linux/drivers/usb/serial/ark3116.c
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   1// SPDX-License-Identifier: GPL-2.0+
   2/*
   3 * Copyright (C) 2009 by Bart Hartgers (bart.hartgers+ark3116@gmail.com)
   4 * Original version:
   5 * Copyright (C) 2006
   6 *   Simon Schulz (ark3116_driver <at> auctionant.de)
   7 *
   8 * ark3116
   9 * - implements a driver for the arkmicro ark3116 chipset (vendor=0x6547,
  10 *   productid=0x0232) (used in a datacable called KQ-U8A)
  11 *
  12 * Supports full modem status lines, break, hardware flow control. Does not
  13 * support software flow control, since I do not know how to enable it in hw.
  14 *
  15 * This driver is a essentially new implementation. I initially dug
  16 * into the old ark3116.c driver and suddenly realized the ark3116 is
  17 * a 16450 with a USB interface glued to it. See comments at the
  18 * bottom of this file.
  19 */
  20
  21#include <linux/kernel.h>
  22#include <linux/ioctl.h>
  23#include <linux/tty.h>
  24#include <linux/slab.h>
  25#include <linux/tty_flip.h>
  26#include <linux/module.h>
  27#include <linux/usb.h>
  28#include <linux/usb/serial.h>
  29#include <linux/serial.h>
  30#include <linux/serial_reg.h>
  31#include <linux/uaccess.h>
  32#include <linux/mutex.h>
  33#include <linux/spinlock.h>
  34
  35#define DRIVER_AUTHOR "Bart Hartgers <bart.hartgers+ark3116@gmail.com>"
  36#define DRIVER_DESC "USB ARK3116 serial/IrDA driver"
  37#define DRIVER_DEV_DESC "ARK3116 RS232/IrDA"
  38#define DRIVER_NAME "ark3116"
  39
  40/* usb timeout of 1 second */
  41#define ARK_TIMEOUT 1000
  42
  43static const struct usb_device_id id_table[] = {
  44        { USB_DEVICE(0x6547, 0x0232) },
  45        { USB_DEVICE(0x18ec, 0x3118) },         /* USB to IrDA adapter */
  46        { },
  47};
  48MODULE_DEVICE_TABLE(usb, id_table);
  49
  50static int is_irda(struct usb_serial *serial)
  51{
  52        struct usb_device *dev = serial->dev;
  53        if (le16_to_cpu(dev->descriptor.idVendor) == 0x18ec &&
  54                        le16_to_cpu(dev->descriptor.idProduct) == 0x3118)
  55                return 1;
  56        return 0;
  57}
  58
  59struct ark3116_private {
  60        int                     irda;   /* 1 for irda device */
  61
  62        /* protects hw register updates */
  63        struct mutex            hw_lock;
  64
  65        int                     quot;   /* baudrate divisor */
  66        __u32                   lcr;    /* line control register value */
  67        __u32                   hcr;    /* handshake control register (0x8)
  68                                         * value */
  69        __u32                   mcr;    /* modem control register value */
  70
  71        /* protects the status values below */
  72        spinlock_t              status_lock;
  73        __u32                   msr;    /* modem status register value */
  74        __u32                   lsr;    /* line status register value */
  75};
  76
  77static int ark3116_write_reg(struct usb_serial *serial,
  78                             unsigned reg, __u8 val)
  79{
  80        int result;
  81         /* 0xfe 0x40 are magic values taken from original driver */
  82        result = usb_control_msg(serial->dev,
  83                                 usb_sndctrlpipe(serial->dev, 0),
  84                                 0xfe, 0x40, val, reg,
  85                                 NULL, 0, ARK_TIMEOUT);
  86        if (result)
  87                return result;
  88
  89        return 0;
  90}
  91
  92static int ark3116_read_reg(struct usb_serial *serial,
  93                            unsigned reg, unsigned char *buf)
  94{
  95        int result;
  96        /* 0xfe 0xc0 are magic values taken from original driver */
  97        result = usb_control_msg(serial->dev,
  98                                 usb_rcvctrlpipe(serial->dev, 0),
  99                                 0xfe, 0xc0, 0, reg,
 100                                 buf, 1, ARK_TIMEOUT);
 101        if (result < 1) {
 102                dev_err(&serial->interface->dev,
 103                                "failed to read register %u: %d\n",
 104                                reg, result);
 105                if (result >= 0)
 106                        result = -EIO;
 107
 108                return result;
 109        }
 110
 111        return 0;
 112}
 113
 114static inline int calc_divisor(int bps)
 115{
 116        /* Original ark3116 made some exceptions in rounding here
 117         * because windows did the same. Assume that is not really
 118         * necessary.
