[PATCH v3 0/2] spi: add sysfs interface for userspace device instantiation

[Vishwaroop A <va@nvidia.com>]

Development boards such as the Jetson AGX Orin expose SPI buses on
expansion headers so that users can connect and interact with SPI
peripherals from userspace via /dev/spidevB.C character devices.

Today, instantiating spidev devices on upstream kernels is impractical:
the spidev driver rejects bare "spidev" compatible strings in DT, and
vendor-specific strings have been rejected by DT maintainers.

This series adds an I2C-style new_device/delete_device sysfs interface
to SPI host controllers, enabling runtime device instantiation from
userspace without any device-tree changes.

Patch 1 adds the core sysfs interface (new_device/delete_device).
Patch 2 adds user-facing documentation and ABI entries.

Changes in v3:
  - Made the entire feature conditional on CONFIG_SPI_DYNAMIC using
    #if IS_ENABLED(CONFIG_SPI_DYNAMIC) preprocessor guards, since this
    adds a new way of dynamically instantiating and removing SPI
    devices and the add_lock locking in spi_unregister_controller() is
    already conditional on CONFIG_SPI_DYNAMIC (Mark Brown)
  - Wrapped userspace_clients list, lock, and userspace_node struct
    members in #if IS_ENABLED(CONFIG_SPI_DYNAMIC) in spi.h
  - Started a new thread instead of replying to v2 (Mark Brown)

Changes in v2:
  - Fixed a race between new_device_store() and
    spi_unregister_controller(): hold ctlr->add_lock across both
    __spi_add_device() and the list insertion so the two operations
    are atomic with respect to controller teardown (Mark Brown)

Vishwaroop A (2):
  spi: add new_device/delete_device sysfs interface
  docs: spi: add documentation for userspace device instantiation

 .../ABI/testing/sysfs-class-spi-master        |  34 +++
 Documentation/spi/index.rst                   |   1 +
 Documentation/spi/instantiating-devices.rst   |  88 ++++++++
 drivers/spi/spi.c                             | 206 ++++++++++++++++++
 include/linux/spi/spi.h                       |  12 +
 5 files changed, 341 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-class-spi-master
 create mode 100644 Documentation/spi/instantiating-devices.rst

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2.17.1