Linux: Support getting DeviceDescriptor and DeviceInfo from a opened …

…device

src/descriptors.rs

@@ -181,6 +181,36 @@ macro_rules! descriptor_fields {
     }
 }
 
+/// Check whether the buffer contains a valid device descriptor.
+/// On success, it will return length of the descriptor, or returns `None`.
+pub(crate) fn validate_device_descriptor(buf: &[u8]) -> Option<usize> {
+    if buf.len() < DESCRIPTOR_LEN_DEVICE as usize {
+        if buf.len() != 0 {
+            warn!(
+                "device descriptor buffer is {} bytes, need {}",
+                buf.len(),
+                DESCRIPTOR_LEN_DEVICE
+            );
+        }
+        return None;
+    }
+
+    if buf[0] < DESCRIPTOR_LEN_DEVICE {
+        warn!("invalid device descriptor bLength");
+        return None;
+    }
+
+    if buf[1] != DESCRIPTOR_TYPE_DEVICE {
+        warn!(
+            "device bDescriptorType is {}, not a device descriptor",
+            buf[1]
+        );
+        return None;
+    }
+
+    return Some(buf[0] as usize);
+}
+
 /// Information about a USB device.
 #[derive(Clone)]
 pub struct DeviceDescriptor<'a>(&'a [u8]);

src/device.rs

@@ -1,7 +1,8 @@
 use crate::{
     descriptors::{
-        decode_string_descriptor, validate_string_descriptor, ActiveConfigurationError,
-        Configuration, InterfaceAltSetting, DESCRIPTOR_TYPE_STRING,
+        decode_string_descriptor, validate_device_descriptor, validate_string_descriptor,
+        ActiveConfigurationError, Configuration, DeviceDescriptor, InterfaceAltSetting,
+        DESCRIPTOR_TYPE_STRING,
     },
     platform,
     transfer::{
@@ -341,6 +342,22 @@ impl Device {
         t.submit::<ControlOut>(data);
         TransferFuture::new(t)
     }
+
+    /// Get the device descriptor.
+    ///
+    /// The only situation when it returns `None` is
+    /// that the cached descriptors contain no valid device descriptor.
+    ///
+    /// ### Platform-specific notes
+    ///
+    /// * This is only supported on Linux at present.
+    /// * On Linux, this method uses descriptors cached in memory, instead
+    ///   of sending a request to the device for a descriptor.
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    pub fn device_descriptor(&self) -> Option<DeviceDescriptor> {
+        let buf = self.backend.descriptors();
+        validate_device_descriptor(&buf).map(|len| DeviceDescriptor::new(&buf[0..len]))
+    }
 }
 
 /// An opened interface of a USB device.

src/platform/linux_usbfs/device.rs

@@ -405,6 +405,10 @@ impl LinuxDevice {
         );
         return Err(ErrorKind::Other.into());
     }
+
+    pub(crate) fn descriptors(&self) -> &[u8] {
+        &self.descriptors
+    }
 }
 
 impl Drop for LinuxDevice {