 119         * Crystal is 12MHz, probably because of USB, but we divide by 4?
 120         */
 121        return (12000000 + 2*bps) / (4*bps);
 122}
 123
 124static int ark3116_port_probe(struct usb_serial_port *port)
 125{
 126        struct usb_serial *serial = port->serial;
 127        struct ark3116_private *priv;
 128
 129        priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 130        if (!priv)
 131                return -ENOMEM;
 132
 133        mutex_init(&priv->hw_lock);
 134        spin_lock_init(&priv->status_lock);
 135
 136        priv->irda = is_irda(serial);
 137
 138        usb_set_serial_port_data(port, priv);
 139
 140        /* setup the hardware */
 141        ark3116_write_reg(serial, UART_IER, 0);
 142        /* disable DMA */
 143        ark3116_write_reg(serial, UART_FCR, 0);
 144        /* handshake control */
 145        priv->hcr = 0;
 146        ark3116_write_reg(serial, 0x8     , 0);
 147        /* modem control */
 148        priv->mcr = 0;
 149        ark3116_write_reg(serial, UART_MCR, 0);
 150
 151        if (!(priv->irda)) {
 152                ark3116_write_reg(serial, 0xb , 0);
 153        } else {
 154                ark3116_write_reg(serial, 0xb , 1);
 155                ark3116_write_reg(serial, 0xc , 0);
 156                ark3116_write_reg(serial, 0xd , 0x41);
 157                ark3116_write_reg(serial, 0xa , 1);
 158        }
 159
 160        /* setup baudrate */
 161        ark3116_write_reg(serial, UART_LCR, UART_LCR_DLAB);
 162
 163        /* setup for 9600 8N1 */
 164        priv->quot = calc_divisor(9600);
 165        ark3116_write_reg(serial, UART_DLL, priv->quot & 0xff);
 166        ark3116_write_reg(serial, UART_DLM, (priv->quot>>8) & 0xff);
 167
 168        priv->lcr = UART_LCR_WLEN8;
 169        ark3116_write_reg(serial, UART_LCR, UART_LCR_WLEN8);
 170
 171        ark3116_write_reg(serial, 0xe, 0);
 172
 173        if (priv->irda)
 174                ark3116_write_reg(serial, 0x9, 0);
 175
 176        dev_info(&port->dev, "using %s mode\n", priv->irda ? "IrDA" : "RS232");
 177
 178        return 0;
 179}
 180
 181static int ark3116_port_remove(struct usb_serial_port *port)
 182{
 183        struct ark3116_private *priv = usb_get_serial_port_data(port);
 184
 185        /* device is closed, so URBs and DMA should be down */
 186        mutex_destroy(&priv->hw_lock);
 187        kfree(priv);
 188
 189        return 0;
 190}
 191
 192static void ark3116_set_termios(struct tty_struct *tty,
 193                                struct usb_serial_port *port,
 194                                struct ktermios *old_termios)
 195{
 196        struct usb_serial *serial = port->serial;
 197        struct ark3116_private *priv = usb_get_serial_port_data(port);
 198        struct ktermios *termios = &tty->termios;
 199        unsigned int cflag = termios->c_cflag;
 200        int bps = tty_get_baud_rate(tty);
 201        int quot;
 202        __u8 lcr, hcr, eval;
 203
 204        /* set data bit count */
 205        switch (cflag & CSIZE) {
 206        case CS5:
 207                lcr = UART_LCR_WLEN5;
 208                break;
 209        case CS6:
 210                lcr = UART_LCR_WLEN6;
 211                break;
 212        case CS7:
 213                lcr = UART_LCR_WLEN7;
 214                break;
 215        default:
 216        case CS8:
 217                lcr = UART_LCR_WLEN8;
 218                break;
 219        }
 220        if (cflag & CSTOPB)
 221                lcr |= UART_LCR_STOP;
 222        if (cflag & PARENB)
 223                lcr |= UART_LCR_PARITY;
 224        if (!(cflag & PARODD))
 225                lcr |= UART_LCR_EPAR;
 226#ifdef CMSPAR
 227        if (cflag & CMSPAR)
 228                lcr |= UART_LCR_SPAR;
 229#endif
 230        /* handshake control */
 231        hcr = (cflag & CRTSCTS) ? 0x03 : 0x00;
 232
 233        /* calc baudrate */
 234        dev_dbg(&port->dev, "%s - setting bps to %d\n", __func__, bps);
 235        eval = 0;
 236        switch (bps) {
