0.rst
 create mode 100644 drivers/iio/opensensorfusion/Kconfig
 create mode 100644 drivers/iio/opensensorfusion/Makefile
 create mode 100644 drivers/iio/opensensorfusion/osf_core.c
 create mode 100644 drivers/iio/opensensorfusion/osf_core.h
 create mode 100644 drivers/iio/opensensorfusion/osf_iio.c
 create mode 100644 drivers/iio/opensensorfusion/osf_iio.h
 create mode 100644 drivers/iio/opensensorfusion/osf_protocol.c
 create mode 100644 drivers/iio/opensensorfusion/osf_protocol.h
 create mode 100644 drivers/iio/opensensorfusion/osf_serdev.c
 create mode 100644 drivers/iio/opensensorfusion/osf_stream.c
 create mode 100644 drivers/iio/opensensorfusion/osf_stream.h

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2.43.0