usb: unify ControlHandler+DeviceStateHandler, route all control requests to all handlers.

- Allows classes to handle vendor requests.
- Allows classes to use a single handler for multiple interfaces.
- Allows classes to access the other events (previously only `reset` was available).

11 files changed, 367 insertions, 306 deletions

diff --git a/embassy-usb/Cargo.toml b/embassy-usb/Cargo.toml
index 463e1268c..ae3f3ac37 100644
--- a/embassy-usb/Cargo.toml
+++ b/embassy-usb/Cargo.toml
@@ -27,6 +27,15 @@ max-interface-count-6 = []
 max-interface-count-7 = []
 max-interface-count-8 = []
 
+max-handler-count-1 = []
+max-handler-count-2 = []
+max-handler-count-3 = []
+max-handler-count-4 = [] # Default
+max-handler-count-5 = []
+max-handler-count-6 = []
+max-handler-count-7 = []
+max-handler-count-8 = []
+
 # END AUTOGENERATED CONFIG FEATURES
 
 [dependencies]
diff --git a/embassy-usb/build.rs b/embassy-usb/build.rs
index 524bdc2f6..33d32f7d3 100644
--- a/embassy-usb/build.rs
+++ b/embassy-usb/build.rs
@@ -7,6 +7,7 @@ static CONFIGS: &[(&str, usize)] = &[
     // BEGIN AUTOGENERATED CONFIG FEATURES
     // Generated by gen_config.py. DO NOT EDIT.
     ("MAX_INTERFACE_COUNT", 4),
+    ("MAX_HANDLER_COUNT", 4),
     // END AUTOGENERATED CONFIG FEATURES
 ];
 
diff --git a/embassy-usb/gen_config.py b/embassy-usb/gen_config.py
index 55a7fa3c0..67ce359cd 100644
--- a/embassy-usb/gen_config.py
+++ b/embassy-usb/gen_config.py
@@ -28,6 +28,7 @@ def feature(name, default, min, max, pow2=None):
 
 
 feature("max_interface_count", default=4, min=1, max=8)
+feature("max_handler_count", default=4, min=1, max=8)
 
 # ========= Update Cargo.toml
 
diff --git a/embassy-usb/src/builder.rs b/embassy-usb/src/builder.rs
index d89fc4017..07b2a177c 100644
--- a/embassy-usb/src/builder.rs
+++ b/embassy-usb/src/builder.rs
@@ -1,12 +1,12 @@
 use heapless::Vec;
 
-use crate::control::ControlHandler;
+use crate::config::*;
 use crate::descriptor::{BosWriter, DescriptorWriter};
 use crate::driver::{Driver, Endpoint, EndpointType};
 #[cfg(feature = "msos-descriptor")]
 use crate::msos::{DeviceLevelDescriptor, FunctionLevelDescriptor, MsOsDescriptorWriter};
 use crate::types::*;
-use crate::{DeviceStateHandler, Interface, UsbDevice, MAX_INTERFACE_COUNT, STRING_INDEX_CUSTOM_START};
+use crate::{Handler, Interface, UsbDevice, MAX_INTERFACE_COUNT, STRING_INDEX_CUSTOM_START};
 
 #[derive(Debug, Copy, Clone)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
@@ -122,8 +122,8 @@ impl<'a> Config<'a> {
 /// [`UsbDevice`] builder.
 pub struct Builder<'d, D: Driver<'d>> {
     config: Config<'d>,
-    handler: Option<&'d dyn DeviceStateHandler>,
-    interfaces: Vec<Interface<'d>, MAX_INTERFACE_COUNT>,
+    handlers: Vec<&'d mut dyn Handler, MAX_HANDLER_COUNT>,
+    interfaces: Vec<Interface, MAX_INTERFACE_COUNT>,
     control_buf: &'d mut [u8],
 
     driver: D,
@@ -151,7 +151,6 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
         bos_descriptor_buf: &'d mut [u8],
         #[cfg(feature = "msos-descriptor")] msos_descriptor_buf: &'d mut [u8],
         control_buf: &'d mut [u8],
-        handler: Option<&'d dyn DeviceStateHandler>,
     ) -> Self {
         // Magic values specified in USB-IF ECN on IADs.
         if config.composite_with_iads
@@ -179,9 +178,9 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
 
         Builder {
             driver,
-            handler,
             config,
             interfaces: Vec::new(),
+            handlers: Vec::new(),
             control_buf,
             next_string_index: STRING_INDEX_CUSTOM_START,
 
@@ -205,7 +204,7 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
         UsbDevice::build(
             self.driver,
             self.config,
-            self.handler,
+            self.handlers,
             self.device_descriptor.into_buf(),
             self.config_descriptor.into_buf(),
             self.bos_descriptor.writer.into_buf(),
@@ -248,6 +247,26 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
         }
     }
 
+    /// Add a Handler.
+    ///
+    /// The Handler is called on some USB bus events, and to handle all control requests not already
+    /// handled by the USB stack.
+    pub fn handler(&mut self, handler: &'d mut dyn Handler) {
+        if self.handlers.push(handler).is_err() {
+            panic!(
+                "embassy-usb: handler list full. Increase the `max_handler_count` compile-time setting. Current value: {}",
+                MAX_HANDLER_COUNT
+            )
+        }
+    }
+
+    /// Allocates a new string index.
+    pub fn string(&mut self) -> StringIndex {
+        let index = self.next_string_index;
+        self.next_string_index += 1;
+        StringIndex::new(index)
+    }
+
     #[cfg(feature = "msos-descriptor")]
     /// Add an MS OS 2.0 Descriptor Set.
     ///
@@ -301,10 +320,8 @@ impl<'a, 'd, D: Driver<'d>> FunctionBuilder<'a, 'd, D> {
 
         let number = self.builder.interfaces.len() as _;
         let iface = Interface {
-            handler: None,
             current_alt_setting: 0,
             num_alt_settings: 0,
-            num_strings: 0,
         };
 
         if self.builder.interfaces.push(iface).is_err() {
@@ -350,17 +367,9 @@ impl<'a, 'd, D: Driver<'d>> InterfaceBuilder<'a, 'd, D> {
         self.interface_number
     }
 
