/*
   * Copyright (C) 2009 by Bart Hartgers (bart.hartgers+ark3116@gmail.com)
   * Original version:
   * Copyright (C) 2006
   *   Simon Schulz (ark3116_driver <at> auctionant.de)
   *
   * ark3116
   * - implements a driver for the arkmicro ark3116 chipset (vendor=0x6547,
   *   productid=0x0232) (used in a datacable called KQ-U8A)
   *
   * Supports full modem status lines, break, hardware flow control. Does not
   * support software flow control, since I do not know how to enable it in hw.
   *
   * This driver is a essentially new implementation. I initially dug
   * into the old ark3116.c driver and suddenly realized the ark3116 is
   * a 16450 with a USB interface glued to it. See comments at the
   * bottom of this file.
   *
   * This program is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License as published by the
   * Free Software Foundation; either version 2 of the License, or (at your
   * option) any later version.
   */
  
  #include <linux/kernel.h>
  #include <linux/ioctl.h>
  #include <linux/tty.h>
  #include <linux/slab.h>
  #include <linux/tty_flip.h>
  #include <linux/module.h>
  #include <linux/usb.h>
  #include <linux/usb/serial.h>
  #include <linux/serial.h>
  #include <linux/serial_reg.h>
  #include <linux/uaccess.h>
  #include <linux/mutex.h>
  #include <linux/spinlock.h>
  
  #define DRIVER_AUTHOR "Bart Hartgers <bart.hartgers+ark3116@gmail.com>"
  #define DRIVER_DESC "USB ARK3116 serial/IrDA driver"
  #define DRIVER_DEV_DESC "ARK3116 RS232/IrDA"
  #define DRIVER_NAME "ark3116"
  
  /* usb timeout of 1 second */
  #define ARK_TIMEOUT 1000
  
  static const struct usb_device_id id_table[] = {
  	{ USB_DEVICE(0x6547, 0x0232) },
  	{ USB_DEVICE(0x18ec, 0x3118) },		/* USB to IrDA adapter */
  	{ },
  };
  MODULE_DEVICE_TABLE(usb, id_table);
  
  static int is_irda(struct usb_serial *serial)
  {
  	struct usb_device *dev = serial->dev;
  	if (le16_to_cpu(dev->descriptor.idVendor) == 0x18ec &&
  			le16_to_cpu(dev->descriptor.idProduct) == 0x3118)
  		return 1;
  	return 0;
  }
  
  struct ark3116_private {
  	int			irda;	/* 1 for irda device */
  
  	/* protects hw register updates */
  	struct mutex		hw_lock;
  
  	int			quot;	/* baudrate divisor */
  	__u32			lcr;	/* line control register value */
  	__u32			hcr;	/* handshake control register (0x8)
  					 * value */
  	__u32			mcr;	/* modem control register value */
  
  	/* protects the status values below */
  	spinlock_t		status_lock;
  	__u32			msr;	/* modem status register value */
  	__u32			lsr;	/* line status register value */
  };
  
  static int ark3116_write_reg(struct usb_serial *serial,
  			     unsigned reg, __u8 val)
  {
  	int result;
  	 /* 0xfe 0x40 are magic values taken from original driver */
  	result = usb_control_msg(serial->dev,
  				 usb_sndctrlpipe(serial->dev, 0),
  				 0xfe, 0x40, val, reg,
  				 NULL, 0, ARK_TIMEOUT);
  	return result;
  }
  
  static int ark3116_read_reg(struct usb_serial *serial,
  			    unsigned reg, unsigned char *buf)
  {
  	int result;
  	/* 0xfe 0xc0 are magic values taken from original driver */
  	result = usb_control_msg(serial->dev,
  				 usb_rcvctrlpipe(serial->dev, 0),
  				 0xfe, 0xc0, 0, reg,
  				 buf, 1, ARK_TIMEOUT);
  	if (result < 0)
  		return result;
  	else
  		return buf[0];
  }
  
  static inline int calc_divisor(int bps)
  {
  	/* Original ark3116 made some exceptions in rounding here
  	 * because windows did the same. Assume that is not really
  	 * necessary.
  	 * Crystal is 12MHz, probably because of USB, but we divide by 4?
  	 */
  	return (12000000 + 2*bps) / (4*bps);
  }
  
