rust: add basic serial device bus abstractions

This patch series adds the serdev device bus rust abstraction into the
kernel.

This abstraction will be used by a driver,
which targets the MCU devices in Synology devices.

Kari Argillander also messaged me, stating that he wants to write a
watchdog driver with this abstraction (needing initial device data).

@Rob: Are you willing to maintain these rust abstractions yourself,
as you are the expert on this subsystem, otherwise I would take care of
it with a "SERIAL DEVICE BUS [RUST]" section in the MAINTAINERS file. In
the second case, I assume you are going to pick those patches as-is into
your tree, after they have been reviewed?

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
Changes in v6:
- rebased onto v7.1-rc1
- Link to v5: https://patch.msgid.link/20260420-rust_serdev-v5-0-57e8ba...

Changes in v5:
- fix typo in documentation
- Link to v4: https://lore.kernel.org/r/20260411-rust_serdev-v4-0-845e9...

Changes in v4:
- fixed not selecting rust serdev abstraction in sample
- Link to v3: https://lore.kernel.org/r/20260313-rust_serdev-v3-0-c9a3a...

Changes in v3:
- fix vertical import style
- add Kconfig entry for the rust abstraction
- fix documentation in include/linux/serdev.h
- rename private_data to rust_private_data
- fix `complete_all` <-> `wait_for_completion` typo
- move drvdata_borrow call after the completion
- Link to v2: https://lore.kernel.org/r/20260306-rust_serdev-v2-0-e9b23...

Changes in v2:
- fix documentation in `serdev::Driver::write` and
  `serdev::Driver::write_all`
- remove use of `dev_info` in probe from the sample
- remove `properties_parse` from the sample
- add optional `baudrate` property to the sample
- remove 1. patch
- remove `TryFrom<&device::Device<Ctx>> for &serdev::Device<Ctx>`
  implementation
- fix import style
- add patch to return reference in `devres::register` to fix safety
  issue
- add patch to add private data to serdev_device, to fix
  `Device.drvdata()` from failing
- simplify abstraction by removing ability to receive the initial
  transmission. It may be added later in a separate patch series if
  needed.
- Link to v1: https://lore.kernel.org/r/20251220-rust_serdev-v1-0-e4464...

---
Markus Probst (4):
      rust: devres: return reference in `devres::register`
      serdev: add rust private data to serdev_device
      rust: add basic serial device bus abstractions
      samples: rust: add Rust serial device bus sample device driver

 drivers/tty/serdev/Kconfig         |   7 +
 include/linux/serdev.h             |  15 +-
 rust/bindings/bindings_helper.h    |   1 +
 rust/helpers/helpers.c             |   1 +
 rust/helpers/serdev.c              |  22 ++
 rust/kernel/cpufreq.rs             |   3 +-
 rust/kernel/devres.rs              |  15 +-
 rust/kernel/drm/driver.rs          |   3 +-
 rust/kernel/lib.rs                 |   2 +
 rust/kernel/serdev.rs              | 536 +++++++++++++++++++++++++++++++++++++
 samples/rust/Kconfig               |  11 +
 samples/rust/Makefile              |   1 +
 samples/rust/rust_driver_serdev.rs |  86 ++++++
 13 files changed, 693 insertions(+), 10 deletions(-)
---
base-commit: 5e9b7d093f3f77cb0af4409559e3d139babfb443
change-id: 20251217-rust_serdev-ee5481e9085c