usb: builtin handling of interface alternate settings

The stack reads its own descriptors to figure out which endpoints
are used in which alt settings, and enables/disables them as needed.

The ControlHandler has a callback so it can get notified of alternate
setting changes, which is purely informative (it doesn't have to do anything).

embassy-usb/src/descriptor_reader.rs

@@ -0,0 +1,110 @@
+use crate::descriptor::descriptor_type;
+use crate::types::EndpointAddress;
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct ReadError;
+
+pub struct Reader<'a> {
+    data: &'a [u8],
+}
+
+impl<'a> Reader<'a> {
+    pub fn new(data: &'a [u8]) -> Self {
+        Self { data }
+    }
+
+    pub fn eof(&self) -> bool {
+        self.data.is_empty()
+    }
+
+    pub fn read<const N: usize>(&mut self) -> Result<[u8; N], ReadError> {
+        let n = self.data.get(0..N).ok_or(ReadError)?;
+        self.data = &self.data[N..];
+        Ok(n.try_into().unwrap())
+    }
+
+    pub fn read_u8(&mut self) -> Result<u8, ReadError> {
+        Ok(u8::from_le_bytes(self.read()?))
+    }
+    pub fn read_u16(&mut self) -> Result<u16, ReadError> {
+        Ok(u16::from_le_bytes(self.read()?))
+    }
+
+    pub fn read_slice(&mut self, len: usize) -> Result<&'a [u8], ReadError> {
+        let res = self.data.get(0..len).ok_or(ReadError)?;
+        self.data = &self.data[len..];
+        Ok(res)
+    }
+
+    pub fn read_descriptors(&mut self) -> DescriptorIter<'_, 'a> {
+        DescriptorIter { r: self }
+    }
+}
+
+pub struct DescriptorIter<'a, 'b> {
+    r: &'a mut Reader<'b>,
+}
+
+impl<'a, 'b> Iterator for DescriptorIter<'a, 'b> {
+    type Item = Result<(u8, Reader<'a>), ReadError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.r.eof() {
+            return None;
+        }
+
+        let len = match self.r.read_u8() {
+            Ok(x) => x,
+            Err(e) => return Some(Err(e)),
+        };
+        let type_ = match self.r.read_u8() {
+            Ok(x) => x,
+            Err(e) => return Some(Err(e)),
+        };
+        let data = match self.r.read_slice(len as usize - 2) {
+            Ok(x) => x,
+            Err(e) => return Some(Err(e)),
+        };
+
+        Some(Ok((type_, Reader::new(data))))
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct EndpointInfo {
+    pub configuration: u8,
+    pub interface: u8,
+    pub interface_alt: u8,
+    pub ep_address: EndpointAddress,
+}
+
+pub fn foreach_endpoint(data: &[u8], mut f: impl FnMut(EndpointInfo)) -> Result<(), ReadError> {
+    let mut ep = EndpointInfo {
+        configuration: 0,
+        interface: 0,
+        interface_alt: 0,
+        ep_address: EndpointAddress::from(0),
+    };
+    for res in Reader::new(data).read_descriptors() {
+        let (kind, mut r) = res?;
+        match kind {
+            descriptor_type::CONFIGURATION => {
+                let _total_length = r.read_u16()?;
+                let _total_length = r.read_u8()?;
+                ep.configuration = r.read_u8()?;
+            }
+            descriptor_type::INTERFACE => {
+                ep.interface = r.read_u8()?;
+                ep.interface_alt = r.read_u8()?;
+            }
+            descriptor_type::ENDPOINT => {
+                ep.ep_address = EndpointAddress::from(r.read_u8()?);
+                f(ep)
+            }
+            _ => {}
+        }
+    }
+    Ok(())
+}