[PATCH v4 0/3] i3c: Introduce i3c device userspace interface

[Sam Agazaryan <samagazaryan@google.com>]

This series revives the I3C userspace character device interface (i3cdev),
originally proposed by Vitor Soares in 2019/2020 [1][2].

1. Motivation & Production Use Case:
Previously, one of the reservations against introducing an i3cdev module
was the lack of a standardized production userspace use case beyond
controller development and bringup.

We now have a standardized industry use case: the OCP Secure Firmware
Recovery Specification (v1.1) and Open-Source Silicon Root of Trust
(Caliptra) recovery flows over I3C. When an I3C target device (SoC, ASIC,
or SRoT) is held in ROM or bootloader recovery mode, no functional
in-kernel driver is bound to the target. A userspace recovery daemon on
the BMC must perform private SDR transfers to interact with Recovery
Control & Status Registers (CSRs) and stream recovery firmware images.

Exposing an interface modeled after i2c-dev/spidev allows userspace
recovery tools to operate directly on unbound I3C devices without
requiring rigid or proprietary in-kernel recovery drivers.

2. Changes since v3 (Feb 2020):
- Rebased onto upstream i3c/next and adapted to the unified i3c_xfer API
  (replacing deprecated i3c_priv_xfer / i3c_device_do_priv_xfers with
  i3c_xfer / i3c_device_do_xfers(..., I3C_SDR)).
- Addressed UAPI structure feedback from Greg KH and Arnd Bergmann:
  * Used explicit __u64 for user data buffer addresses.
  * Added fixed-width explicit padding to ensure consistent 32-bit /
    64-bit ABI alignment.
- Wired up .compat_ioctl = compat_ptr_ioctl in file_operations.
- Fixed device lifecycle and concurrency:
  * Switched to cdev_device_add() and cdev_device_del() with an embedded
    struct device and a release callback, preventing use-after-free and
    races on driver unbind/detach (incorporating fix from Oleksandr
    Shulzhenko).
  * Cleared i3cdev->i3c under xfer_lock on detach so concurrent/subsequent
    file operations safely return -ENODEV.
- Fixed read transfer buffer allocation:
  * Replaced unconditional memdup_user() with kzalloc() for read
    transfers (rnw == true), avoiding copying uninitialized userspace
    memory into the kernel buffer.
- Fixed mutex leak in i3cdev_read() and i3cdev_write() error paths.
- Switched to static const struct class with class_register().
- Switched minor number allocation to the IDA allocator.
- Added include/uapi/linux/i3c/ to MAINTAINERS under I3C SUBSYSTEM.

[1] https://lore.kernel.org/all/cover.1575977795.git.vitor.soares@synopsys.com/
[2] https://github.com/vitor-soares-snps/i3c-tools

Vitor Soares (3):
  i3c: master: export i3c_masterdev_type
  i3c: master: add i3c_for_each_dev helper
  i3c: add i3cdev module to expose i3c dev in /dev

 MAINTAINERS                     |   1 +
 drivers/i3c/Kconfig             |  11 +
 drivers/i3c/Makefile            |   1 +
 drivers/i3c/i3cdev.c            | 445 ++++++++++++++++++++++++++++++++
 drivers/i3c/internals.h         |   4 +
 drivers/i3c/master.c            |  15 +-
 include/uapi/linux/i3c/i3cdev.h |  37 +++
 7 files changed, 513 insertions(+), 1 deletion(-)
 create mode 100644 drivers/i3c/i3cdev.c
 create mode 100644 include/uapi/linux/i3c/i3cdev.h

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2.55.0.979.g7e5102b832-goog