-    pub fn handler(&mut self, handler: &'d mut dyn ControlHandler) {
-        self.builder.interfaces[self.interface_number.0 as usize].handler = Some(handler);
-    }
-
     /// Allocates a new string index.
     pub fn string(&mut self) -> StringIndex {
-        let index = self.builder.next_string_index;
-        self.builder.next_string_index += 1;
-        self.builder.interfaces[self.interface_number.0 as usize].num_strings += 1;
-
-        StringIndex::new(index)
+        self.builder.string()
     }
 
     /// Add an alternate setting to the interface and write its descriptor.
diff --git a/embassy-usb/src/class/cdc_acm.rs b/embassy-usb/src/class/cdc_acm.rs
index fb9eaeca7..ff82ad40d 100644
--- a/embassy-usb/src/class/cdc_acm.rs
+++ b/embassy-usb/src/class/cdc_acm.rs
@@ -6,10 +6,10 @@ use core::sync::atomic::{AtomicBool, Ordering};
 
 use embassy_sync::blocking_mutex::CriticalSectionMutex;
 
-use crate::control::{self, ControlHandler, InResponse, OutResponse, Request};
+use crate::control::{self, InResponse, OutResponse, Recipient, Request, RequestType};
 use crate::driver::{Driver, Endpoint, EndpointError, EndpointIn, EndpointOut};
 use crate::types::*;
-use crate::Builder;
+use crate::{Builder, Handler};
 
 /// This should be used as `device_class` when building the `UsbDevice`.
 pub const USB_CLASS_CDC: u8 = 0x02;
@@ -67,6 +67,7 @@ pub struct CdcAcmClass<'d, D: Driver<'d>> {
 }
 
 struct Control<'a> {
+    comm_if: InterfaceNumber,
     shared: &'a ControlShared,
 }
 
@@ -98,7 +99,7 @@ impl<'a> Control<'a> {
     }
 }
 
-impl<'d> ControlHandler for Control<'d> {
+impl<'d> Handler for Control<'d> {
     fn reset(&mut self) {
         let shared = self.shared();
         shared.line_coding.lock(|x| x.set(LineCoding::default()));
@@ -106,12 +107,18 @@ impl<'d> ControlHandler for Control<'d> {
         shared.rts.store(false, Ordering::Relaxed);
     }
 
-    fn control_out(&mut self, req: control::Request, data: &[u8]) -> OutResponse {
+    fn control_out(&mut self, req: control::Request, data: &[u8]) -> Option<OutResponse> {
+        if (req.request_type, req.recipient, req.index)
+            != (RequestType::Class, Recipient::Interface, self.comm_if.0 as u16)
+        {
+            return None;
+        }
+
         match req.request {
             REQ_SEND_ENCAPSULATED_COMMAND => {
                 // We don't actually support encapsulated commands but pretend we do for standards
                 // compatibility.
-                OutResponse::Accepted
+                Some(OutResponse::Accepted)
             }
             REQ_SET_LINE_CODING if data.len() >= 7 => {
                 let coding = LineCoding {
@@ -123,7 +130,7 @@ impl<'d> ControlHandler for Control<'d> {
                 self.shared().line_coding.lock(|x| x.set(coding));
                 debug!("Set line coding to: {:?}", coding);
 
-                OutResponse::Accepted
+                Some(OutResponse::Accepted)
             }
             REQ_SET_CONTROL_LINE_STATE => {
                 let dtr = (req.value & 0x0001) != 0;
@@ -134,13 +141,19 @@ impl<'d> ControlHandler for Control<'d> {
                 shared.rts.store(rts, Ordering::Relaxed);
                 debug!("Set dtr {}, rts {}", dtr, rts);
 
-                OutResponse::Accepted
+                Some(OutResponse::Accepted)
             }
-            _ => OutResponse::Rejected,
+            _ => Some(OutResponse::Rejected),
         }
     }
 
-    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
+    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> Option<InResponse<'a>> {
+        if (req.request_type, req.recipient, req.index)
+            != (RequestType::Class, Recipient::Interface, self.comm_if.0 as u16)
+        {
+            return None;
+        }
+
         match req.request {
             // REQ_GET_ENCAPSULATED_COMMAND is not really supported - it will be rejected below.
             REQ_GET_LINE_CODING if req.length == 7 => {
@@ -151,9 +164,9 @@ impl<'d> ControlHandler for Control<'d> {
                 buf[4] = coding.stop_bits as u8;
                 buf[5] = coding.parity_type as u8;
                 buf[6] = coding.data_bits;
-                InResponse::Accepted(&buf[0..7])
+                Some(InResponse::Accepted(&buf[0..7]))
             }
-            _ => InResponse::Rejected,
+            _ => Some(InResponse::Rejected),
         }
     }
 }
@@ -162,17 +175,12 @@ impl<'d, D: Driver<'d>> CdcAcmClass<'d, D> {
     /// Creates a new CdcAcmClass with the provided UsbBus and max_packet_size in bytes. For
     /// full-speed devices, max_packet_size has to be one of 8, 16, 32 or 64.
     pub fn new(builder: &mut Builder<'d, D>, state: &'d mut State<'d>, max_packet_size: u16) -> Self {
-        let control = state.control.write(Control { shared: &state.shared });
-
-        let control_shared = &state.shared;
-
         assert!(builder.control_buf_len() >= 7);
 
         let mut func = builder.function(USB_CLASS_CDC, CDC_SUBCLASS_ACM, CDC_PROTOCOL_NONE);
 
         // Control interface
         let mut iface = func.interface();
-        iface.handler(control);
         let comm_if = iface.interface_number();
         let data_if = u8::from(comm_if) + 1;
         let mut alt = iface.alt_setting(USB_CLASS_CDC, CDC_SUBCLASS_ACM, CDC_PROTOCOL_NONE, None);
@@ -213,6 +221,16 @@ impl<'d, D: Driver<'d>> CdcAcmClass<'d, D> {
         let read_ep = alt.endpoint_bulk_out(max_packet_size);
         let write_ep = alt.endpoint_bulk_in(max_packet_size);
 
+        drop(func);
+
+        let control = state.control.write(Control {
+            shared: &state.shared,
+            comm_if,
+        });
+        builder.handler(control);
+
+        let control_shared = &state.shared;
+
         CdcAcmClass {
             _comm_ep: comm_ep,
             _data_if: data_if,
diff --git a/embassy-usb/src/class/cdc_ncm/mod.rs b/embassy-usb/src/class/cdc_ncm/mod.rs
index d6c7d37e6..262499ccb 100644
--- a/embassy-usb/src/class/cdc_ncm/mod.rs
+++ b/embassy-usb/src/class/cdc_ncm/mod.rs
@@ -17,10 +17,10 @@
 use core::intrinsics::copy_nonoverlapping;
 use core::mem::{size_of, MaybeUninit};
 
