[PATCH v4 0/2] spi: add new_device/delete_device sysfs interface

[Vishwaroop A <va@nvidia.com>]

Add I2C-style new_device/delete_device sysfs attributes to SPI host
controllers, allowing userspace to instantiate and remove SPI devices
at runtime without device-tree changes.

Changes since v3:
  - Replaced holding add_lock across __spi_add_device() + list
    insertion (which caused an ABBA deadlock between add_lock and the
    kernfs active reference during concurrent unbind) with:
    * A 'dead' flag on spi_controller, set in
      spi_unregister_controller() under both add_lock and
      userspace_clients_lock.
    * __spi_add_device() checks ctlr->dead under add_lock to reject
      new devices after teardown begins.
    * new_device_store() checks ctlr->dead under userspace_clients_lock
      before list insertion, falling back to cleanup + ENODEV.
    * add_lock is released before device_del() so in-flight sysfs
      stores can drain without deadlocking.
    * get_device() taken before spi_add_device() prevents
      use-after-free if __unregister runs concurrently.
  - Used #if IS_ENABLED() preprocessor guard (not runtime IS_ENABLED())
    for the ctlr->dead check in __spi_add_device(), since the dead
    field is conditionally compiled.

Changes since v2:
  - Gated sysfs attributes and locking on CONFIG_SPI_DYNAMIC.

Changes since v1:
  - Added locking to prevent races between new_device_store() and
    concurrent spi_unregister_controller().

Link: https://lore.kernel.org/linux-tegra/909f0c92-d110-4253-903e-5c81e21e12c9@nvidia.com/

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                             | 216 +++++++++++++++++-
 include/linux/spi/spi.h                       |  13 +
 5 files changed, 346 insertions(+), 6 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-class-spi-master
 create mode 100644 Documentation/spi/instantiating-devices.rst

-- 
2.17.1