[PATCH RFC v2 0/4] rust: usb: abstractions towards the port of usbsevseg.c to Rust

[Alexandru Radovici]

My end goal is to port the usbsevseg.c USB driver to Rust. That driver is
small enough to make a useful first target; the remaining piece it
needs is sysfs attribute support for USB devices, which I plan to send
separately.

Porting the driver requires:
- retrieving endpoints and sending control messages API (this series)
- enabling sysfs attributes API for USB devices (future patch)

These patches add the USB and sysfs abstractions needed to port
drivers/usb/misc/usbsevseg.c to Rust: retrieving an interface's
endpoints, and sending and receiving control messages.

Patch 1 adds `AlternateEndpoint`, along with `AlternateSetting`
and the accessors needed to reach one. Patch 2 adds the
control transfer methods and the `Request` type describing a setup packet.
Patch 3 adds the sysfs abstractions. Patch 4 adds the sysfs abstractions
to `usb::Driver`.

The part I would most like feedback on is the typestate design in patch
1. `HostEndpoint` is generic over a direction and a transfer type, both
sealed 1-ZST markers, so a function can require e.g.
`&HostEndpoint<In, Bulk>` and skip checking the descriptor itself. Control
endpoints get a `Bidirectional` marker rather than a direction, since
bit 7 of bEndpointAddress is ignored for them. I am not sure whether
this earns its complexity for the drivers we expect, or whether plain
run-time check would be better.

Only `as_control()`, `as_in()` and `as_out()` are implemented so far;
`as_bulk()`, `as_interrupt()` and `as_isochronous()` are skiped for now 
as no current user needs them.

I am not sure about the soundness of the sysfs absgtractions, especially
the lifetime annotations of `DEVICE_GROUPS`.
Feedback for this would be greatly appreciated.

Signed-off-by: Alexandru Radovici <alexandru.radovici@xxxxxxxxxxxxx>
---
Changes in v2:
- add sysfs abstractions
- add DEVICE_GROUPS abstractions to usb::Driver using sysfs
- rename the `control_` methods correctly
- use `Duration` instead of `i32` for control functions timeout
- fix control function 0 value timeout from "for ever" to `USB_MAX_SYNCHRONOUS_TIMEOUT` 
- add `#[inline]` to short functions
- use `u16::from_le` to read `wMaxPacketSize`
- rename `Generic` generic type to `Dir`
- impl `max_nak_rate` for `HostEndpoint<Bidirectional, Control>`
- make data `&mut [u8]` for `control_message_receive`
- Link to v1: https://lore.kernel.org/r/20260801-rust-usb_control_msg-v1-0-655bb444b52c@xxxxxxxxxxxxx

---
Alexandru Radovici (4):
      rust: usb: add endpoint abstraction
      rust: usb: add control message send and receive
      rust: sysfs: add abstractions for device attributes
      rust: usb: allow drivers to expose sysfs attributes

 rust/kernel/device.rs           |   2 +-
 rust/kernel/lib.rs              |   2 +
 rust/kernel/sysfs.rs            | 602 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/usb.rs              | 227 ++++++++++++++-
 rust/kernel/usb/control.rs      | 367 ++++++++++++++++++++++++
 rust/kernel/usb/endpoint.rs     | 516 ++++++++++++++++++++++++++++++++++
 samples/rust/rust_driver_usb.rs |  14 +
 7 files changed, 1725 insertions(+), 5 deletions(-)
---
base-commit: 8ba098e6b6ff0db8edf28528d1552be261af30d4
change-id: 20260801-rust-usb_control_msg-ee34e002b818

Best regards,
-- 
Alexandru Radovici <alexandru.radovici@xxxxxxxxxxxxx>