  static int ark3116_attach(struct usb_serial *serial)
  {
  	/* make sure we have our end-points */
  	if ((serial->num_bulk_in == 0) ||
  	    (serial->num_bulk_out == 0) ||
  	    (serial->num_interrupt_in == 0)) {
  		dev_err(&serial->dev->dev,
  			"%s - missing endpoint - "
  			"bulk in: %d, bulk out: %d, int in %d
  ",
  			KBUILD_MODNAME,
  			serial->num_bulk_in,
  			serial->num_bulk_out,
  			serial->num_interrupt_in);
  		return -EINVAL;
  	}
  
  	return 0;
  }
  
  static int ark3116_port_probe(struct usb_serial_port *port)
  {
  	struct usb_serial *serial = port->serial;
  	struct ark3116_private *priv;
  
  	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
  	if (!priv)
  		return -ENOMEM;
  
  	mutex_init(&priv->hw_lock);
  	spin_lock_init(&priv->status_lock);
  
  	priv->irda = is_irda(serial);
  
  	usb_set_serial_port_data(port, priv);
  
  	/* setup the hardware */
  	ark3116_write_reg(serial, UART_IER, 0);
  	/* disable DMA */
  	ark3116_write_reg(serial, UART_FCR, 0);
  	/* handshake control */
  	priv->hcr = 0;
  	ark3116_write_reg(serial, 0x8     , 0);
  	/* modem control */
  	priv->mcr = 0;
  	ark3116_write_reg(serial, UART_MCR, 0);
  
  	if (!(priv->irda)) {
  		ark3116_write_reg(serial, 0xb , 0);
  	} else {
  		ark3116_write_reg(serial, 0xb , 1);
  		ark3116_write_reg(serial, 0xc , 0);
  		ark3116_write_reg(serial, 0xd , 0x41);
  		ark3116_write_reg(serial, 0xa , 1);
  	}
  
  	/* setup baudrate */
  	ark3116_write_reg(serial, UART_LCR, UART_LCR_DLAB);
  
  	/* setup for 9600 8N1 */
  	priv->quot = calc_divisor(9600);
  	ark3116_write_reg(serial, UART_DLL, priv->quot & 0xff);
  	ark3116_write_reg(serial, UART_DLM, (priv->quot>>8) & 0xff);
  
  	priv->lcr = UART_LCR_WLEN8;
  	ark3116_write_reg(serial, UART_LCR, UART_LCR_WLEN8);
  
  	ark3116_write_reg(serial, 0xe, 0);
  
  	if (priv->irda)
  		ark3116_write_reg(serial, 0x9, 0);
  
  	dev_info(&serial->dev->dev,
  		"%s using %s mode
  ",
  		KBUILD_MODNAME,
  		priv->irda ? "IrDA" : "RS232");
  	return 0;
  }
  
  static int ark3116_port_remove(struct usb_serial_port *port)
  {
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  
  	/* device is closed, so URBs and DMA should be down */
  	mutex_destroy(&priv->hw_lock);
  	kfree(priv);
  
  	return 0;
  }
  
  static void ark3116_init_termios(struct tty_struct *tty)
  {
  	struct ktermios *termios = &tty->termios;
  	*termios = tty_std_termios;
  	termios->c_cflag = B9600 | CS8
  				      | CREAD | HUPCL | CLOCAL;
  	termios->c_ispeed = 9600;
  	termios->c_ospeed = 9600;
  }
  
  static void ark3116_set_termios(struct tty_struct *tty,
  				struct usb_serial_port *port,
  				struct ktermios *old_termios)
  {
  	struct usb_serial *serial = port->serial;
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  	struct ktermios *termios = &tty->termios;
  	unsigned int cflag = termios->c_cflag;
  	int bps = tty_get_baud_rate(tty);
  	int quot;
  	__u8 lcr, hcr, eval;
  
