Support for multiple device configurations

[ia0]

Am I understanding correctly that the UsbBusAllocator and UsbDeviceBuilder APIs don't support creating a device with multiple USB configurations? For example a device that would look like this:

• Configuration 1:
  • Interface 1
• Configuration 2:
  • Interface 1
  • Interface 2

[mvirkkunen]

Correct, multiple configurations are not supported because I originally deemed that something that is not very useful in practice and I left out the complexity.

What were you looking to implement with multiple configurations?

[ia0]

Thanks for the quick answer!

The use-case I have in mind is a device with the default configuration for applet interfaces (not familiar enough with USB to know if there is such a notion of default configuration) and an alternative configuration with a platform interface. Applet interfaces could be like USB serial or some HID interface, while the platform interface provides a way to install/uninstall/update applets or update the platform itself (or other platform-level operations). Currently, I have a single configuration with all interfaces (applet interfaces and platform interface). This works fine, but I was wondering if I should also provide a way to have the platform interface in its own configuration, and was looking if it was at all possible.

As you can see, this is not motivated by a concrete use-case, but just as an exploration of what would be possible. I understand the goal of keeping usb-device simple, so this is fine for me to close this issue. If I ever have a concrete use-case I would reopen or create a new issue.

[mvirkkunen]

Most practical devices I've seen that completely switch modes for e.g. firmware update (kind of what you're planning) just de-enumerate and then re-enumerate as a different device altogether (different VID:PID, different singular configuration). I think that has a better chance of working with operating systems and drivers and it is doable with usb-device.

[ia0]

Yes, I also thought about doing that. The problem is that this needs cooperation from the applets (which are not necessarily trusted or considered reliable). So for it work, there must be some physical input (like a button or a power cycle) to tell the platform to switch the exposed device from applets to platform. But I agree that this would probably be a better alternative even with this issue.

[mvirkkunen]

If you assume running those applets might make your device not respond to USB events anymore, it wouldn't be able to respond to the request to change configuration either though.

[ia0]

The USB drivers are implemented in the platform (applets can only select from a subset of those and can't influence the USB behavior outside of the API provided by the driver). So I don't think that this should be an issue.

[mvirkkunen]

If the platform thing is always available, you vould make it always respond to e.g. a specific control transfer that resets the device into "platform mode"

[ia0]

If I understand correctly, this would essentially be a vendor way of implementing configurations. SetConfiguration would just be a control transfer asking to the device to USB reset into a different USB device.

I think I get the high-level idea, but I'm still missing the details. Here is how I see it:

• The host sends a (Host to Device, Vendor, Device) Setup packet with some data asking to change "configuration". I see how to do that with the rusb crate which I use.
• On the device, whenever I poll the UsbDevice, regardless of its result, I should read the control pipe if something was returned. To do that, I use the UsbDevice::bus() function to access the bus and then UsbBus::read() to read the control pipe directly from the bus.
• If it's a request to reset, I drop everything (the UsbDevice and the UsbBusAllocator) and recreate a new UsbBus (cheating on ownership since the device was lost with the UsbBusAllocator), UsbBusAllocator, and finally UsbDevice, then call UsbDevice::force_reset() to make sure the device gets re-enumerated.

Does this sound like what you had in mind?