-use crate::control::{self, ControlHandler, InResponse, OutResponse, Request};
+use crate::control::{self, InResponse, OutResponse, Recipient, Request, RequestType};
 use crate::driver::{Driver, Endpoint, EndpointError, EndpointIn, EndpointOut};
 use crate::types::*;
-use crate::Builder;
+use crate::{Builder, Handler};
 
 pub mod embassy_net;
 
@@ -117,8 +117,7 @@ fn byteify<T>(buf: &mut [u8], data: T) -> &[u8] {
 
 /// Internal state for the CDC-NCM class.
 pub struct State<'a> {
-    comm_control: MaybeUninit<CommControl<'a>>,
-    data_control: MaybeUninit<DataControl>,
+    control: MaybeUninit<Control<'a>>,
     shared: ControlShared,
 }
 
@@ -126,8 +125,7 @@ impl<'a> State<'a> {
     /// Create a new `State`.
     pub fn new() -> Self {
         Self {
-            comm_control: MaybeUninit::uninit(),
-            data_control: MaybeUninit::uninit(),
+            control: MaybeUninit::uninit(),
             shared: Default::default(),
         }
     }
@@ -144,29 +142,55 @@ impl Default for ControlShared {
     }
 }
 
-struct CommControl<'a> {
+struct Control<'a> {
     mac_addr_string: StringIndex,
     shared: &'a ControlShared,
     mac_addr_str: [u8; 12],
+    comm_if: InterfaceNumber,
+    data_if: InterfaceNumber,
 }
 
-impl<'d> ControlHandler for CommControl<'d> {
-    fn control_out(&mut self, req: control::Request, _data: &[u8]) -> OutResponse {
+impl<'d> Handler for Control<'d> {
+    fn set_alternate_setting(&mut self, iface: InterfaceNumber, alternate_setting: u8) {
+        if iface != self.data_if {
+            return;
+        }
+
+        match alternate_setting {
+            ALTERNATE_SETTING_ENABLED => info!("ncm: interface enabled"),
+            ALTERNATE_SETTING_DISABLED => info!("ncm: interface disabled"),
+            _ => unreachable!(),
+        }
+    }
+
+    fn control_out(&mut self, req: control::Request, _data: &[u8]) -> Option<OutResponse> {
+        if (req.request_type, req.recipient, req.index)
+            != (RequestType::Class, Recipient::Interface, self.comm_if.0 as u16)
+        {
+            return None;
+        }
+
         match req.request {
             REQ_SEND_ENCAPSULATED_COMMAND => {
                 // We don't actually support encapsulated commands but pretend we do for standards
                 // compatibility.
-                OutResponse::Accepted
+                Some(OutResponse::Accepted)
             }
             REQ_SET_NTB_INPUT_SIZE => {
                 // TODO
-                OutResponse::Accepted
+                Some(OutResponse::Accepted)
             }
-            _ => OutResponse::Rejected,
+            _ => Some(OutResponse::Rejected),
         }
     }
 
-    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
+    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> Option<InResponse<'a>> {
+        if (req.request_type, req.recipient, req.index)
+            != (RequestType::Class, Recipient::Interface, self.comm_if.0 as u16)
+        {
+            return None;
+        }
+
         match req.request {
             REQ_GET_NTB_PARAMETERS => {
                 let res = NtbParameters {
@@ -187,9 +211,9 @@ impl<'d> ControlHandler for CommControl<'d> {
                         max_datagram_count: 1, // We only decode 1 packet per NTB
                     },
                 };
-                InResponse::Accepted(byteify(buf, res))
+                Some(InResponse::Accepted(byteify(buf, res)))
             }
-            _ => InResponse::Rejected,
+            _ => Some(InResponse::Rejected),
         }
     }
 
@@ -214,18 +238,6 @@ impl<'d> ControlHandler for CommControl<'d> {
     }
 }
 
-struct DataControl {}
-
-impl ControlHandler for DataControl {
-    fn set_alternate_setting(&mut self, alternate_setting: u8) {
-        match alternate_setting {
-            ALTERNATE_SETTING_ENABLED => info!("ncm: interface enabled"),
-            ALTERNATE_SETTING_DISABLED => info!("ncm: interface disabled"),
-            _ => unreachable!(),
-        }
-    }
-}
-
 /// CDC-NCM class
 pub struct CdcNcmClass<'d, D: Driver<'d>> {
     _comm_if: InterfaceNumber,
@@ -253,11 +265,6 @@ impl<'d, D: Driver<'d>> CdcNcmClass<'d, D> {
         // Control interface
         let mut iface = func.interface();
         let mac_addr_string = iface.string();
-        iface.handler(state.comm_control.write(CommControl {
-            mac_addr_string,
-            shared: &state.shared,
-            mac_addr_str: [0; 12],
-        }));
         let comm_if = iface.interface_number();
         let mut alt = iface.alt_setting(USB_CLASS_CDC, CDC_SUBCLASS_NCM, CDC_PROTOCOL_NONE, None);
 
@@ -307,13 +314,23 @@ impl<'d, D: Driver<'d>> CdcNcmClass<'d, D> {
 
         // Data interface
         let mut iface = func.interface();
-        iface.handler(state.data_control.write(DataControl {}));
         let data_if = iface.interface_number();
         let _alt = iface.alt_setting(USB_CLASS_CDC_DATA, 0x00, CDC_PROTOCOL_NTB, None);
         let mut alt = iface.alt_setting(USB_CLASS_CDC_DATA, 0x00, CDC_PROTOCOL_NTB, None);
         let read_ep = alt.endpoint_bulk_out(max_packet_size);
         let write_ep = alt.endpoint_bulk_in(max_packet_size);
 