  	/* set data bit count */
  	switch (cflag & CSIZE) {
  	case CS5:
  		lcr = UART_LCR_WLEN5;
  		break;
  	case CS6:
  		lcr = UART_LCR_WLEN6;
  		break;
  	case CS7:
  		lcr = UART_LCR_WLEN7;
  		break;
  	default:
  	case CS8:
  		lcr = UART_LCR_WLEN8;
  		break;
  	}
  	if (cflag & CSTOPB)
  		lcr |= UART_LCR_STOP;
  	if (cflag & PARENB)
  		lcr |= UART_LCR_PARITY;
  	if (!(cflag & PARODD))
  		lcr |= UART_LCR_EPAR;
  #ifdef CMSPAR
  	if (cflag & CMSPAR)
  		lcr |= UART_LCR_SPAR;
  #endif
  	/* handshake control */
  	hcr = (cflag & CRTSCTS) ? 0x03 : 0x00;
  
  	/* calc baudrate */
  	dev_dbg(&port->dev, "%s - setting bps to %d
  ", __func__, bps);
  	eval = 0;
  	switch (bps) {
  	case 0:
  		quot = calc_divisor(9600);
  		break;
  	default:
  		if ((bps < 75) || (bps > 3000000))
  			bps = 9600;
  		quot = calc_divisor(bps);
  		break;
  	case 460800:
  		eval = 1;
  		quot = calc_divisor(bps);
  		break;
  	case 921600:
  		eval = 2;
  		quot = calc_divisor(bps);
  		break;
  	}
  
  	/* Update state: synchronize */
  	mutex_lock(&priv->hw_lock);
  
  	/* keep old LCR_SBC bit */
  	lcr |= (priv->lcr & UART_LCR_SBC);
  
  	dev_dbg(&port->dev, "%s - setting hcr:0x%02x,lcr:0x%02x,quot:%d
  ",
  		__func__, hcr, lcr, quot);
  
  	/* handshake control */
  	if (priv->hcr != hcr) {
  		priv->hcr = hcr;
  		ark3116_write_reg(serial, 0x8, hcr);
  	}
  
  	/* baudrate */
  	if (priv->quot != quot) {
  		priv->quot = quot;
  		priv->lcr = lcr; /* need to write lcr anyway */
  
  		/* disable DMA since transmit/receive is
  		 * shadowed by UART_DLL
  		 */
  		ark3116_write_reg(serial, UART_FCR, 0);
  
  		ark3116_write_reg(serial, UART_LCR,
  				  lcr|UART_LCR_DLAB);
  		ark3116_write_reg(serial, UART_DLL, quot & 0xff);
  		ark3116_write_reg(serial, UART_DLM, (quot>>8) & 0xff);
  
  		/* restore lcr */
  		ark3116_write_reg(serial, UART_LCR, lcr);
  		/* magic baudrate thingy: not sure what it does,
  		 * but windows does this as well.
  		 */
  		ark3116_write_reg(serial, 0xe, eval);
  
  		/* enable DMA */
  		ark3116_write_reg(serial, UART_FCR, UART_FCR_DMA_SELECT);
  	} else if (priv->lcr != lcr) {
  		priv->lcr = lcr;
  		ark3116_write_reg(serial, UART_LCR, lcr);
  	}
  
  	mutex_unlock(&priv->hw_lock);
  
  	/* check for software flow control */
  	if (I_IXOFF(tty) || I_IXON(tty)) {
  		dev_warn(&serial->dev->dev,
  			 "%s: don't know how to do software flow control
  ",
  			 KBUILD_MODNAME);
  	}
  
  	/* Don't rewrite B0 */
  	if (tty_termios_baud_rate(termios))
  		tty_termios_encode_baud_rate(termios, bps, bps);
  }
  
  static void ark3116_close(struct usb_serial_port *port)
  {
  	struct usb_serial *serial = port->serial;
  
  	/* disable DMA */
  	ark3116_write_reg(serial, UART_FCR, 0);
  
