[RFC PATCH 0/3] rust: power_supply class abstraction and SMB347 charger driver

[Bruce Robertson <brucer42@gmail.com>]

This series adds a Rust abstraction for the power supply class -- which
does not currently exist in mainline or rust-next -- together with the
SMBus helpers needed to drive an I2C charger from Rust, and a Rust port
of the Summit SMB347 battery charger as the first consumer.

Motivation
----------
Rust drivers for power supply hardware are currently blocked: there is
no safe Rust interface to the power supply class, and the I2cClient
abstraction exposes no register I/O. This series provides a minimal,
self-contained path from binding an I2C charger to reporting charging
state through sysfs, entirely in safe Rust, with the unsafe FFI confined
to the two abstraction layers.

The series is structured abstraction-first:

  1/3  Safe SMBus read/write/update_bits over i2c::I2cClient.
  2/3  A power_supply Driver trait, a generic get_property trampoline,
       and an RAII Registration that owns the descriptor lifetime.
  3/3  A Rust SMB347 charger driver consuming both: it binds over I2C
       and reports STATUS, ONLINE and CHARGE_TYPE.

Testing
-------
Built and exercised against an emulated SMB347 using i2c-stub: seeding
the chip's status registers and reading back the corresponding sysfs
attributes (status, online, charge_type) confirms the full C->Rust->C
path. No physical hardware or interrupt path has been tested.

Open questions / known limitations (hence RFC)
----------------------------------------------
  - get_property recovers the driver's private data via the parent
    device's drvdata. The current code relies on the observed ordering
    (callbacks only fire after probe() has set drvdata); the contract
    should be made explicit.
  - smbus_update_bits() is not atomic against concurrent callers; a lock
    will be required before a charger IRQ handler is added (not yet
    implemented).
  - Only get_property and a handful of properties are wired up;
    set_property, property_is_writeable and the IRQ-driven
    power_supply_changed() notification are future work.

Feedback on the abstraction's shape -- especially the descriptor
lifetime and the drvdata recovery -- would be very welcome.

checkpatch emits one MAINTAINERS warning on patch 3/3 (new driver file);
it is a false positive -- the driver is covered by the existing POWER
SUPPLY CLASS "F: drivers/power/supply/" glob and needs no new entry.
Patch 2/3 adds the one file outside any existing glob
(rust/kernel/power_supply.rs) and updates MAINTAINERS accordingly.

Based on rust-next (v7.2-rc1).

Bruce Robertson (3):
  rust: i2c: add SMBus byte transfer helpers
  rust: power_supply: add power supply class abstraction
  power: supply: add Rust SMB347 charger driver

 MAINTAINERS                                 |   1 +
 drivers/power/supply/Kconfig                |  14 ++
 drivers/power/supply/Makefile               |   1 +
 drivers/power/supply/smb347-charger_rust.rs | 180 ++++++++++++++++++++
 rust/bindings/bindings_helper.h             |   1 +
 rust/kernel/i2c.rs                          |  42 +++++
 rust/kernel/lib.rs                          |   1 +
 rust/kernel/power_supply.rs                 | 133 +++++++++++++++
 8 files changed, 373 insertions(+)
 create mode 100644 drivers/power/supply/smb347-charger_rust.rs
 create mode 100644 rust/kernel/power_supply.rs

-- 
2.43.0