+        drop(func);
+
+        let control = state.control.write(Control {
+            mac_addr_string,
+            shared: &state.shared,
+            mac_addr_str: [0; 12],
+            comm_if,
+            data_if,
+        });
+        builder.handler(control);
+
         CdcNcmClass {
             _comm_if: comm_if,
             comm_ep,
diff --git a/embassy-usb/src/class/hid.rs b/embassy-usb/src/class/hid.rs
index 0283c1124..974268c62 100644
--- a/embassy-usb/src/class/hid.rs
+++ b/embassy-usb/src/class/hid.rs
@@ -9,9 +9,10 @@ use ssmarshal::serialize;
 #[cfg(feature = "usbd-hid")]
 use usbd_hid::descriptor::AsInputReport;
 
-use crate::control::{ControlHandler, InResponse, OutResponse, Request, RequestType};
+use crate::control::{InResponse, OutResponse, Recipient, Request, RequestType};
 use crate::driver::{Driver, Endpoint, EndpointError, EndpointIn, EndpointOut};
-use crate::Builder;
+use crate::types::InterfaceNumber;
+use crate::{Builder, Handler};
 
 const USB_CLASS_HID: u8 = 0x03;
 const USB_SUBCLASS_NONE: u8 = 0x00;
@@ -100,17 +101,11 @@ fn build<'d, D: Driver<'d>>(
     config: Config<'d>,
     with_out_endpoint: bool,
 ) -> (Option<D::EndpointOut>, D::EndpointIn, &'d AtomicUsize) {
-    let control = state.control.write(Control::new(
-        config.report_descriptor,
-        config.request_handler,
-        &state.out_report_offset,
-    ));
-
     let len = config.report_descriptor.len();
 
     let mut func = builder.function(USB_CLASS_HID, USB_SUBCLASS_NONE, USB_PROTOCOL_NONE);
     let mut iface = func.interface();
-    iface.handler(control);
+    let if_num = iface.interface_number();
     let mut alt = iface.alt_setting(USB_CLASS_HID, USB_SUBCLASS_NONE, USB_PROTOCOL_NONE, None);
 
     // HID descriptor
@@ -139,6 +134,16 @@ fn build<'d, D: Driver<'d>>(
         None
     };
 
+    drop(func);
+
+    let control = state.control.write(Control::new(
+        if_num,
+        config.report_descriptor,
+        config.request_handler,
+        &state.out_report_offset,
+    ));
+    builder.handler(control);
+
     (ep_out, ep_in, &state.out_report_offset)
 }
 
@@ -400,6 +405,7 @@ pub trait RequestHandler {
 }
 
 struct Control<'d> {
+    if_num: InterfaceNumber,
     report_descriptor: &'d [u8],
     request_handler: Option<&'d dyn RequestHandler>,
     out_report_offset: &'d AtomicUsize,
@@ -408,11 +414,13 @@ struct Control<'d> {
 
 impl<'d> Control<'d> {
     fn new(
+        if_num: InterfaceNumber,
         report_descriptor: &'d [u8],
         request_handler: Option<&'d dyn RequestHandler>,
         out_report_offset: &'d AtomicUsize,
     ) -> Self {
         Control {
+            if_num,
             report_descriptor,
             request_handler,
             out_report_offset,
@@ -438,88 +446,100 @@ impl<'d> Control<'d> {
     }
 }
 
-impl<'d> ControlHandler for Control<'d> {
+impl<'d> Handler for Control<'d> {
     fn reset(&mut self) {
         self.out_report_offset.store(0, Ordering::Release);
     }
 
-    fn get_descriptor<'a>(&'a mut self, req: Request, _buf: &'a mut [u8]) -> InResponse<'a> {
-        match (req.value >> 8) as u8 {
-            HID_DESC_DESCTYPE_HID_REPORT => InResponse::Accepted(self.report_descriptor),
-            HID_DESC_DESCTYPE_HID => InResponse::Accepted(&self.hid_descriptor),
-            _ => InResponse::Rejected,
+    fn control_out(&mut self, req: Request, data: &[u8]) -> Option<OutResponse> {
+        if (req.request_type, req.recipient, req.index)
+            != (RequestType::Class, Recipient::Interface, self.if_num.0 as u16)
+        {
+            return None;
         }
-    }
 
-    fn control_out(&mut self, req: Request, data: &[u8]) -> OutResponse {
         trace!("HID control_out {:?} {=[u8]:x}", req, data);
-        if let RequestType::Class = req.request_type {
-            match req.request {
-                HID_REQ_SET_IDLE => {
-                    if let Some(handler) = self.request_handler {
-                        let id = req.value as u8;
-                        let id = (id != 0).then(|| ReportId::In(id));
-                        let dur = u32::from(req.value >> 8);
-                        let dur = if dur == 0 { u32::MAX } else { 4 * dur };
-                        handler.set_idle_ms(id, dur);
-                    }
-                    OutResponse::Accepted
+        match req.request {
+            HID_REQ_SET_IDLE => {
+                if let Some(handler) = self.request_handler {
+                    let id = req.value as u8;
+                    let id = (id != 0).then(|| ReportId::In(id));
+                    let dur = u32::from(req.value >> 8);
+                    let dur = if dur == 0 { u32::MAX } else { 4 * dur };
+                    handler.set_idle_ms(id, dur);
                 }
-                HID_REQ_SET_REPORT => match (ReportId::try_from(req.value), self.request_handler) {
-                    (Ok(id), Some(handler)) => handler.set_report(id, data),
-                    _ => OutResponse::Rejected,
-                },
-                HID_REQ_SET_PROTOCOL => {
-                    if req.value == 1 {
-                        OutResponse::Accepted
-                    } else {
-                        warn!("HID Boot Protocol is unsupported.");
-                        OutResponse::Rejected // UNSUPPORTED: Boot Protocol
-                    }
+                Some(OutResponse::Accepted)
+            }
+            HID_REQ_SET_REPORT => match (ReportId::try_from(req.value), self.request_handler) {
+                (Ok(id), Some(handler)) => Some(handler.set_report(id, data)),
+                _ => Some(OutResponse::Rejected),
+            },
+            HID_REQ_SET_PROTOCOL => {
+                if req.value == 1 {
+                    Some(OutResponse::Accepted)
+                } else {
+                    warn!("HID Boot Protocol is unsupported.");
+                    Some(OutResponse::Rejected) // UNSUPPORTED: Boot Protocol
                 }
-                _ => OutResponse::Rejected,
             }
-        } else {
-            OutResponse::Rejected // UNSUPPORTED: SET_DESCRIPTOR
+            _ => Some(OutResponse::Rejected),
         }
     }
 
