[PATCH v3 5/5] rust: usb: let a driver keep its interface usable while unbinding

[Mike Lothian <mike@fireburn.co.uk>]

The USB core kills every outstanding URB and disables an interface's
endpoints before it calls any driver callback. A driver with something
left to say to the device on the way out therefore cannot say it: the
transfer names an endpoint that no longer exists and is refused.

Expose the core's soft-unbind flag as a driver constant. Setting it
defers that teardown until the callbacks return, which makes cancelling
outstanding transfers the driver's own responsibility.

This matters to a driver that leaves the device in a state a user can
see. A display bridge that simply stops sending pixels leaves the
monitor lit on whatever it decoded last.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <mike@fireburn.co.uk>
---
 rust/kernel/usb.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/rust/kernel/usb.rs b/rust/kernel/usb.rs
index b2a0fb104ddf..1d8f5bc8a545 100644
--- a/rust/kernel/usb.rs
+++ b/rust/kernel/usb.rs
@@ -107,6 +107,7 @@ unsafe fn register(
             (*udrv.get()).pre_reset = Some(Self::pre_reset_callback);
             (*udrv.get()).post_reset = Some(Self::post_reset_callback);
             (*udrv.get()).id_table = T::ID_TABLE.as_ptr();
+            (*udrv.get()).set_soft_unbind(T::SOFT_UNBIND as core::ffi::c_uint);
         }
 
         // SAFETY: `udrv` is guaranteed to be a valid `DriverType`.
@@ -474,6 +475,19 @@ pub trait Driver {
     /// The table of device ids supported by the driver.
     const ID_TABLE: IdTable<Self::IdInfo>;
 
+    /// Whether the USB core must leave this interface usable until the driver has let go of it.
+    ///
+    /// By default the core kills every outstanding URB and disables the interface's endpoints
+    /// *before* it calls any driver callback, so a driver that has something to say to the device
+    /// on the way out cannot say it: the transfer is refused with [`ENOENT`] because the endpoint
+    /// it names no longer exists. Setting this defers that teardown until after the callbacks
+    /// return, which makes cancelling outstanding transfers the driver's own responsibility.
+    ///
+    /// Only useful to a driver that leaves the device in a state a user can see -- a display that
+    /// otherwise goes on scanning out its last frame, an interface that must be told to power
+    /// down.
+    const SOFT_UNBIND: bool = false;
+
     /// USB driver probe.
     ///
     /// Called when a new USB interface is bound to this driver.