trezor-firmware/bootloader/usb_send.h

static void send_msg_success(usbd_device *dev)
{
	uint8_t response[64];
	memzero(response, sizeof(response));
	// response: Success message (id 2), payload len 0
	memcpy(response,
		// header
		"?##"
		// msg_id
		"\x00\x02"
		// msg_size
		"\x00\x00\x00\x00",
		9);
	while (usbd_ep_write_packet(dev, ENDPOINT_ADDRESS_IN, response, 64) != 64) {}
}

static void send_msg_failure(usbd_device *dev)
{
	uint8_t response[64];
	memzero(response, sizeof(response));
	// response: Failure message (id 3), payload len 2
	//           - code = 99 (Failure_FirmwareError)
	memcpy(response,
		// header
		"?##"
		// msg_id
		"\x00\x03"
		// msg_size
		"\x00\x00\x00\x02"
		// data
		"\x08" "\x63",
		11);
	while (usbd_ep_write_packet(dev, ENDPOINT_ADDRESS_IN, response, 64) != 64) {}
}

static void send_msg_features(usbd_device *dev)
{
	uint8_t response[64];
	memzero(response, sizeof(response));
	// response: Features message (id 17), payload len 25
	//           - vendor = "trezor.io"
	//           - major_version = VERSION_MAJOR
	//           - minor_version = VERSION_MINOR
	//           - patch_version = VERSION_PATCH
	//           - bootloader_mode = True
	//           - firmware_present = True/False
	//           - model = "1"
	memcpy(response,
		// header
		"?##"
		// msg_id
		"\x00\x11"
		// msg_size
		"\x00\x00\x00\x16"
		// data
		"\x0a" "\x09" "trezor.io"
		"\x10" VERSION_MAJOR_CHAR
		"\x18" VERSION_MINOR_CHAR
		"\x20" VERSION_PATCH_CHAR
		"\x28" "\x01"
		"\x90\x01" "\x00"
		"\xaa" "\x01" "1",
		34);
	response[30] = firmware_present_new() ? 0x01 : 0x00;
	while (usbd_ep_write_packet(dev, ENDPOINT_ADDRESS_IN, response, 64) != 64) {}
}

static void send_msg_buttonrequest_firmwarecheck(usbd_device *dev)
{
	uint8_t response[64];
	memzero(response, sizeof(response));
	// response: ButtonRequest message (id 26), payload len 2
	//           - code = ButtonRequest_FirmwareCheck (9)
	memcpy(response,
		// header
		"?##"
		// msg_id
		"\x00\x1a"
		// msg_size
		"\x00\x00\x00\x02"
		// data
		"\x08" "\x09",
		11
	);
	while (usbd_ep_write_packet(dev, ENDPOINT_ADDRESS_IN, response, 64) != 64) {}
}