-    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
-        trace!("HID control_in {:?}", req);
-        match req.request {
-            HID_REQ_GET_REPORT => {
-                let size = match ReportId::try_from(req.value) {
-                    Ok(id) => self.request_handler.and_then(|x| x.get_report(id, buf)),
-                    Err(_) => None,
-                };
-
-                if let Some(size) = size {
-                    InResponse::Accepted(&buf[0..size])
-                } else {
-                    InResponse::Rejected
-                }
-            }
-            HID_REQ_GET_IDLE => {
-                if let Some(handler) = self.request_handler {
-                    let id = req.value as u8;
-                    let id = (id != 0).then(|| ReportId::In(id));
-                    if let Some(dur) = handler.get_idle_ms(id) {
-                        let dur = u8::try_from(dur / 4).unwrap_or(0);
-                        buf[0] = dur;
-                        InResponse::Accepted(&buf[0..1])
-                    } else {
-                        InResponse::Rejected
+    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> Option<InResponse<'a>> {
+        if req.index != self.if_num.0 as u16 {
+            return None;
+        }
+
+        match (req.request_type, req.recipient) {
+            (RequestType::Standard, Recipient::Interface) => match req.request {
+                Request::GET_DESCRIPTOR => match (req.value >> 8) as u8 {
+                    HID_DESC_DESCTYPE_HID_REPORT => Some(InResponse::Accepted(self.report_descriptor)),
+                    HID_DESC_DESCTYPE_HID => Some(InResponse::Accepted(&self.hid_descriptor)),
+                    _ => Some(InResponse::Rejected),
+                },
+
+                _ => Some(InResponse::Rejected),
+            },
+            (RequestType::Class, Recipient::Interface) => {
+                trace!("HID control_in {:?}", req);
+                match req.request {
+                    HID_REQ_GET_REPORT => {
+                        let size = match ReportId::try_from(req.value) {
+                            Ok(id) => self.request_handler.and_then(|x| x.get_report(id, buf)),
+                            Err(_) => None,
+                        };
+
+                        if let Some(size) = size {
+                            Some(InResponse::Accepted(&buf[0..size]))
+                        } else {
+                            Some(InResponse::Rejected)
+                        }
                     }
-                } else {
-                    InResponse::Rejected
+                    HID_REQ_GET_IDLE => {
+                        if let Some(handler) = self.request_handler {
+                            let id = req.value as u8;
+                            let id = (id != 0).then(|| ReportId::In(id));
+                            if let Some(dur) = handler.get_idle_ms(id) {
+                                let dur = u8::try_from(dur / 4).unwrap_or(0);
+                                buf[0] = dur;
+                                Some(InResponse::Accepted(&buf[0..1]))
+                            } else {
+                                Some(InResponse::Rejected)
+                            }
+                        } else {
+                            Some(InResponse::Rejected)
+                        }
+                    }
+                    HID_REQ_GET_PROTOCOL => {
+                        // UNSUPPORTED: Boot Protocol
+                        buf[0] = 1;
+                        Some(InResponse::Accepted(&buf[0..1]))
+                    }
+                    _ => Some(InResponse::Rejected),
                 }
             }
-            HID_REQ_GET_PROTOCOL => {
-                // UNSUPPORTED: Boot Protocol
-                buf[0] = 1;
-                InResponse::Accepted(&buf[0..1])
-            }
-            _ => InResponse::Rejected,
+            _ => None,
         }
     }
 }
diff --git a/embassy-usb/src/control.rs b/embassy-usb/src/control.rs
index 39b499f03..ceccfd85b 100644
--- a/embassy-usb/src/control.rs
+++ b/embassy-usb/src/control.rs
@@ -2,7 +2,6 @@
 use core::mem;
 
 use crate::driver::Direction;
-use crate::types::StringIndex;
 
 /// Control request type.
 #[repr(u8)]
@@ -145,60 +144,3 @@ pub enum InResponse<'a> {
     /// The request was rejected.
     Rejected,
 }
-
-/// Handler for control requests.
-///
-/// All methods are optional callbacks that will be called by
-/// [`UsbDevice::run()`](crate::UsbDevice::run)
-pub trait ControlHandler {
-    /// Called after a USB reset after the bus reset sequence is complete.
-    fn reset(&mut self) {}
-
-    /// Called when a "set alternate setting" control request is done on the interface.
-    fn set_alternate_setting(&mut self, alternate_setting: u8) {
-        let _ = alternate_setting;
-    }
-
-    /// Called when a control request is received with direction HostToDevice.
-    ///
-    /// # Arguments
-    ///
-    /// * `req` - The request from the SETUP packet.
-    /// * `data` - The data from the request.
-    fn control_out(&mut self, req: Request, data: &[u8]) -> OutResponse {
-        let _ = (req, data);
-        OutResponse::Rejected
-    }
-
-    /// Called when a control request is received with direction DeviceToHost.
-    ///
-    /// You should write the response somewhere (usually to `buf`, but you may use another buffer
-    /// owned by yourself, or a static buffer), then return `InResponse::Accepted(data)`.
-    ///
-    /// # Arguments
-    ///
-    /// * `req` - The request from the SETUP packet.
-    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
-        let _ = (req, buf);
-        InResponse::Rejected
-    }
-
-    /// Called when a GET DESCRIPTOR control request is received on the interface.
-    ///
-    /// You should write the response somewhere (usually to `buf`, but you may use another buffer
-    /// owned by yourself, or a static buffer), then return `InResponse::Accepted(data)`.
-    ///
-    /// # Arguments
-    ///
-    /// * `req` - The request from the SETUP packet.
-    fn get_descriptor<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
-        let _ = (req, buf);
-        InResponse::Rejected
-    }
-
-    /// Called when a GET_DESCRIPTOR STRING control request is received.
-    fn get_string(&mut self, index: StringIndex, lang_id: u16) -> Option<&str> {
-        let _ = (index, lang_id);
-        None
-    }
-}
diff --git a/embassy-usb/src/descriptor_reader.rs b/embassy-usb/src/descriptor_reader.rs
index d64bcb73b..05adcce60 100644
--- a/embassy-usb/src/descriptor_reader.rs
+++ b/embassy-usb/src/descriptor_reader.rs
@@ -1,5 +1,6 @@
 use crate::descriptor::descriptor_type;
 use crate::driver::EndpointAddress;
+use crate::types::InterfaceNumber;
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
@@ -75,7 +76,7 @@ impl<'a, 'b> Iterator for DescriptorIter<'a, 'b> {
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
 pub struct EndpointInfo {
     pub configuration: u8,
-    pub interface: u8,
+    pub interface: InterfaceNumber,
     pub interface_alt: u8,
     pub ep_address: EndpointAddress,
 }
@@ -83,7 +84,7 @@ pub struct EndpointInfo {
 pub fn foreach_endpoint(data: &[u8], mut f: impl FnMut(EndpointInfo)) -> Result<(), ReadError> {
     let mut ep = EndpointInfo {
         configuration: 0,
-        interface: 0,
+        interface: InterfaceNumber(0),
         interface_alt: 0,
         ep_address: EndpointAddress::from(0),
     };
@@ -96,7 +97,7 @@ pub fn foreach_endpoint(data: &[u8], mut f: impl FnMut(EndpointInfo)) -> Result<
                 ep.configuration = r.read_u8()?;
             }
             descriptor_type::INTERFACE => {
-                ep.interface = r.read_u8()?;
+                ep.interface = InterfaceNumber(r.read_u8()?);
                 ep.interface_alt = r.read_u8()?;
             }
             descriptor_type::ENDPOINT => {
diff --git a/embassy-usb/src/lib.rs b/embassy-usb/src/lib.rs
index f8983318e..bfeccd5fe 100644
--- a/embassy-usb/src/lib.rs
+++ b/embassy-usb/src/lib.rs
@@ -82,32 +82,87 @@ const STRING_INDEX_PRODUCT: u8 = 2;
 const STRING_INDEX_SERIAL_NUMBER: u8 = 3;
 const STRING_INDEX_CUSTOM_START: u8 = 4;
 
