[PATCH RFC 0/2] rust: usb: abstractions towards the port of usbsevseg.c to Rust

[Alexandru Radovici <alexandru.radovici@wyliodrin.com>]

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 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.

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.

Signed-off-by: Alexandru Radovici <alexandru.radovici@wyliodrin.com>
---
Alexandru Radovici (2):
      rust: usb: add endpoint abstraction
      rust: usb: add control message send and receive

 rust/kernel/usb.rs          | 127 ++++++++++-
 rust/kernel/usb/control.rs  | 353 ++++++++++++++++++++++++++++++
 rust/kernel/usb/endpoint.rs | 508 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 987 insertions(+), 1 deletion(-)
---
base-commit: 8ba098e6b6ff0db8edf28528d1552be261af30d4
change-id: 20260801-rust-usb_control_msg-ee34e002b818

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