  	/* deactivate interrupts */
  	ark3116_write_reg(serial, UART_IER, 0);
  
  	usb_serial_generic_close(port);
  	if (serial->num_interrupt_in)
  		usb_kill_urb(port->interrupt_in_urb);
  }
  
  static int ark3116_open(struct tty_struct *tty, struct usb_serial_port *port)
  {
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  	struct usb_serial *serial = port->serial;
  	unsigned char *buf;
  	int result;
  
  	buf = kmalloc(1, GFP_KERNEL);
  	if (buf == NULL)
  		return -ENOMEM;
  
  	result = usb_serial_generic_open(tty, port);
  	if (result) {
  		dev_dbg(&port->dev,
  			"%s - usb_serial_generic_open failed: %d
  ",
  			__func__, result);
  		goto err_out;
  	}
  
  	/* remove any data still left: also clears error state */
  	ark3116_read_reg(serial, UART_RX, buf);
  
  	/* read modem status */
  	priv->msr = ark3116_read_reg(serial, UART_MSR, buf);
  	/* read line status */
  	priv->lsr = ark3116_read_reg(serial, UART_LSR, buf);
  
  	result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
  	if (result) {
  		dev_err(&port->dev, "submit irq_in urb failed %d
  ",
  			result);
  		ark3116_close(port);
  		goto err_out;
  	}
  
  	/* activate interrupts */
  	ark3116_write_reg(port->serial, UART_IER, UART_IER_MSI|UART_IER_RLSI);
  
  	/* enable DMA */
  	ark3116_write_reg(port->serial, UART_FCR, UART_FCR_DMA_SELECT);
  
  	/* setup termios */
  	if (tty)
  		ark3116_set_termios(tty, port, NULL);
  
  err_out:
  	kfree(buf);
  	return result;
  }
  
  static int ark3116_ioctl(struct tty_struct *tty,
  			 unsigned int cmd, unsigned long arg)
  {
  	struct usb_serial_port *port = tty->driver_data;
  	struct serial_struct serstruct;
  	void __user *user_arg = (void __user *)arg;
  
  	switch (cmd) {
  	case TIOCGSERIAL:
  		/* XXX: Some of these values are probably wrong. */
  		memset(&serstruct, 0, sizeof(serstruct));
  		serstruct.type = PORT_16654;
  		serstruct.line = port->minor;
  		serstruct.port = port->port_number;
  		serstruct.custom_divisor = 0;
  		serstruct.baud_base = 460800;
  
  		if (copy_to_user(user_arg, &serstruct, sizeof(serstruct)))
  			return -EFAULT;
  
  		return 0;
  	case TIOCSSERIAL:
  		if (copy_from_user(&serstruct, user_arg, sizeof(serstruct)))
  			return -EFAULT;
  		return 0;
  	}
  
  	return -ENOIOCTLCMD;
  }
  
  static int ark3116_tiocmget(struct tty_struct *tty)
  {
  	struct usb_serial_port *port = tty->driver_data;
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  	__u32 status;
  	__u32 ctrl;
  	unsigned long flags;
  
  	mutex_lock(&priv->hw_lock);
  	ctrl = priv->mcr;
  	mutex_unlock(&priv->hw_lock);
  
  	spin_lock_irqsave(&priv->status_lock, flags);
  	status = priv->msr;
  	spin_unlock_irqrestore(&priv->status_lock, flags);
  
  	return  (status & UART_MSR_DSR  ? TIOCM_DSR  : 0) |
  		(status & UART_MSR_CTS  ? TIOCM_CTS  : 0) |
  		(status & UART_MSR_RI   ? TIOCM_RI   : 0) |
  		(status & UART_MSR_DCD  ? TIOCM_CD   : 0) |
  		(ctrl   & UART_MCR_DTR  ? TIOCM_DTR  : 0) |
  		(ctrl   & UART_MCR_RTS  ? TIOCM_RTS  : 0) |
  		(ctrl   & UART_MCR_OUT1 ? TIOCM_OUT1 : 0) |
  		(ctrl   & UART_MCR_OUT2 ? TIOCM_OUT2 : 0);
  }
  
  static int ark3116_tiocmset(struct tty_struct *tty,
  			unsigned set, unsigned clr)
  {
  	struct usb_serial_port *port = tty->driver_data;
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  
  	/* we need to take the mutex here, to make sure that the value
  	 * in priv->mcr is actually the one that is in the hardware
  	 */
  
  	mutex_lock(&priv->hw_lock);
  
