usb: add support for custom string descriptors.

embassy-usb/src/builder.rs

@@ -1,6 +1,6 @@
 use heapless::Vec;
 
-use crate::Interface;
+use crate::{Interface, STRING_INDEX_CUSTOM_START};
 
 use super::control::ControlHandler;
 use super::descriptor::{BosWriter, DescriptorWriter};
@@ -181,7 +181,7 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
             config,
             interfaces: Vec::new(),
             control_buf,
-            next_string_index: 4,
+            next_string_index: STRING_INDEX_CUSTOM_START,
 
             device_descriptor,
             config_descriptor,
@@ -212,14 +212,6 @@ impl<'d, D: Driver<'d>> Builder<'d, D> {
         self.control_buf.len()
     }
 
-    /// Allocates a new string index.
-    pub fn alloc_string(&mut self) -> StringIndex {
-        let index = self.next_string_index;
-        self.next_string_index += 1;
-
-        StringIndex::new(index)
-    }
-
     /// Add an USB function.
     ///
     /// If [`Config::composite_with_iads`] is set, this will add an IAD descriptor
@@ -277,6 +269,7 @@ impl<'a, 'd, D: Driver<'d>> FunctionBuilder<'a, 'd, D> {
             handler: None,
             current_alt_setting: 0,
             num_alt_settings: 0,
+            num_strings: 0,
         };
 
         if self.builder.interfaces.push(iface).is_err() {
@@ -308,6 +301,15 @@ impl<'a, 'd, D: Driver<'d>> InterfaceBuilder<'a, 'd, D> {
         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)
+    }
+
     /// Add an alternate setting to the interface and write its descriptor.
     ///
     /// Alternate setting numbers are guaranteed to be allocated consecutively, starting from 0.

embassy-usb/src/control.rs

@@ -189,6 +189,20 @@ pub trait ControlHandler {
         let _ = (req, buf);
         InResponse::Rejected
     }
+
+    /// Called when a GET_DESCRIPTOR STRING control request is received.
+    ///
+    /// Write the response string somewhere (usually to `buf`, but you may use another buffer
+    /// owned by yourself, or a static buffer), then return it.
+    fn get_string<'a>(
+        &'a mut self,
+        index: StringIndex,
+        lang_id: u16,
+        buf: &'a mut [u8],
+    ) -> Option<&'a str> {
+        let _ = (index, lang_id, buf);
+        None
+    }
 }
 
 /// Typestate representing a ControlPipe in the DATA IN stage

embassy-usb/src/lib.rs

@@ -66,6 +66,11 @@ pub const CONFIGURATION_VALUE: u8 = 1;
 
 pub const MAX_INTERFACE_COUNT: usize = 4;
 
+const STRING_INDEX_MANUFACTURER: u8 = 1;
+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 {
     /// Called when the USB device has been enabled or disabled.
@@ -91,6 +96,7 @@ struct Interface<'d> {
     handler: Option<&'d mut dyn ControlHandler>,
     current_alt_setting: u8,
     num_alt_settings: u8,
+    num_strings: u8,
 }
 
 pub struct UsbDevice<'d, D: Driver<'d>> {
@@ -540,14 +546,34 @@ impl<'d, D: Driver<'d>> UsbDevice<'d, D> {
                         .await
                 } else {
                     let s = match index {
-                        1 => self.config.manufacturer,
-                        2 => self.config.product,
-                        3 => self.config.serial_number,
+                        STRING_INDEX_MANUFACTURER => self.config.manufacturer,
+                        STRING_INDEX_PRODUCT => self.config.product,
+                        STRING_INDEX_SERIAL_NUMBER => self.config.serial_number,
                         _ => {
-                            let _index = StringIndex::new(index);
-                            let _lang_id = req.index;
-                            // TODO
-                            None
+                            // 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);
+                                    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, self.control_buf)
+                                } else {
+                                    warn!("String requested to an interface with no handler.");
+                                    None
+                                }
+                            } else {
+                                warn!("String requested but didn't match to an interface.");
+                                None
+                            }
                         }
                     };