What is needed to make a composite device with two Serials?

[csarn]

I'm trying what others seem to have done before, but I can't find any examples online.
I want to have two serial CDC-ACM interfaces, so that one can stream data continuously while the other can be used for interaction with the device.
Calling the second SerialPort::new() panics at https://github.com/rust-embedded-community/usbd-serial/blob/master/src/cdc_acm.rs#L60 with alloc_ep failed: EndpointOverflow.
My microcontroller is the stm32f411. This is supported by the blackmagic probe firmware, which also offers two Serial interfaces, so I assume that the microcontroller should be able to handle it:
https://github.com/blackmagic-debug/blackmagic/blob/df0c092165a5e6784ba94a0eaa0ec08a71c3a0c7/src/platforms/common/usb_serial.c#L30-L39
Is it maybe relevant that the blackmagic uses an IN endpoint for one CDC CTRL endpoint and an OUT endpoint for the other one? Is this somehow possible with usbd-serial?

[ololoshka2871]

I found the solution:
The problem is that the 128 byte buffer (default) is not enough for the descriptors of 2 CDC-acm devices, you need to enable the 256 byte buffer:

1. Cargo.toml

usb-device = { version = "...", features = ["control-buffer-256"] }

2. Initialise two separate ports

static mut USB_BUS: Option<usb_device::bus::UsbBusAllocator<UsbBusType>> = None;

let usb = Peripheral {
      usb: ctx.device.USB,
      pin_dm: gpioa.pa11,
      pin_dp: gpioa.pa12,
};

unsafe {
    USB_BUS.replace(UsbBus::new(usb));
}

let serial1 = SerialPort::new(unsafe { USB_BUS.as_ref().unwrap_unchecked() });
let serial2 = SerialPort::new(unsafe { USB_BUS.as_ref().unwrap_unchecked() });

let usb_dev = UsbDeviceBuilder::new(
      unsafe { USB_BUS.as_ref().unwrap_unchecked() },
      usb_device::prelude::UsbVidPid(0x0001, 0x1111),
  )
  .manufacturer("myvendor")
  .product("maydevice")
  .serial_number("1")
  .composite_with_iads() # <- enable composite device
  .build();

[NickCao]

Is it maybe relevant that the blackmagic uses an IN endpoint for one CDC CTRL endpoint and an OUT endpoint for the other one? Is this somehow possible with usbd-serial?

It is. stm32f411 only has 4 bi-directional endpoints. 1 is used for EP0, only 3 left for serial ports. Every serial port uses 1.5 endpoints (1 for data, 0.5 for control). However the control endpoints of the two serial ports have to be created in different directions just like what blackmagic is doing.

I've implemented the idea in NickCao@41faf42

Edit: According to https://stackoverflow.com/questions/61386938/use-cdc-acm-without-interrupt-endpoint, this can cause some issues due to lost interrupt data, but we don't seem to be handling interrupt data anyway.