[question] The interface is not loaded in time when hot plugging the device

[LinusChen-yf]

platform: Windows

When hot-plugging is detected, sometimes the interfaces of the device are empty, and it is often necessary to wait for 50 to 100 milliseconds to retry to list_devices and obtain the interface.
I think this is not a good way to rely on experience to judge adding delays and re-acquiring devices. Is this a bug? Or is there any better way to deal with this problem? Thank you!

[kevinmehall]

It's a documented limitation, but could be considered a bug that should be fixed if we can find a better way to do this.

This only occurs for a composite device using usbccgp.sys. We get the notification from the Windows API for the new "Composite Group Parent" device. But the interfaces are considered entirely separate devices by Windows with separate drivers that are created later.

We could potentially populate DeviceInfo::interfaces() using the interfaces from the configuration descriptor, though it would be missing the interface strings. But you still wouldn't be able to claim the interface until the Windows device nodes for the interfaces exist. And it would be hard to wait for them because they don't necessarily directly correspond to the interfaces in the interface descriptor.

If your device is entirely accessed by nusb and don't need to use other drivers on other interfaces, one workaround is to bind the entire device to WinUSB, and then there is a single Windows device node that is ready when the notification is received. Note that v0.1 has an issue where interfaces other than the first one don't work in this setup; fixed in v0.2 and pending backport in #145.

[LinusChen-yf]

Thanks for your answer, I'm sorry I didn't read the document carefully enough.

[mfreeborn]

May I ask, is it necessary to re-fetch the DeviceInfo struct using list_devices in order to be able to access all the interfaces, or can we just wait a bit with the original DeviceInfo before calling open?

e.g. this pseudocode

let watcher =  nusb::watch_devices().unwrap();

match watcher.select_next_some() {
    HotplugEvent::Connected(device_info) => {
    // say the device_info here hasn't had its interfaces fully populated yet
    // can we just stick a sleep here and use the same device_info object to subsequently access the interfaces
    tokio::time::sleep(Duration::from_millis(250)).await;
    let interface = device_info.open()...claim_interface(interface_num);

    // _or_ do we need to run nusb::list_devices after the delay to get a fresh new `DeviceInfo`
    let fresh_device_info = nusb::list_devices()...find(/* device_info */);
    let interface = fresh_device_info.open()...etc;
    }
    HotplugEvent::Disconnected(_) => todo!()
}

[kevinmehall]

That's fine. You can even .open() the device and just delay or retry the .claim_interface().

DeviceInfo::open() only uses the ID, not any of the interface related information from the DeviceInfo.

[LinusChen-yf]

For me, I need to call list_devices again. Because I need to get the interface number I want based on the interface name. If I don't call list_devices, I can't get the interface number.

[mfreeborn]

Thanks for the clarification.