Add UVC video gadget support (#12)

Tested with https://gitlab.freedesktop.org/camera/uvc-gadget. Creating the gadget, then running uvc-gadget uvc.usb-gadget0-0 (where 'uvc.usb-gadget0-0' is the function dir) will enumerate with my macOS host. One can open a camera application and open the device. The default just shows a test pattern.

README.md

@@ -25,6 +25,7 @@ The following pre-defined USB functions, implemented by kernel drivers, are avai
 * mass-storage device (MSD)
 * musical instrument digital interface (MIDI)
 * audio device (UAC2)
+* video device (UVC)
 
 In addition fully custom USB functions can be implemented in user-mode Rust code.
 
@@ -62,6 +63,7 @@ The following Linux kernel configuration options should be enabled for full func
   * `CONFIG_USB_CONFIGFS_F_HID`
   * `CONFIG_USB_CONFIGFS_F_MIDI`
   * `CONFIG_USB_CONFIGFS_F_UAC2`
+  * `CONFIG_USB_CONFIGFS_F_UVC`
 
 root permissions are required to configure USB gadgets on Linux and
 the `configfs` filesystem needs to be mounted.

src/function/mod.rs

@@ -9,6 +9,7 @@ pub mod net;
 pub mod other;
 pub mod serial;
 pub mod util;
+pub mod video;
 
 use std::{cmp, hash, hash::Hash, sync::Arc};
 
@@ -63,4 +64,5 @@ impl Hash for Handle {
 fn register_remove_handlers() {
     register_remove_handler(custom::driver(), custom::remove_handler);
     register_remove_handler(msd::driver(), msd::remove_handler);
+    register_remove_handler(video::driver(), video::remove_handler);
 }

src/function/util.rs

@@ -205,6 +205,13 @@ impl FunctionDir {
         fs::create_dir(path)
     }
 
+    /// Create a subdirectory and its parent directories.
+    pub fn create_dir_all(&self, name: impl AsRef<Path>) -> Result<()> {
+        let path = self.property_path(name)?;
+        log::debug!("creating directorys {}", path.display());
+        fs::create_dir_all(path)
+    }
+
     /// Remove a subdirectory.
     pub fn remove_dir(&self, name: impl AsRef<Path>) -> Result<()> {
         let path = self.property_path(name)?;
@@ -249,6 +256,14 @@ impl FunctionDir {
         log::debug!("setting property {} to {}", path.display(), String::from_utf8_lossy(value));
         fs::write(path, value)
     }
+
+    /// Create a symbolic link.
+    pub fn symlink(&self, target: impl AsRef<Path>, link: impl AsRef<Path>) -> Result<()> {
+        let target = self.property_path(target)?;
+        let link = self.property_path(link)?;
+        log::debug!("creating symlink {} -> {}", link.display(), target.display());
+        std::os::unix::fs::symlink(target, link)
+    }
 }
 
 /// Split configfs function directory path into driver name and instance name.