 237        case 0:
 238                quot = calc_divisor(9600);
 239                break;
 240        default:
 241                if ((bps < 75) || (bps > 3000000))
 242                        bps = 9600;
 243                quot = calc_divisor(bps);
 244                break;
 245        case 460800:
 246                eval = 1;
 247                quot = calc_divisor(bps);
 248                break;
 249        case 921600:
 250                eval = 2;
 251                quot = calc_divisor(bps);
 252                break;
 253        }
 254
 255        /* Update state: synchronize */
 256        mutex_lock(&priv->hw_lock);
 257
 258        /* keep old LCR_SBC bit */
 259        lcr |= (priv->lcr & UART_LCR_SBC);
 260
 261        dev_dbg(&port->dev, "%s - setting hcr:0x%02x,lcr:0x%02x,quot:%d\n",
 262                __func__, hcr, lcr, quot);
 263
 264        /* handshake control */
 265        if (priv->hcr != hcr) {
 266                priv->hcr = hcr;
 267                ark3116_write_reg(serial, 0x8, hcr);
 268        }
 269
 270        /* baudrate */
 271        if (priv->quot != quot) {
 272                priv->quot = quot;
 273                priv->lcr = lcr; /* need to write lcr anyway */
 274
 275                /* disable DMA since transmit/receive is
 276                 * shadowed by UART_DLL
 277                 */
 278                ark3116_write_reg(serial, UART_FCR, 0);
 279
 280                ark3116_write_reg(serial, UART_LCR,
 281                                  lcr|UART_LCR_DLAB);
 282                ark3116_write_reg(serial, UART_DLL, quot & 0xff);
 283                ark3116_write_reg(serial, UART_DLM, (quot>>8) & 0xff);
 284
 285                /* restore lcr */
 286                ark3116_write_reg(serial, UART_LCR, lcr);
 287                /* magic baudrate thingy: not sure what it does,
 288                 * but windows does this as well.
 289                 */
 290                ark3116_write_reg(serial, 0xe, eval);
 291
 292                /* enable DMA */
 293                ark3116_write_reg(serial, UART_FCR, UART_FCR_DMA_SELECT);
 294        } else if (priv->lcr != lcr) {
 295                priv->lcr = lcr;
 296                ark3116_write_reg(serial, UART_LCR, lcr);
 297        }
 298
 299        mutex_unlock(&priv->hw_lock);
 300
 301        /* check for software flow control */
 302        if (I_IXOFF(tty) || I_IXON(tty)) {
 303                dev_warn(&port->dev,
 304                                "software flow control not implemented\n");
 305        }
 306
 307        /* Don't rewrite B0 */
 308        if (tty_termios_baud_rate(termios))
 309                tty_termios_encode_baud_rate(termios, bps, bps);
 310}
 311
 312static void ark3116_close(struct usb_serial_port *port)
 313{
 314        struct usb_serial *serial = port->serial;
 315
 316        /* disable DMA */
 317        ark3116_write_reg(serial, UART_FCR, 0);
 318
 319        /* deactivate interrupts */
 320        ark3116_write_reg(serial, UART_IER, 0);
 321
 322        usb_serial_generic_close(port);
 323
 324        usb_kill_urb(port->interrupt_in_urb);
 325}
 326
 327static int ark3116_open(struct tty_struct *tty, struct usb_serial_port *port)
 328{
 329        struct ark3116_private *priv = usb_get_serial_port_data(port);
 330        struct usb_serial *serial = port->serial;
 331        unsigned char *buf;
 332        int result;
 333
 334        buf = kmalloc(1, GFP_KERNEL);
 335        if (buf == NULL)
 336                return -ENOMEM;
 337
 338        result = usb_serial_generic_open(tty, port);
 339        if (result) {
 340                dev_dbg(&port->dev,
 341                        "%s - usb_serial_generic_open failed: %d\n",
 342                        __func__, result);
 343                goto err_free;
 344        }
 345
 346        /* remove any data still left: also clears error state */
 347        ark3116_read_reg(serial, UART_RX, buf);
 348
 349        /* read modem status */
