Linux: Support getting device speed from the kernel

This is done through an ioctl `USBDEVFS_GET_SPEED`. Judging from
the kernel souce code, it just returns a cached value, and should
not make any request to the device.

src/device.rs

@@ -9,7 +9,7 @@ use crate::{
         Control, ControlIn, ControlOut, EndpointType, Queue, RequestBuffer, TransferError,
         TransferFuture,
     },
-    DeviceInfo, Error,
+    DeviceInfo, Error, Speed,
 };
 use log::error;
 use std::{io::ErrorKind, sync::Arc, time::Duration};
@@ -358,6 +358,19 @@ impl Device {
         let buf = self.backend.descriptors();
         validate_device_descriptor(&buf).map(|len| DeviceDescriptor::new(&buf[0..len]))
     }
+
+    /// Get device speed.
+    ///
+    /// On most platforms, the speed is stored by the OS, and calling this method should not
+    /// make any request to the device.
+    ///
+    /// ### Platform-specific notes
+    ///
+    /// * This is only supported on Linux at present.
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    pub fn get_speed(&self) -> Result<Option<Speed>, Error> {
+        self.backend.get_speed()
+    }
 }
 
 /// An opened interface of a USB device.

src/platform/linux_usbfs/device.rs

@@ -33,7 +33,7 @@ use crate::{
     transfer::{
         notify_completion, Control, Direction, EndpointType, TransferError, TransferHandle,
     },
-    DeviceInfo, Error,
+    DeviceInfo, Error, Speed,
 };
 
 pub(crate) struct LinuxDevice {
@@ -409,6 +409,20 @@ impl LinuxDevice {
     pub(crate) fn descriptors(&self) -> &[u8] {
         &self.descriptors
     }
+
+    pub(crate) fn get_speed(&self) -> Result<Option<Speed>, Error> {
+        usbfs::get_speed(&self.fd)
+            .map_err(|errno| errno.into())
+            .map(|raw_speed| match raw_speed {
+                1 => Some(Speed::Low),
+                2 => Some(Speed::Full),
+                3 => Some(Speed::High),
+                // 4 is wireless USB, but we don't support it
+                5 => Some(Speed::Super),
+                6 => Some(Speed::SuperPlus),
+                _ => None,
+            })
+    }
 }
 
 impl Drop for LinuxDevice {

src/platform/linux_usbfs/usbfs.rs

@@ -286,3 +286,10 @@ pub fn clear_halt<Fd: AsFd>(fd: Fd, endpoint: u8) -> io::Result<()> {
         ioctl::ioctl(fd, ctl)
     }
 }
+
+pub fn get_speed<Fd: AsFd>(fd: Fd) -> io::Result<usize> {
+    unsafe {
+        let ctl = Transfer::<ioctl::NoneOpcode<b'U', 31, ()>, ()>::new(());
+        ioctl::ioctl(fd, ctl)
+    }
+}