-/// A handler trait for changes in the device state of the [UsbDevice].
-pub trait DeviceStateHandler {
+/// Handler for device events and control requests.
+///
+/// All methods are optional callbacks that will be called by
+/// [`UsbDevice::run()`](crate::UsbDevice::run)
+pub trait Handler {
     /// Called when the USB device has been enabled or disabled.
-    fn enabled(&self, _enabled: bool) {}
+    fn enabled(&mut self, _enabled: bool) {}
 
-    /// Called when the host resets the device.
-    fn reset(&self) {}
+    /// Called after a USB reset after the bus reset sequence is complete.
+    fn reset(&mut self) {}
 
     /// Called when the host has set the address of the device to `addr`.
-    fn addressed(&self, _addr: u8) {}
+    fn addressed(&mut self, _addr: u8) {}
 
     /// Called when the host has enabled or disabled the configuration of the device.
-    fn configured(&self, _configured: bool) {}
+    fn configured(&mut self, _configured: bool) {}
 
     /// Called when the bus has entered or exited the suspend state.
-    fn suspended(&self, _suspended: bool) {}
+    fn suspended(&mut self, _suspended: bool) {}
 
     /// Called when remote wakeup feature is enabled or disabled.
-    fn remote_wakeup_enabled(&self, _enabled: bool) {}
+    fn remote_wakeup_enabled(&mut self, _enabled: bool) {}
+
+    /// Called when a "set alternate setting" control request is done on the interface.
+    fn set_alternate_setting(&mut self, iface: InterfaceNumber, alternate_setting: u8) {
+        let _ = iface;
+        let _ = alternate_setting;
+    }
+
+    /// Called when a control request is received with direction HostToDevice.
+    ///
+    /// # Arguments
+    ///
+    /// * `req` - The request from the SETUP packet.
+    /// * `data` - The data from the request.
+    ///
+    /// # Returns
+    ///
+    /// If you didn't handle this request (for example if it's for the wrong interface), return
+    /// `None`. In this case, the the USB stack will continue calling the other handlers, to see
+    /// if another handles it.
+    ///
+    /// If you did, return `Some` with either `Accepted` or `Rejected`. This will make the USB stack
+    /// respond to the control request, and stop calling other handlers.
+    fn control_out(&mut self, req: Request, data: &[u8]) -> Option<OutResponse> {
+        let _ = (req, data);
+        None
+    }
+
+    /// Called when a control request is received with direction DeviceToHost.
+    ///
+    /// You should write the response somewhere (usually to `buf`, but you may use another buffer
+    /// owned by yourself, or a static buffer), then return `InResponse::Accepted(data)`.
+    ///
+    /// # Arguments
+    ///
+    /// * `req` - The request from the SETUP packet.
+    ///
+    /// # Returns
+    ///
+    /// If you didn't handle this request (for example if it's for the wrong interface), return
+    /// `None`. In this case, the the USB stack will continue calling the other handlers, to see
+    /// if another handles it.
+    ///
+    /// If you did, return `Some` with either `Accepted` or `Rejected`. This will make the USB stack
+    /// respond to the control request, and stop calling other handlers.
+    fn control_in<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> Option<InResponse<'a>> {
+        let _ = (req, buf);
+        None
+    }
+
+    /// Called when a GET_DESCRIPTOR STRING control request is received.
+    fn get_string(&mut self, index: StringIndex, lang_id: u16) -> Option<&str> {
+        let _ = (index, lang_id);
+        None
+    }
 }
 
-struct Interface<'d> {
-    handler: Option<&'d mut dyn ControlHandler>,
+struct Interface {
     current_alt_setting: u8,
     num_alt_settings: u8,
-    num_strings: u8,
 }
 
 /// Main struct for the USB device stack.
@@ -119,7 +174,6 @@ pub struct UsbDevice<'d, D: Driver<'d>> {
 
 struct Inner<'d, D: Driver<'d>> {
     bus: D::Bus,
-    handler: Option<&'d dyn DeviceStateHandler>,
 
     config: Config<'d>,
     device_descriptor: &'d [u8],
@@ -138,7 +192,9 @@ struct Inner<'d, D: Driver<'d>> {
     /// instead of regular `accept()`.
     set_address_pending: bool,
 