 350        result = ark3116_read_reg(serial, UART_MSR, buf);
 351        if (result)
 352                goto err_close;
 353        priv->msr = *buf;
 354
 355        /* read line status */
 356        result = ark3116_read_reg(serial, UART_LSR, buf);
 357        if (result)
 358                goto err_close;
 359        priv->lsr = *buf;
 360
 361        result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
 362        if (result) {
 363                dev_err(&port->dev, "submit irq_in urb failed %d\n",
 364                        result);
 365                goto err_close;
 366        }
 367
 368        /* activate interrupts */
 369        ark3116_write_reg(port->serial, UART_IER, UART_IER_MSI|UART_IER_RLSI);
 370
 371        /* enable DMA */
 372        ark3116_write_reg(port->serial, UART_FCR, UART_FCR_DMA_SELECT);
 373
 374        /* setup termios */
 375        if (tty)
 376                ark3116_set_termios(tty, port, NULL);
 377
 378        kfree(buf);
 379
 380        return 0;
 381
 382err_close:
 383        usb_serial_generic_close(port);
 384err_free:
 385        kfree(buf);
 386
 387        return result;
 388}
 389
 390static int ark3116_tiocmget(struct tty_struct *tty)
 391{
 392        struct usb_serial_port *port = tty->driver_data;
 393        struct ark3116_private *priv = usb_get_serial_port_data(port);
 394        __u32 status;
 395        __u32 ctrl;
 396        unsigned long flags;
 397
 398        mutex_lock(&priv->hw_lock);
 399        ctrl = priv->mcr;
 400        mutex_unlock(&priv->hw_lock);
 401
 402        spin_lock_irqsave(&priv->status_lock, flags);
 403        status = priv->msr;
 404        spin_unlock_irqrestore(&priv->status_lock, flags);
 405
 406        return  (status & UART_MSR_DSR  ? TIOCM_DSR  : 0) |
 407                (status & UART_MSR_CTS  ? TIOCM_CTS  : 0) |
 408                (status & UART_MSR_RI   ? TIOCM_RI   : 0) |
 409                (status & UART_MSR_DCD  ? TIOCM_CD   : 0) |
 410                (ctrl   & UART_MCR_DTR  ? TIOCM_DTR  : 0) |
 411                (ctrl   & UART_MCR_RTS  ? TIOCM_RTS  : 0) |
 412                (ctrl   & UART_MCR_OUT1 ? TIOCM_OUT1 : 0) |
 413                (ctrl   & UART_MCR_OUT2 ? TIOCM_OUT2 : 0);
 414}
 415
 416static int ark3116_tiocmset(struct tty_struct *tty,
 417                        unsigned set, unsigned clr)
 418{
 419        struct usb_serial_port *port = tty->driver_data;
 420        struct ark3116_private *priv = usb_get_serial_port_data(port);
 421
 422        /* we need to take the mutex here, to make sure that the value
 423         * in priv->mcr is actually the one that is in the hardware
 424         */
 425
 426        mutex_lock(&priv->hw_lock);
 427
 428        if (set & TIOCM_RTS)
 429                priv->mcr |= UART_MCR_RTS;
 430        if (set & TIOCM_DTR)
 431                priv->mcr |= UART_MCR_DTR;
 432        if (set & TIOCM_OUT1)
 433                priv->mcr |= UART_MCR_OUT1;
 434        if (set & TIOCM_OUT2)
 435                priv->mcr |= UART_MCR_OUT2;
 436        if (clr & TIOCM_RTS)
 437                priv->mcr &= ~UART_MCR_RTS;
 438        if (clr & TIOCM_DTR)
 439                priv->mcr &= ~UART_MCR_DTR;
 440        if (clr & TIOCM_OUT1)
 441                priv->mcr &= ~UART_MCR_OUT1;
 442        if (clr & TIOCM_OUT2)
 443                priv->mcr &= ~UART_MCR_OUT2;
 444
 445        ark3116_write_reg(port->serial, UART_MCR, priv->mcr);
 446
 447        mutex_unlock(&priv->hw_lock);
 448
 449        return 0;
 450}
 451
 452static void ark3116_break_ctl(struct tty_struct *tty, int break_state)
 453{
 454        struct usb_serial_port *port = tty->driver_data;
 455        struct ark3116_private *priv = usb_get_serial_port_data(port);
 456
 457        /* LCR is also used for other things: protect access */
 458        mutex_lock(&priv->hw_lock);
 459
 460        if (break_state)
 461                priv->lcr |= UART_LCR_SBC;
 462        else
 463                priv->lcr &= ~UART_LCR_SBC;
 464