  	if (set & TIOCM_RTS)
  		priv->mcr |= UART_MCR_RTS;
  	if (set & TIOCM_DTR)
  		priv->mcr |= UART_MCR_DTR;
  	if (set & TIOCM_OUT1)
  		priv->mcr |= UART_MCR_OUT1;
  	if (set & TIOCM_OUT2)
  		priv->mcr |= UART_MCR_OUT2;
  	if (clr & TIOCM_RTS)
  		priv->mcr &= ~UART_MCR_RTS;
  	if (clr & TIOCM_DTR)
  		priv->mcr &= ~UART_MCR_DTR;
  	if (clr & TIOCM_OUT1)
  		priv->mcr &= ~UART_MCR_OUT1;
  	if (clr & TIOCM_OUT2)
  		priv->mcr &= ~UART_MCR_OUT2;
  
  	ark3116_write_reg(port->serial, UART_MCR, priv->mcr);
  
  	mutex_unlock(&priv->hw_lock);
  
  	return 0;
  }
  
  static void ark3116_break_ctl(struct tty_struct *tty, int break_state)
  {
  	struct usb_serial_port *port = tty->driver_data;
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  
  	/* LCR is also used for other things: protect access */
  	mutex_lock(&priv->hw_lock);
  
  	if (break_state)
  		priv->lcr |= UART_LCR_SBC;
  	else
  		priv->lcr &= ~UART_LCR_SBC;
  
  	ark3116_write_reg(port->serial, UART_LCR, priv->lcr);
  
  	mutex_unlock(&priv->hw_lock);
  }
  
  static void ark3116_update_msr(struct usb_serial_port *port, __u8 msr)
  {
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  	unsigned long flags;
  
  	spin_lock_irqsave(&priv->status_lock, flags);
  	priv->msr = msr;
  	spin_unlock_irqrestore(&priv->status_lock, flags);
  
  	if (msr & UART_MSR_ANY_DELTA) {
  		/* update input line counters */
  		if (msr & UART_MSR_DCTS)
  			port->icount.cts++;
  		if (msr & UART_MSR_DDSR)
  			port->icount.dsr++;
  		if (msr & UART_MSR_DDCD)
  			port->icount.dcd++;
  		if (msr & UART_MSR_TERI)
  			port->icount.rng++;
  		wake_up_interruptible(&port->port.delta_msr_wait);
  	}
  }
  
  static void ark3116_update_lsr(struct usb_serial_port *port, __u8 lsr)
  {
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  	unsigned long flags;
  
  	spin_lock_irqsave(&priv->status_lock, flags);
  	/* combine bits */
  	priv->lsr |= lsr;
  	spin_unlock_irqrestore(&priv->status_lock, flags);
  
  	if (lsr&UART_LSR_BRK_ERROR_BITS) {
  		if (lsr & UART_LSR_BI)
  			port->icount.brk++;
  		if (lsr & UART_LSR_FE)
  			port->icount.frame++;
  		if (lsr & UART_LSR_PE)
  			port->icount.parity++;
  		if (lsr & UART_LSR_OE)
  			port->icount.overrun++;
  	}
  }
  
  static void ark3116_read_int_callback(struct urb *urb)
  {
  	struct usb_serial_port *port = urb->context;
  	int status = urb->status;
  	const __u8 *data = urb->transfer_buffer;
  	int result;
  
  	switch (status) {
  	case -ECONNRESET:
  	case -ENOENT:
  	case -ESHUTDOWN:
  		/* this urb is terminated, clean up */
  		dev_dbg(&port->dev, "%s - urb shutting down with status: %d
  ",
  			__func__, status);
  		return;
  	default:
  		dev_dbg(&port->dev, "%s - nonzero urb status received: %d
  ",
  			__func__, status);
  		break;
  	case 0: /* success */
  		/* discovered this by trail and error... */
  		if ((urb->actual_length == 4) && (data[0] == 0xe8)) {
  			const __u8 id = data[1]&UART_IIR_ID;
  			dev_dbg(&port->dev, "%s: iir=%02x
  ", __func__, data[1]);
  			if (id == UART_IIR_MSI) {
  				dev_dbg(&port->dev, "%s: msr=%02x
  ",
  					__func__, data[3]);
  				ark3116_update_msr(port, data[3]);
  				break;
  			} else if (id == UART_IIR_RLSI) {
  				dev_dbg(&port->dev, "%s: lsr=%02x
  ",
  					__func__, data[2]);
  				ark3116_update_lsr(port, data[2]);
  				break;
  			}
  		}
  		/*
  		 * Not sure what this data meant...
  		 */
  		usb_serial_debug_data(&port->dev, __func__,
  				      urb->actual_length,
  				      urb->transfer_buffer);
  		break;
  	}
  