-    interfaces: Vec<Interface<'d>, MAX_INTERFACE_COUNT>,
+    interfaces: Vec<Interface, MAX_INTERFACE_COUNT>,
+    handlers: Vec<&'d mut dyn Handler, MAX_HANDLER_COUNT>,
+
     #[cfg(feature = "msos-descriptor")]
     msos_descriptor: crate::msos::MsOsDescriptorSet<'d>,
 }
@@ -147,11 +203,11 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
     pub(crate) fn build(
         driver: D,
         config: Config<'d>,
-        handler: Option<&'d dyn DeviceStateHandler>,
+        handlers: Vec<&'d mut dyn Handler, MAX_HANDLER_COUNT>,
         device_descriptor: &'d [u8],
         config_descriptor: &'d [u8],
         bos_descriptor: &'d [u8],
-        interfaces: Vec<Interface<'d>, MAX_INTERFACE_COUNT>,
+        interfaces: Vec<Interface, MAX_INTERFACE_COUNT>,
         control_buf: &'d mut [u8],
         #[cfg(feature = "msos-descriptor")] msos_descriptor: crate::msos::MsOsDescriptorSet<'d>,
     ) -> UsbDevice<'d, D> {
@@ -165,7 +221,6 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
             inner: Inner {
                 bus,
                 config,
-                handler,
                 device_descriptor,
                 config_descriptor,
                 bos_descriptor,
@@ -177,6 +232,7 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
                 address: 0,
                 set_address_pending: false,
                 interfaces,
+                handlers,
                 #[cfg(feature = "msos-descriptor")]
                 msos_descriptor,
             },
@@ -221,7 +277,7 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
             self.inner.suspended = false;
             self.inner.remote_wakeup_enabled = false;
 
-            if let Some(h) = &self.inner.handler {
+            for h in &mut self.inner.handlers {
                 h.enabled(false);
             }
         }
@@ -250,7 +306,7 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
             self.inner.bus.remote_wakeup().await?;
             self.inner.suspended = false;
 
-            if let Some(h) = &self.inner.handler {
+            for h in &mut self.inner.handlers {
                 h.suspended(false);
             }
 
@@ -361,29 +417,29 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                 self.remote_wakeup_enabled = false;
                 self.address = 0;
 
-                for iface in self.interfaces.iter_mut() {
-                    iface.current_alt_setting = 0;
-                    if let Some(h) = &mut iface.handler {
-                        h.reset();
-                        h.set_alternate_setting(0);
-                    }
+                for h in &mut self.handlers {
+                    h.reset();
                 }
 
-                if let Some(h) = &self.handler {
-                    h.reset();
+                for (i, iface) in self.interfaces.iter_mut().enumerate() {
+                    iface.current_alt_setting = 0;
+
+                    for h in &mut self.handlers {
+                        h.set_alternate_setting(InterfaceNumber::new(i as _), 0);
+                    }
                 }
             }
             Event::Resume => {
                 trace!("usb: resume");
                 self.suspended = false;
-                if let Some(h) = &self.handler {
+                for h in &mut self.handlers {
                     h.suspended(false);
                 }
             }
             Event::Suspend => {
                 trace!("usb: suspend");
                 self.suspended = true;
-                if let Some(h) = &self.handler {
+                for h in &mut self.handlers {
                     h.suspended(true);
                 }
             }
@@ -392,7 +448,7 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                 self.bus.enable().await;
                 self.device_state = UsbDeviceState::Default;
 
-                if let Some(h) = &self.handler {
+                for h in &mut self.handlers {
                     h.enabled(true);
                 }
             }
@@ -401,7 +457,7 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                 self.bus.disable().await;
                 self.device_state = UsbDeviceState::Unpowered;
 
-                if let Some(h) = &self.handler {
+                for h in &mut self.handlers {
                     h.enabled(false);
                 }
             }
@@ -416,14 +472,14 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
             (RequestType::Standard, Recipient::Device) => match (req.request, req.value) {
                 (Request::CLEAR_FEATURE, Request::FEATURE_DEVICE_REMOTE_WAKEUP) => {
                     self.remote_wakeup_enabled = false;
-                    if let Some(h) = &self.handler {
+                    for h in &mut self.handlers {
                         h.remote_wakeup_enabled(false);
                     }
                     OutResponse::Accepted
                 }
                 (Request::SET_FEATURE, Request::FEATURE_DEVICE_REMOTE_WAKEUP) => {
                     self.remote_wakeup_enabled = true;
-                    if let Some(h) = &self.handler {
+                    for h in &mut self.handlers {
                         h.remote_wakeup_enabled(true);
                     }
                     OutResponse::Accepted
@@ -432,7 +488,7 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                     self.address = addr as u8;
                     self.set_address_pending = true;
                     self.device_state = UsbDeviceState::Addressed;
-                    if let Some(h) = &self.handler {
+                    for h in &mut self.handlers {
                         h.addressed(self.address);
                     }
                     OutResponse::Accepted
@@ -443,14 +499,14 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
 
                     // Enable all endpoints of selected alt settings.
                     foreach_endpoint(self.config_descriptor, |ep| {
-                        let iface = &self.interfaces[ep.interface as usize];
+                        let iface = &self.interfaces[ep.interface.0 as usize];
                         self.bus
                             .endpoint_set_enabled(ep.ep_address, iface.current_alt_setting == ep.interface_alt);
                     })
                     .unwrap();
 
-                    // Notify handler.
-                    if let Some(h) = &self.handler {
+                    // Notify handlers.
+                    for h in &mut self.handlers {
                         h.configured(true);
                     }
 
@@ -468,8 +524,8 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                         })
                         .unwrap();
 
-                        // Notify handler.
-                        if let Some(h) = &self.handler {
+                        // Notify handlers.
+                        for h in &mut self.handlers {
                             h.configured(false);
                         }
 
@@ -479,7 +535,8 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                 _ => OutResponse::Rejected,
             },
             (RequestType::Standard, Recipient::Interface) => {
-                let iface = match self.interfaces.get_mut(req.index as usize) {
+                let iface_num = InterfaceNumber::new(req.index as _);
+                let iface = match self.interfaces.get_mut(iface_num.0 as usize) {
                     Some(iface) => iface,
                     None => return OutResponse::Rejected,
                 };
@@ -497,7 +554,7 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
 
                         // Enable/disable EPs of this interface as needed.
                         foreach_endpoint(self.config_descriptor, |ep| {
-                            if ep.interface == req.index as u8 {
+                            if ep.interface == iface_num {
                                 self.bus
                                     .endpoint_set_enabled(ep.ep_address, iface.current_alt_setting == ep.interface_alt);
                             }
@@ -506,8 +563,8 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
 
