Document and warn on case where WinUSB changes the passed control transfer index field to the interface number

src/device.rs

@@ -346,8 +346,13 @@ impl Interface {
     ///
     /// ### Platform-specific notes
     ///
-    /// * On Linux, this takes a device-wide lock, so if you have multiple threads, you
-    ///   are better off using the async methods.
+    /// * On Linux, this takes a device-wide lock, so if you have multiple
+    ///   threads, you are better off using the async methods.
+    /// * On Windows, if the `recipient` is `Interface`, the WinUSB driver sends
+    ///   the interface number in the least significant byte of `index`,
+    ///   overriding any value passed. A warning is logged if the passed `index`
+    ///   least significant byte differs from the interface number, and this may
+    ///   become an error in the future.
     pub fn control_in_blocking(
         &self,
         control: Control,
@@ -361,8 +366,13 @@ impl Interface {
     ///
     /// ### Platform-specific notes
     ///
-    /// * On Linux, this takes a device-wide lock, so if you have multiple threads, you
-    ///   are better off using the async methods.
+    /// * On Linux, this takes a device-wide lock, so if you have multiple
+    ///   threads, you are better off using the async methods.
+    /// * On Windows, if the `recipient` is `Interface`, the WinUSB driver sends
+    ///   the interface number in the least significant byte of `index`,
+    ///   overriding any value passed. A warning is logged if the passed `index`
+    ///   least significant byte differs from the interface number, and this may
+    ///   become an error in the future.
     pub fn control_out_blocking(
         &self,
         control: Control,
@@ -394,6 +404,13 @@ impl Interface {
     /// })).into_result()?;
     /// # Ok(()) }
     /// ```
+    ///
+    /// ### Platform-specific notes
+    /// * On Windows, if the `recipient` is `Interface`, the WinUSB driver sends
+    ///   the interface number in the least significant byte of `index`,
+    ///   overriding any value passed. A warning is logged if the passed `index`
+    ///   least significant byte differs from the interface number, and this may
+    ///   become an error in the future.
     pub fn control_in(&self, data: ControlIn) -> TransferFuture<ControlIn> {
         let mut t = self.backend.make_transfer(0, EndpointType::Control);
         t.submit::<ControlIn>(data);
@@ -422,6 +439,13 @@ impl Interface {
     /// })).into_result()?;
     /// # Ok(()) }
     /// ```
+    ///
+    /// ### Platform-specific notes
+    /// * On Windows, if the `recipient` is `Interface`, the WinUSB driver sends
+    ///   the interface number in the least significant byte of `index`,
+    ///   overriding any value passed. A warning is logged if the passed `index`
+    ///   least significant byte differs from the interface number, and this may
+    ///   become an error in the future.
     pub fn control_out(&self, data: ControlOut) -> TransferFuture<ControlOut> {
         let mut t = self.backend.make_transfer(0, EndpointType::Control);
         t.submit::<ControlOut>(data);

src/platform/windows_winusb/device.rs

@@ -8,7 +8,7 @@ use std::{
     time::Duration,
 };
 
-use log::{debug, error, info};
+use log::{debug, error, info, warn};
 use windows_sys::Win32::{
     Devices::Usb::{
         WinUsb_ControlTransfer, WinUsb_Free, WinUsb_Initialize, WinUsb_SetCurrentAlternateSetting,
@@ -19,7 +19,7 @@ use windows_sys::Win32::{
 
 use crate::{
     descriptors::{validate_config_descriptor, DESCRIPTOR_TYPE_CONFIGURATION},
-    transfer::{Control, Direction, EndpointType, TransferError, TransferHandle},
+    transfer::{Control, Direction, EndpointType, Recipient, TransferError, TransferHandle},
     DeviceInfo, Error,
 };
 
@@ -125,6 +125,7 @@ impl WindowsDevice {
 
         Ok(Arc::new(WindowsInterface {
             handle,
+            interface,
             winusb_handle,
         }))
     }
@@ -139,6 +140,7 @@ impl WindowsDevice {
 
 pub(crate) struct WindowsInterface {
     pub(crate) handle: OwnedHandle,
+    pub(crate) interface: u8,
     pub(crate) winusb_handle: WINUSB_INTERFACE_HANDLE,
 }
 
@@ -161,6 +163,11 @@ impl WindowsInterface {
         timeout: Duration,
     ) -> Result<usize, TransferError> {
         info!("Blocking control {direction:?}, {len} bytes");
+
+        if control.recipient == Recipient::Interface && control.index as u8 != self.interface {
+            warn!("WinUSB sends interface number instead of passed `index` when performing a control transfer with `Recipient::Interface`");
+        }
+
         let timeout_ms = timeout.as_millis().min(u32::MAX as u128) as u32;
         let r = WinUsb_SetPipePolicy(
             self.winusb_handle,

src/platform/windows_winusb/transfer.rs

@@ -6,7 +6,7 @@ use std::{
     sync::Arc,
 };
 
-use log::{debug, error};
+use log::{debug, error, warn};
 use windows_sys::Win32::{
     Devices::Usb::{
         WinUsb_ControlTransfer, WinUsb_GetOverlappedResult, WinUsb_ReadPipe, WinUsb_WritePipe,
@@ -22,7 +22,7 @@ use windows_sys::Win32::{
 
 use crate::transfer::{
     notify_completion, Completion, ControlIn, ControlOut, EndpointType, PlatformSubmit,
-    PlatformTransfer, RequestBuffer, ResponseBuffer, TransferError,
+    PlatformTransfer, Recipient, RequestBuffer, ResponseBuffer, TransferError,
 };
 
 use super::util::raw_handle;
@@ -228,6 +228,11 @@ impl PlatformSubmit<ControlIn> for TransferData {
     unsafe fn submit(&mut self, data: ControlIn, user_data: *mut c_void) {
         assert_eq!(self.endpoint, 0);
         assert_eq!(self.ep_type, EndpointType::Control);
+
+        if data.recipient == Recipient::Interface && data.index as u8 != self.interface.interface {
+            warn!("WinUSB sends interface number instead of passed `index` when performing a control transfer with `Recipient::Interface`");
+        }
+
         addr_of_mut!((*self.event).ptr).write(user_data);
 
         let mut buf = ManuallyDrop::new(Vec::with_capacity(data.length as usize));
@@ -270,6 +275,11 @@ impl PlatformSubmit<ControlOut<'_>> for TransferData {
     unsafe fn submit(&mut self, data: ControlOut, user_data: *mut c_void) {
         assert_eq!(self.endpoint, 0);
         assert_eq!(self.ep_type, EndpointType::Control);
+
+        if data.recipient == Recipient::Interface && data.index as u8 != self.interface.interface {
+            warn!("WinUSB sends interface number instead of passed `index` when performing a control transfer with `Recipient::Interface`");
+        }
+
         addr_of_mut!((*self.event).ptr).write(user_data);
 
         let mut buf = ManuallyDrop::new(data.data.to_vec());