Add support for proxying isochronous OUT endpoints

With this change, I'm able to proxy one of Logitech USB web cameras at a
low resolution (160x120 pixels) via dwc3.

Proxying with higher resolutions fails to produce a picture on the host:
I think because the transfer rate is too low and all the frames get dropped.

device-libusb.cpp

@@ -299,18 +299,51 @@ int send_data(uint8_t endpoint, uint8_t attributes, uint8_t *dataptr,
 	return result;
 }
 
+void iso_transfer_callback(struct libusb_transfer *transfer) {
+	int *iso_completed = (int *)transfer->user_data;
+	*iso_completed = 1;
+}
+
 int receive_data(uint8_t endpoint, uint8_t attributes, uint16_t maxPacketSize,
 			uint8_t **dataptr, int *length, int timeout) {
 	int result = LIBUSB_SUCCESS;
+	struct libusb_transfer *transfer;
+	int iso_completed, iso_packets;
 
 	int attempt = 0;
 	switch (attributes & USB_ENDPOINT_XFERTYPE_MASK) {
 	case USB_ENDPOINT_XFER_CONTROL:
 		fprintf(stderr, "Can't read on a control endpoint.\n");
 		break;
 	case USB_ENDPOINT_XFER_ISOC:
-		if (verbose_level)
-			fprintf(stderr, "Isochronous(read) endpoint EP%02x unhandled.\n", endpoint);
+		*dataptr = new uint8_t[maxPacketSize];
+		// We could retrieve multiple packets at a time, but then we
+		// would need to split the received data submit each packet
+		// separately via Raw Gadget. So retrieve only one packet for
+		// simplicity.
+		iso_packets = 1;
+		transfer = libusb_alloc_transfer(iso_packets);
+		if (!transfer) {
+			fprintf(stderr, "Failed to allocate libusb_transfer.\n");
+			result = LIBUSB_ERROR_OTHER;
+		}
+		iso_completed = 0;
+		libusb_fill_iso_transfer(transfer, dev_handle, endpoint, *dataptr, maxPacketSize,
+					iso_packets, iso_transfer_callback, &iso_completed, timeout);
+		libusb_set_iso_packet_lengths(transfer, maxPacketSize / iso_packets);
+		result = libusb_submit_transfer(transfer);
+		if (result != LIBUSB_SUCCESS) {
+			libusb_free_transfer(transfer);
+			break;
+		}
+		while (!iso_completed)
+			libusb_handle_events_completed(NULL, &iso_completed);
+		*length = 0;
+		for (int i = 0; i < iso_packets; i++)
+			*length += transfer->iso_packet_desc[i].actual_length;
+		if (result == LIBUSB_SUCCESS && verbose_level > 2)
+			printf("Received iso data(%d) bytes\n", *length);
+		libusb_free_transfer(transfer);
 		break;
 	case USB_ENDPOINT_XFER_BULK:
 		*dataptr = new uint8_t[maxPacketSize * 8];

host-raw-gadget.cpp

@@ -140,6 +140,11 @@ int usb_raw_ep_write(int fd, struct usb_raw_ep_io *io) {
 			// endpoint threads when shutting them down.
 			return rv;
 		}
+		else if (errno == EXDEV) {
+			// Ignore failures caused by sending an isochronous
+			// transfer too late (dwc3 returns EXDEV).
+			return rv;
+		}
 		else if (errno == EBUSY)
 			return rv;
 		perror("ioctl(USB_RAW_IOCTL_EP_WRITE)");

proxy.cpp

@@ -145,6 +145,11 @@ void *ep_loop_write(void *arg) {
 					ep.bEndpointAddress, transfer_type.c_str(), dir.c_str());
 				break;
 			}
+			if (rv < 0 && errno == EXDEV) {
+				printf("EP%x(%s_%s): missed isochronous timing, ignoring transfer\n",
+					ep.bEndpointAddress, transfer_type.c_str(), dir.c_str());
+				continue;
+			}
 			else if (rv < 0) {
 				perror("usb_raw_ep_write()");
 				exit(EXIT_FAILURE);