  	result = usb_submit_urb(urb, GFP_ATOMIC);
  	if (result)
  		dev_err(&urb->dev->dev,
  			"%s - Error %d submitting interrupt urb
  ",
  			__func__, result);
  }
  
  
  /* Data comes in via the bulk (data) URB, errors/interrupts via the int URB.
   * This means that we cannot be sure which data byte has an associated error
   * condition, so we report an error for all data in the next bulk read.
   *
   * Actually, there might even be a window between the bulk data leaving the
   * ark and reading/resetting the lsr in the read_bulk_callback where an
   * interrupt for the next data block could come in.
   * Without somekind of ordering on the ark, we would have to report the
   * error for the next block of data as well...
   * For now, let's pretend this can't happen.
   */
  static void ark3116_process_read_urb(struct urb *urb)
  {
  	struct usb_serial_port *port = urb->context;
  	struct ark3116_private *priv = usb_get_serial_port_data(port);
  	unsigned char *data = urb->transfer_buffer;
  	char tty_flag = TTY_NORMAL;
  	unsigned long flags;
  	__u32 lsr;
  
  	/* update line status */
  	spin_lock_irqsave(&priv->status_lock, flags);
  	lsr = priv->lsr;
  	priv->lsr &= ~UART_LSR_BRK_ERROR_BITS;
  	spin_unlock_irqrestore(&priv->status_lock, flags);
  
  	if (!urb->actual_length)
  		return;
  
  	if (lsr & UART_LSR_BRK_ERROR_BITS) {
  		if (lsr & UART_LSR_BI)
  			tty_flag = TTY_BREAK;
  		else if (lsr & UART_LSR_PE)
  			tty_flag = TTY_PARITY;
  		else if (lsr & UART_LSR_FE)
  			tty_flag = TTY_FRAME;
  
  		/* overrun is special, not associated with a char */
  		if (lsr & UART_LSR_OE)
  			tty_insert_flip_char(&port->port, 0, TTY_OVERRUN);
  	}
  	tty_insert_flip_string_fixed_flag(&port->port, data, tty_flag,
  							urb->actual_length);
  	tty_flip_buffer_push(&port->port);
  }
  
  static struct usb_serial_driver ark3116_device = {
  	.driver = {
  		.owner =	THIS_MODULE,
  		.name =		"ark3116",
  	},
  	.id_table =		id_table,
  	.num_ports =		1,
  	.attach =		ark3116_attach,
  	.port_probe =		ark3116_port_probe,
  	.port_remove =		ark3116_port_remove,
  	.set_termios =		ark3116_set_termios,
  	.init_termios =		ark3116_init_termios,
  	.ioctl =		ark3116_ioctl,
  	.tiocmget =		ark3116_tiocmget,
  	.tiocmset =		ark3116_tiocmset,
  	.tiocmiwait =		usb_serial_generic_tiocmiwait,
  	.get_icount =		usb_serial_generic_get_icount,
  	.open =			ark3116_open,
  	.close =		ark3116_close,
  	.break_ctl = 		ark3116_break_ctl,
  	.read_int_callback = 	ark3116_read_int_callback,
  	.process_read_urb =	ark3116_process_read_urb,
  };
  
  static struct usb_serial_driver * const serial_drivers[] = {
  	&ark3116_device, NULL
  };
  
  module_usb_serial_driver(serial_drivers, id_table);
  
  MODULE_LICENSE("GPL");
  
  MODULE_AUTHOR(DRIVER_AUTHOR);
  MODULE_DESCRIPTION(DRIVER_DESC);
  
