Added "usb-send-delay" quirk

This quirk introduces some configurable pause after each USB URB has been
sent to the device.

Looks like some HP devices may fail large jobs in raster format, if data
is transmitted too quickly from the host to the printer.

This quirk effectively adds some transmission throttling.

See #95 for discussion and details.

Author: alexpevzner

quirks.go

@@ -47,6 +47,7 @@ const (
 	QuirkNmUsbMaxInterfaces  = "usb-max-interfaces"
 	QuirkNmZlpRecvHack       = "zlp-recv-hack"
 	QuirkNmZlpSend           = "zlp-send"
+	QuirkNmUSBSendDelay      = "usb-send-delay"
 )
 
 // quirkParse maps quirk names into appropriate parsing methods,
@@ -64,6 +65,7 @@ var quirkParse = map[string]func(*Quirk) error{
 	QuirkNmUsbMaxInterfaces:  (*Quirk).parseUint,
 	QuirkNmZlpRecvHack:       (*Quirk).parseBool,
 	QuirkNmZlpSend:           (*Quirk).parseBool,
+	QuirkNmUSBSendDelay:      (*Quirk).parseDuration,
 }
 
 // quirkDefaultStrings contains default values for quirks, in
@@ -371,6 +373,12 @@ func (quirks Quirks) GetZlpSend() bool {
 	return quirks.Get(QuirkNmZlpSend).Parsed.(bool)
 }
 
+// GetUSBSendDelay returns effective "usb-send-delay" parameter
+// taking the whole set into consideration.
+func (quirks Quirks) GetUSBSendDelay() time.Duration {
+	return quirks.Get(QuirkNmUSBSendDelay).Parsed.(time.Duration)
+}
+
 // QuirksSet represents collection of quirks
 type QuirksSet []*Quirks
 

usbio_libusb.go

@@ -809,6 +809,11 @@ func (iface *UsbInterface) Send(ctx context.Context,
 	<-doneChan
 	n, err = libusbTransferStatusDecode(ctx, xfer)
 
+	// Introduce inter-URB send delay, if configured
+	if delay := iface.quirks.GetUSBSendDelay(); delay != 0 {
+		time.Sleep(delay)
+	}
+
 	return
 }