# USB-3: DATA Stage

The next step is to respond to the GET_DESCRIPTOR request with a device descriptor. To do this we'll use the `dk::usb::Ep0In` abstraction -- we'll look into what the abstraction does in a future section; for now we'll just use it.

An instance of this abstraction is available in the `board` value (`#[init]` function). The first step is to make this `Ep0In` instance available to the `on_event` function.

The `Ep0In` API has two methods: `start` and `end` (also see their API documentation). `start` is used to start a DATA stage; this method takes a *slice of bytes* (`[u8]`) as argument; this argument is the response data. The `end` method needs to be called after `start`, when the EP0DATADONE event is raised, to complete the control transfer. `Ep0In` will automatically issue the STATUS stage that must follow the DATA stage.

To implement responding to a GET_DESCRIPTOR request, do the following:

1. **Extend the parser implementation to handle a GET_DESCRIPTOR request:** make the `common/usb/lib.rs::get_descriptor_configuration()` test run successfully.
2. **Answer the Descriptor Request:** extend `usb-3.rs` so that it uses `Ep0In` to respond to the `GET_DESCRIPTOR Device` request (and only to that request). The response must be a device descriptor with its fields set to these values:

- `bDeviceClass = bDeviceSubClass = bDeviceProtocol = 0`, these are unimportant for enumeration
- `bMaxPacketSize0 = 64`, this is the most performant option (minimizes exchanges between the device and the host) and it's assumed by the `Ep0In` abstraction
- `idVendor = consts::VID`, value expected by the `usb-list` tool (\*)
- `idProduct = consts::PID`, value expected by the `usb-list` tool (\*)
- `bcdDevice = 0x0100`, this means version 1.0 but any value should do
- `iManufacturer = iProduct = iSerialNumber = None`, string descriptors not supported
- `bNumConfigurations = 1`, must be at least `1` so this is the minimum value

>(\*) the `common` crate refers to the crate in the `advanced/common` folder. It is already part of the `firmware` crate dependencies.

Although you can create the device descriptor by hand as an array filled with magic values we *strongly* recommend you use the `usb2::device::Descriptor` abstraction. The crate is already in the dependency list of the project; you can open its API documentation with the following command: `cargo doc -p usb2 --open`.

Note that the device descriptor is 18 bytes long but the host may ask for fewer bytes (see `wlength` field in the SETUP data). In that case you must respond with the amount of bytes the host asked for. The opposite may also happen: `wlength` may be larger than the size of the device descriptor; in this case your answer must be 18 bytes long (do *not* pad the response with zeroes).

Don't forget to also handle the `EP0DATADONE` event!

Once you have successfully responded to the GET_DESCRIPTOR Device request you should get logs like these (if you are logging like `usb-3` does):

``` console
INFO:usb_3 -- USB: UsbReset @ 342.071532ms
INFO:usb_3 -- USB: UsbEp0Setup @ 414.855956ms
INFO:usb_3 -- SETUP: bmrequesttype: 128, brequest: 6, wlength: 64, windex: 0, wvalue: 256
INFO:usb_3 -- GET_DESCRIPTOR Device [length=64]
INFO:dk::usbd -- EP0IN: start 18B transfer
INFO:usb_3 -- USB: UsbEp0DataDone @ 415.222166ms
INFO:dk::usbd -- EP0IN: transfer done
INFO:usb_3 -- USB: UsbReset @ 465.637206ms
INFO:usb_3 -- USB: UsbEp0Setup @ 538.208007ms
INFO:usb_3 -- SETUP: bmrequesttype: 0, brequest: 5, wlength: 0, windex: 0, wvalue: 27
ERROR:usb_3 -- unknown request (goal achieved if GET_DESCRIPTOR Device was handled)
INFO:dk -- `dk::exit() called; exiting ...`
```

A solution to this exercise can be found in `src/bin/usb-3-solution.rs`.