  /*
   * The following describes what I learned from studying the old
   * ark3116.c driver, disassembling the windows driver, and some lucky
   * guesses. Since I do not have any datasheet or other
   * documentation, inaccuracies are almost guaranteed.
   *
   * Some specs for the ARK3116 can be found here:
   * http://web.archive.org/web/20060318000438/
   *   www.arkmicro.com/en/products/view.php?id=10
   * On that page, 2 GPIO pins are mentioned: I assume these are the
   * OUT1 and OUT2 pins of the UART, so I added support for those
   * through the MCR. Since the pins are not available on my hardware,
   * I could not verify this.
   * Also, it states there is "on-chip hardware flow control". I have
   * discovered how to enable that. Unfortunately, I do not know how to
   * enable XON/XOFF (software) flow control, which would need support
   * from the chip as well to work. Because of the wording on the web
   * page there is a real possibility the chip simply does not support
   * software flow control.
   *
   * I got my ark3116 as part of a mobile phone adapter cable. On the
   * PCB, the following numbered contacts are present:
   *
   *  1:- +5V
   *  2:o DTR
   *  3:i RX
   *  4:i DCD
   *  5:o RTS
   *  6:o TX
   *  7:i RI
   *  8:i DSR
   * 10:- 0V
   * 11:i CTS
   *
   * On my chip, all signals seem to be 3.3V, but 5V tolerant. But that
   * may be different for the one you have ;-).
   *
   * The windows driver limits the registers to 0-F, so I assume there
   * are actually 16 present on the device.
   *
   * On an UART interrupt, 4 bytes of data come in on the interrupt
   * endpoint. The bytes are 0xe8 IIR LSR MSR.
   *
   * The baudrate seems to be generated from the 12MHz crystal, using
   * 4-times subsampling. So quot=12e6/(4*baud). Also see description
   * of register E.
   *
   * Registers 0-7:
   * These seem to be the same as for a regular 16450. The FCR is set
   * to UART_FCR_DMA_SELECT (0x8), I guess to enable transfers between
   * the UART and the USB bridge/DMA engine.
   *
   * Register 8:
   * By trial and error, I found out that bit 0 enables hardware CTS,
   * stopping TX when CTS is +5V. Bit 1 does the same for RTS, making
   * RTS +5V when the 3116 cannot transfer the data to the USB bus
   * (verified by disabling the reading URB). Note that as far as I can
   * tell, the windows driver does NOT use this, so there might be some
   * hardware bug or something.
   *
   * According to a patch provided here
   * (http://lkml.org/lkml/2009/7/26/56), the ARK3116 can also be used
   * as an IrDA dongle. Since I do not have such a thing, I could not
   * investigate that aspect. However, I can speculate ;-).
   *
   * - IrDA encodes data differently than RS232. Most likely, one of
   *   the bits in registers 9..E enables the IR ENDEC (encoder/decoder).
   * - Depending on the IR transceiver, the input and output need to be
   *   inverted, so there are probably bits for that as well.
   * - IrDA is half-duplex, so there should be a bit for selecting that.
   *
   * This still leaves at least two registers unaccounted for. Perhaps
   * The chip can do XON/XOFF or CRC in HW?
   *
   * Register 9:
   * Set to 0x00 for IrDA, when the baudrate is initialised.
   *
   * Register A:
   * Set to 0x01 for IrDA, at init.
   *
   * Register B:
   * Set to 0x01 for IrDA, 0x00 for RS232, at init.
   *
   * Register C:
   * Set to 00 for IrDA, at init.
   *
   * Register D:
   * Set to 0x41 for IrDA, at init.
   *
   * Register E:
   * Somekind of baudrate override. The windows driver seems to set
   * this to 0x00 for normal baudrates, 0x01 for 460800, 0x02 for 921600.
   * Since 460800 and 921600 cannot be obtained by dividing 3MHz by an integer,
   * it could be somekind of subdivisor thingy.
   * However,it does not seem to do anything: selecting 921600 (divisor 3,
   * reg E=2), still gets 1 MHz. I also checked if registers 9, C or F would
   * work, but they don't.
   *
   * Register F: unknown
   */