Add support for modifying the receive latency.

Currently this isn't used or needed anywhere, but the research has been
done, and it would be silly to drop the knowledge. We may need it in the
future.

src/serial.h

@@ -93,6 +93,8 @@ int serial_set_queue_size (serial_t *device, unsigned int input, unsigned int ou
 
 int serial_set_halfduplex (serial_t *device, int value);
 
+int serial_set_latency (serial_t *device, unsigned int milliseconds);
+
 int serial_read (serial_t *device, void* data, unsigned int size);
 int serial_write (serial_t *device, const void* data, unsigned int size);
 

src/serial_posix.c

@@ -512,6 +512,45 @@ serial_set_halfduplex (serial_t *device, int value)
 	return 0;
 }
 
+int
+serial_set_latency (serial_t *device, unsigned int milliseconds)
+{
+	if (device == NULL)
+		return -1; // EINVAL (Invalid argument)
+
+#if defined(TIOCGSERIAL) && defined(TIOCSSERIAL)
+	// Get the current settings.
+	struct serial_struct ss;
+	if (ioctl (device->fd, TIOCGSERIAL, &ss) != 0 && NOPTY) {
+		SYSERROR (device->context, errno);
+		return -1;
+	}
+
+	// Set or clear the low latency flag.
+	if (milliseconds == 0) {
+		ss.flags |= ASYNC_LOW_LATENCY;
+	} else {
+		ss.flags &= ~ASYNC_LOW_LATENCY;
+	}
+
+	// Apply the new settings.
+	if (ioctl (device->fd, TIOCSSERIAL, &ss) != 0 && NOPTY) {
+		SYSERROR (device->context, errno);
+		return -1;
+	}
+#elif defined(IOSSDATALAT)
+	// Set the receive latency in microseconds. Serial drivers use this
+	// value to determine how often to dequeue characters received by
+	// the hardware. A value of zero restores the default value.
+	unsigned long usec = (milliseconds == 0 ? 1 : milliseconds * 1000);
+	if (ioctl (device->fd, IOSSDATALAT, &usec) != 0 && NOPTY) {
+		SYSERROR (device->context, errno);
+		return -1;
+	}
+#endif
+
+	return 0;
+}
 
 int
 serial_read (serial_t *device, void *data, unsigned int size)

src/serial_win32.c

@@ -389,6 +389,15 @@ serial_set_halfduplex (serial_t *device, int value)
 }
 
 
+int
+serial_set_latency (serial_t *device, unsigned int milliseconds)
+{
+	if (device == NULL)
+		return -1; // ERROR_INVALID_PARAMETER (The parameter is incorrect)
+
+	return 0;
+}
+
 int
 serial_read (serial_t *device, void* data, unsigned int size)
 {