                         // TODO check it is valid (not out of range)
 
-                        if let Some(handler) = &mut iface.handler {
-                            handler.set_alternate_setting(new_altsetting);
+                        for h in &mut self.handlers {
+                            h.set_alternate_setting(iface_num, new_altsetting);
                         }
                         OutResponse::Accepted
                     }
@@ -527,17 +584,7 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                 }
                 _ => OutResponse::Rejected,
             },
-            (RequestType::Class, Recipient::Interface) => {
-                let iface = match self.interfaces.get_mut(req.index as usize) {
-                    Some(iface) => iface,
-                    None => return OutResponse::Rejected,
-                };
-                match &mut iface.handler {
-                    Some(handler) => handler.control_out(req, data),
-                    None => OutResponse::Rejected,
-                }
-            }
-            _ => OutResponse::Rejected,
+            _ => self.handle_control_out_delegated(req, data),
         }
     }
 
@@ -582,11 +629,7 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                         buf[0] = iface.current_alt_setting;
                         InResponse::Accepted(&buf[..1])
                     }
-                    Request::GET_DESCRIPTOR => match &mut iface.handler {
-                        Some(handler) => handler.get_descriptor(req, buf),
-                        None => InResponse::Rejected,
-                    },
-                    _ => InResponse::Rejected,
+                    _ => self.handle_control_in_delegated(req, buf),
                 }
             }
             (RequestType::Standard, Recipient::Endpoint) => match req.request {
@@ -601,32 +644,46 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                 }
                 _ => InResponse::Rejected,
             },
-            (RequestType::Class, Recipient::Interface) => {
-                let iface = match self.interfaces.get_mut(req.index as usize) {
-                    Some(iface) => iface,
-                    None => return InResponse::Rejected,
-                };
-
-                match &mut iface.handler {
-                    Some(handler) => handler.control_in(req, buf),
-                    None => InResponse::Rejected,
-                }
-            }
             #[cfg(feature = "msos-descriptor")]
             (RequestType::Vendor, Recipient::Device) => {
-                if !self.msos_descriptor.is_empty() {
-                    if req.request == self.msos_descriptor.vendor_code() && req.index == 7 {
-                        // Index 7 retrieves the MS OS Descriptor Set
-                        InResponse::Accepted(self.msos_descriptor.descriptor())
-                    } else {
-                        InResponse::Rejected
-                    }
+                if !self.msos_descriptor.is_empty()
+                    && req.request == self.msos_descriptor.vendor_code()
+                    && req.index == 7
+                {
+                    // Index 7 retrieves the MS OS Descriptor Set
+                    InResponse::Accepted(self.msos_descriptor.descriptor())
                 } else {
-                    InResponse::Rejected
+                    self.handle_control_in_delegated(req, buf)
                 }
             }
-            _ => InResponse::Rejected,
+            _ => self.handle_control_in_delegated(req, buf),
+        }
+    }
+
+    fn handle_control_out_delegated(&mut self, req: Request, data: &[u8]) -> OutResponse {
+        for h in &mut self.handlers {
+            if let Some(res) = h.control_out(req, data) {
+                return res;
+            }
         }
+        OutResponse::Rejected
+    }
+
+    fn handle_control_in_delegated<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
+        unsafe fn extend_lifetime<'x, 'y>(r: InResponse<'x>) -> InResponse<'y> {
+            core::mem::transmute(r)
+        }
+
+        for h in &mut self.handlers {
+            if let Some(res) = h.control_in(req, buf) {
+                // safety: the borrow checker isn't smart enough to know this pattern (returning a
+                // borrowed value from inside the loop) is sound. Workaround by unsafely extending lifetime.
+                // Also, Polonius (the WIP new borrow checker) does accept it.
+
+                return unsafe { extend_lifetime(res) };
+            }
+        }
+        InResponse::Rejected
     }
 
     fn handle_get_descriptor<'a>(&'a mut self, req: Request, buf: &'a mut [u8]) -> InResponse<'a> {
@@ -649,30 +706,16 @@ impl<'d, D: Driver<'d>> Inner<'d, D> {
                         STRING_INDEX_PRODUCT => self.config.product,
                         STRING_INDEX_SERIAL_NUMBER => self.config.serial_number,
                         _ => {
-                            // Find out which iface owns this string index.
-                            let mut index_left = index - STRING_INDEX_CUSTOM_START;
-                            let mut the_iface = None;
-                            for iface in &mut self.interfaces {
-                                if index_left < iface.num_strings {
-                                    the_iface = Some(iface);
+                            let mut s = None;
+                            for handler in &mut self.handlers {
+                                let index = StringIndex::new(index);
+                                let lang_id = req.index;
+                                if let Some(res) = handler.get_string(index, lang_id) {
+                                    s = Some(res);
                                     break;
                                 }
-                                index_left -= iface.num_strings;
-                            }
-
-                            if let Some(iface) = the_iface {
-                                if let Some(handler) = &mut iface.handler {
-                                    let index = StringIndex::new(index);
-                                    let lang_id = req.index;
-                                    handler.get_string(index, lang_id)
-                                } else {
-                                    warn!("String requested to an interface with no handler.");
-                                    None
-                                }
-                            } else {
-                                warn!("String requested but didn't match to an interface.");
-                                None
                             }
+                            s
                         }
                     };
 
diff --git a/embassy-usb/src/types.rs b/embassy-usb/src/types.rs
index 1743e61ff..15d195002 100644
--- a/embassy-usb/src/types.rs
+++ b/embassy-usb/src/types.rs
@@ -1,9 +1,9 @@
 //! USB types.
 
 /// A handle for a USB interface that contains its number.
-#[derive(Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
-pub struct InterfaceNumber(pub(crate) u8);
+pub struct InterfaceNumber(pub u8);
 
 impl InterfaceNumber {
     pub(crate) fn new(index: u8) -> InterfaceNumber {
@@ -20,7 +20,7 @@ impl From<InterfaceNumber> for u8 {
 /// A handle for a USB string descriptor that contains its index.
 #[derive(Copy, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
-pub struct StringIndex(u8);
+pub struct StringIndex(pub u8);
 
 impl StringIndex {
     pub(crate) fn new(index: u8) -> StringIndex {