[RFC PATCH 2/9] rust: usb: add synchronous control transfer support

[Mike Lothian]

Building on the bulk transfer support, add synchronous control IN/OUT
transfers on the default control endpoint (pipe 0) as safe methods on
`usb::Device`.

`control_send()` and `control_recv()` wrap `usb_control_msg_send()` and
`usb_control_msg_recv()`. The `bRequest`, `bmRequestType`, `wValue` and
`wIndex` setup fields are taken as arguments; `wLength` is the buffer
length. Both copy the buffer internally, so unlike the bulk path the
caller's buffer need not be DMA-capable. They block and sleep, so must be
called from process context; the timeout is a `Delta` (zero waits
indefinitely).

This is what a driver needs for the standard control-request preamble at
device bring-up before bulk traffic begins.

Signed-off-by: Mike Lothian <mike@xxxxxxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8 [Claude-Code]
---
 rust/kernel/usb.rs | 75 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/rust/kernel/usb.rs b/rust/kernel/usb.rs
index c9acadb2deaf..e1fb50fd6997 100644
--- a/rust/kernel/usb.rs
+++ b/rust/kernel/usb.rs
@@ -530,6 +530,81 @@ pub fn bulk_recv(&self, endpoint: u8, data: &mut [u8], timeout: Delta) -> Result
         data[..n].copy_from_slice(&buf[..n]);
         Ok(n)
     }
+
+    /// Issues a synchronous control OUT transfer on the default control endpoint.
+    ///
+    /// Wraps [`usb_control_msg_send()`]; `request`, `request_type`, `value` and
+    /// `index` are the `bRequest`, `bmRequestType`, `wValue` and `wIndex` setup
+    /// fields. `data` is the payload (`wLength` is its length).
+    ///
+    /// This is a blocking, sleeping call and must only be invoked from process
+    /// context. Unlike the bulk path, the buffer is copied internally, so `data`
+    /// need not reside in DMA-capable memory. `timeout` is the maximum time to
+    /// wait, rounded down to whole milliseconds; a [`Delta`] of zero — or any
+    /// non-zero value below 1 ms — waits indefinitely.
+    ///
+    /// [`usb_control_msg_send()`]: https://docs.kernel.org/driver-api/usb/usb.html#c.usb_control_msg_send
+    pub fn control_send(
+        &self,
+        request: u8,
+        request_type: u8,
+        value: u16,
+        index: u16,
+        data: &[u8],
+        timeout: Delta,
+    ) -> Result {
+        // SAFETY: `self.as_raw()` is valid by the type invariant; `data` is valid for
+        // reads of `data.len()` bytes; `usb_control_msg_send()` copies the buffer.
+        to_result(unsafe {
+            bindings::usb_control_msg_send(
+                self.as_raw(),
+                0,
+                request,
+                request_type,
+                value,
+                index,
+                data.as_ptr().cast::<kernel::ffi::c_void>(),
+                data.len().try_into()?,
+                timeout.as_millis().try_into()?,
+                bindings::GFP_KERNEL,
+            )
+        })
+    }
+
+    /// Issues a synchronous control IN transfer on the default control endpoint,
+    /// filling `data` with exactly `data.len()` bytes.
+    ///
+    /// Wraps [`usb_control_msg_recv()`], which fails the transfer if the device
+    /// returns fewer than `data.len()` bytes. The setup fields and context/buffer
+    /// rules are as for [`Device::control_send`].
+    ///
+    /// [`usb_control_msg_recv()`]: https://docs.kernel.org/driver-api/usb/usb.html#c.usb_control_msg_recv
+    pub fn control_recv(
+        &self,
+        request: u8,
+        request_type: u8,
+        value: u16,
+        index: u16,
+        data: &mut [u8],
+        timeout: Delta,
+    ) -> Result {
+        // SAFETY: `self.as_raw()` is valid by the type invariant; `data` is valid for
+        // writes of `data.len()` bytes; `usb_control_msg_recv()` copies into the buffer.
+        to_result(unsafe {
+            bindings::usb_control_msg_recv(
+                self.as_raw(),
+                0,
+                request,
+                request_type,
+                value,
+                index,
+                data.as_mut_ptr().cast::<kernel::ffi::c_void>(),
+                data.len().try_into()?,
+                timeout.as_millis().try_into()?,
+                bindings::GFP_KERNEL,
+            )
+        })
+    }
 }
 
 // SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
-- 
2.54.0