ark3116.c

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/*
 * Copyright (C) 2009 by Bart Hartgers ([email protected])
 * 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/init.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>

/*
 * Version information
 */

#define DRIVER_VERSION "v0.7"
#define DRIVER_AUTHOR "Bart Hartgers <[email protected]>"
#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 (1*HZ)

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 {
    wait_queue_head_t       delta_msr_wait;
    struct async_icount icount;
    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 contol 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\n",
            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;

    init_waitqueue_head(&priv->delta_msr_wait);
    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\n",
        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\n", __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\n",
        __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\n",
             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;

    if (serial->dev) {
        /* 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\n",
            __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\n",
            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_get_icount(struct tty_struct *tty,
                    struct serial_icounter_struct *icount)
{
    struct usb_serial_port *port = tty->driver_data;
    struct ark3116_private *priv = usb_get_serial_port_data(port);
    struct async_icount cnow = priv->icount;
    icount->cts = cnow.cts;
    icount->dsr = cnow.dsr;
    icount->rng = cnow.rng;
    icount->dcd = cnow.dcd;
    icount->rx = cnow.rx;
    icount->tx = cnow.tx;
    icount->frame = cnow.frame;
    icount->overrun = cnow.overrun;
    icount->parity = cnow.parity;
    icount->brk = cnow.brk;
    icount->buf_overrun = cnow.buf_overrun;
    return 0;
}

static int ark3116_ioctl(struct tty_struct *tty,
             unsigned int cmd, unsigned long arg)
{
    struct usb_serial_port *port = tty->driver_data;
    struct ark3116_private *priv = usb_get_serial_port_data(port);
    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->serial->minor;
        serstruct.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;
    case TIOCMIWAIT:
        for (;;) {
            struct async_icount prev = priv->icount;
            interruptible_sleep_on(&priv->delta_msr_wait);
            /* see if a signal did it */
            if (signal_pending(current))
                return -ERESTARTSYS;
            if ((prev.rng == priv->icount.rng) &&
                (prev.dsr == priv->icount.dsr) &&
                (prev.dcd == priv->icount.dcd) &&
                (prev.cts == priv->icount.cts))
                return -EIO;
            if ((arg & TIOCM_RNG &&
                 (prev.rng != priv->icount.rng)) ||
                (arg & TIOCM_DSR &&
                 (prev.dsr != priv->icount.dsr)) ||
                (arg & TIOCM_CD  &&
                 (prev.dcd != priv->icount.dcd)) ||
                (arg & TIOCM_CTS &&
                 (prev.cts != priv->icount.cts)))
                return 0;
        }
        break;
    }

    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)
            priv->icount.cts++;
        if (msr & UART_MSR_DDSR)
            priv->icount.dsr++;
        if (msr & UART_MSR_DDCD)
            priv->icount.dcd++;
        if (msr & UART_MSR_TERI)
            priv->icount.rng++;
        wake_up_interruptible(&priv->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)
            priv->icount.brk++;
        if (lsr & UART_LSR_FE)
            priv->icount.frame++;
        if (lsr & UART_LSR_PE)
            priv->icount.parity++;
        if (lsr & UART_LSR_OE)
            priv->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\n",
            __func__, status);
        return;
    default:
        dev_dbg(&port->dev, "%s - nonzero urb status received: %d\n",
            __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\n", __func__, data[1]);
            if (id == UART_IIR_MSI) {
                dev_dbg(&port->dev, "%s: msr=%02x\n",
                    __func__, data[3]);
                ark3116_update_msr(port, data[3]);
                break;
            } else if (id == UART_IIR_RLSI) {
                dev_dbg(&port->dev, "%s: lsr=%02x\n",
                    __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\n",
            __func__, result);
}


/* Data comes in via the bulk (data) URB, erors/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);
    struct tty_struct *tty;
    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;

    tty = tty_port_tty_get(&port->port);
    if (!tty)
        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(tty, 0, TTY_OVERRUN);
    }
    tty_insert_flip_string_fixed_flag(tty, data, tty_flag,
                            urb->actual_length);
    tty_flip_buffer_push(tty);
    tty_kref_put(tty);
}

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,
    .get_icount =       ark3116_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
 */