Pass the max buffer size down to the alloc_transfer. Use the number

of iso packets matching the buffer size. This guarantees that the
transfer size is smaller or equal to the buffer.
Device will transfer less data if iso transfer response does not utilize the
maximum available number of iso packets per microframe or doesn't fill
the packets entirely.

usb/iso.go

@@ -37,18 +37,25 @@ func iso_callback(cptr unsafe.Pointer) {
 	close(ch)
 }
 
-func (end *endpoint) allocTransfer() *Transfer {
-	const (
-		iso_packets = 8 // 128 // 242
-	)
-
-	xfer := C.libusb_alloc_transfer(C.int(iso_packets))
+func (end *endpoint) allocTransfer(maxLen int) *Transfer {
+	isoPacketSize := end.EndpointInfo.MaxIsoPacket
+	// the larget the input buffer, the more packets we use in a single
+	// transfer.
+	numIsoPackets := maxLen / int(isoPacketSize)
+	if numIsoPackets*int(isoPacketSize) < maxLen {
+		numIsoPackets++
+	}
+	// arbitrary limit
+	if numIsoPackets > 200 {
+		numIsoPackets = 200
+	}
+	xfer := C.libusb_alloc_transfer(C.int(numIsoPackets))
 	if xfer == nil {
 		log.Printf("usb: transfer allocation failed?!")
 		return nil
 	}
 
-	buf := make([]byte, iso_packets*end.EndpointInfo.MaxIsoPacket)
+	buf := make([]byte, numIsoPackets*int(end.EndpointInfo.MaxIsoPacket))
 	done := make(chan struct{}, 1)
 
 	xfer.dev_handle = end.Device.handle
@@ -57,7 +64,7 @@ func (end *endpoint) allocTransfer() *Transfer {
 
 	xfer.buffer = (*C.uchar)((unsafe.Pointer)(&buf[0]))
 	xfer.length = C.int(len(buf))
-	xfer.num_iso_packets = iso_packets
+	xfer.num_iso_packets = C.int(numIsoPackets)
 
 	C.libusb_set_iso_packet_lengths(xfer, C.uint(end.EndpointInfo.MaxIsoPacket))
 	/*
@@ -130,7 +137,7 @@ func (t *Transfer) Close() error {
 }
 
 func isochronous_xfer(e *endpoint, buf []byte, timeout time.Duration) (int, error) {
-	t := e.allocTransfer()
+	t := e.allocTransfer(len(buf))
 	defer t.Close()
 
 	if err := t.Submit(timeout); err != nil {