[RFC PATCH 6/9] rust: usb: add usb::Device::interrupt_recv()

[Mike Lothian]

Add a safe wrapper for synchronous interrupt IN transfers, mirroring
bulk_recv() but using usb_interrupt_msg() over an interrupt pipe so the
URB's transfer type matches the endpoint. This is the correctly-typed API
for reading a device's interrupt-IN status endpoints; usb_bulk_msg() would
also work (it detects an interrupt endpoint and rewrites the pipe to
PIPE_INTERRUPT), but interrupt_recv() exists precisely so callers do not
have to rely on that legacy fixup.

The interrupt pipe is built with a new rust_helper_usb_rcvintpipe() shim,
matching the existing bulk-pipe helpers (usb_rcvintpipe() is a
function-like macro). It sleeps, so it must be called from process
context, and like bulk_recv() the buffer must be DMA-capable.

Signed-off-by: Mike Lothian <mike@xxxxxxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8 [Claude-Code]
---
 rust/helpers/usb.c |  6 ++++++
 rust/kernel/usb.rs | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/rust/helpers/usb.c b/rust/helpers/usb.c
index d398eb2f6669..ac7b30334882 100644
--- a/rust/helpers/usb.c
+++ b/rust/helpers/usb.c
@@ -19,3 +19,9 @@ rust_helper_usb_rcvbulkpipe(struct usb_device *dev, unsigned int endpoint)
 {
 	return usb_rcvbulkpipe(dev, endpoint);
 }
+
+__rust_helper unsigned int
+rust_helper_usb_rcvintpipe(struct usb_device *dev, unsigned int endpoint)
+{
+	return usb_rcvintpipe(dev, endpoint);
+}
diff --git a/rust/kernel/usb.rs b/rust/kernel/usb.rs
index a151a120e82e..c7bf4637ee90 100644
--- a/rust/kernel/usb.rs
+++ b/rust/kernel/usb.rs
@@ -566,6 +566,50 @@ pub fn bulk_recv(&self, endpoint: u8, data: &mut [u8], timeout: Delta) -> Result
         Ok(n)
     }
 
+    /// Issues a synchronous interrupt IN transfer on `endpoint`, returning the number of bytes
+    /// received. Uses `usb_interrupt_msg()` over an interrupt pipe (built with `usb_rcvintpipe()`)
+    /// so the URB's transfer type matches the endpoint — the correctly-typed counterpart to
+    /// [`bulk_recv`] for interrupt-IN status endpoints.
+    ///
+    /// This is a blocking, sleeping call and must only be invoked from process context. `endpoint`
+    /// is the endpoint's `bEndpointAddress` (e.g. `0x83`); only its low four bits — the endpoint
+    /// number — are used and the direction is fixed by the method (IN). As for [`bulk_recv`], the
+    /// data is received into a kmalloc'd bounce buffer internally and copied into `data`, so `data`
+    /// need not be DMA-capable. `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.
+    ///
+    /// [`bulk_recv`]: Self::bulk_recv
+    pub fn interrupt_recv(&self, endpoint: u8, data: &mut [u8], timeout: Delta) -> Result<usize> {
+        let mut actual: kernel::ffi::c_int = 0;
+
+        // `usb_interrupt_msg()` requires a DMA-capable buffer; receive into a kmalloc'd
+        // bounce buffer and copy out, so `data` need not be DMA-capable itself.
+        let mut buf = KVec::from_elem(0u8, data.len(), GFP_KERNEL)?;
+
+        // SAFETY: `self.as_raw()` is a valid `struct usb_device` by the type invariant.
+        let pipe = unsafe { bindings::usb_rcvintpipe(self.as_raw(), endpoint.into()) };
+
+        // SAFETY: `self.as_raw()` is valid by the type invariant; `buf` is a kmalloc'd buffer
+        // valid for writes of `buf.len()` bytes; `actual` is a valid out-pointer.
+        // `usb_interrupt_msg()` fills an interrupt URB for `pipe` and blocks until it completes
+        // or `timeout` elapses.
+        to_result(unsafe {
+            bindings::usb_interrupt_msg(
+                self.as_raw(),
+                pipe,
+                buf.as_mut_ptr().cast::<kernel::ffi::c_void>(),
+                buf.len().try_into()?,
+                &mut actual,
+                timeout.as_millis().try_into()?,
+            )
+        })?;
+
+        // `usb_interrupt_msg()` never reports more than the requested length.
+        let n = (actual as usize).min(data.len());
+        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
-- 
2.54.0