 465        ark3116_write_reg(port->serial, UART_LCR, priv->lcr);
 466
 467        mutex_unlock(&priv->hw_lock);
 468}
 469
 470static void ark3116_update_msr(struct usb_serial_port *port, __u8 msr)
 471{
 472        struct ark3116_private *priv = usb_get_serial_port_data(port);
 473        unsigned long flags;
 474
 475        spin_lock_irqsave(&priv->status_lock, flags);
 476        priv->msr = msr;
 477        spin_unlock_irqrestore(&priv->status_lock, flags);
 478
 479        if (msr & UART_MSR_ANY_DELTA) {
 480                /* update input line counters */
 481                if (msr & UART_MSR_DCTS)
 482                        port->icount.cts++;
 483                if (msr & UART_MSR_DDSR)
 484                        port->icount.dsr++;
 485                if (msr & UART_MSR_DDCD)
 486                        port->icount.dcd++;
 487                if (msr & UART_MSR_TERI)
 488                        port->icount.rng++;
 489                wake_up_interruptible(&port->port.delta_msr_wait);
 490        }
 491}
 492
 493static void ark3116_update_lsr(struct usb_serial_port *port, __u8 lsr)
 494{
 495        struct ark3116_private *priv = usb_get_serial_port_data(port);
 496        unsigned long flags;
 497
 498        spin_lock_irqsave(&priv->status_lock, flags);
 499        /* combine bits */
 500        priv->lsr |= lsr;
 501        spin_unlock_irqrestore(&priv->status_lock, flags);
 502
 503        if (lsr&UART_LSR_BRK_ERROR_BITS) {
 504                if (lsr & UART_LSR_BI)
 505                        port->icount.brk++;
 506                if (lsr & UART_LSR_FE)
 507                        port->icount.frame++;
 508                if (lsr & UART_LSR_PE)
 509                        port->icount.parity++;
 510                if (lsr & UART_LSR_OE)
 511                        port->icount.overrun++;
 512        }
 513}
 514
 515static void ark3116_read_int_callback(struct urb *urb)
 516{
 517        struct usb_serial_port *port = urb->context;
 518        int status = urb->status;
 519        const __u8 *data = urb->transfer_buffer;
 520        int result;
 521
 522        switch (status) {
 523        case -ECONNRESET:
 524        case -ENOENT:
 525        case -ESHUTDOWN:
 526                /* this urb is terminated, clean up */
 527                dev_dbg(&port->dev, "%s - urb shutting down with status: %d\n",
 528                        __func__, status);
 529                return;
 530        default:
 531                dev_dbg(&port->dev, "%s - nonzero urb status received: %d\n",
 532                        __func__, status);
 533                break;
 534        case 0: /* success */
 535                /* discovered this by trail and error... */
 536                if ((urb->actual_length == 4) && (data[0] == 0xe8)) {
 537                        const __u8 id = data[1]&UART_IIR_ID;
 538                        dev_dbg(&port->dev, "%s: iir=%02x\n", __func__, data[1]);
 539                        if (id == UART_IIR_MSI) {
 540                                dev_dbg(&port->dev, "%s: msr=%02x\n",
 541                                        __func__, data[3]);
 542                                ark3116_update_msr(port, data[3]);
 543                                break;
 544                        } else if (id == UART_IIR_RLSI) {
 545                                dev_dbg(&port->dev, "%s: lsr=%02x\n",
 546                                        __func__, data[2]);
 547                                ark3116_update_lsr(port, data[2]);
 548                                break;
 549                        }
 550                }
 551                /*
 552                 * Not sure what this data meant...
 553                 */
 554                usb_serial_debug_data(&port->dev, __func__,
 555                                      urb->actual_length,
 556                                      urb->transfer_buffer);
 557                break;
 558        }
 559
 560        result = usb_submit_urb(urb, GFP_ATOMIC);
 561        if (result)
 562                dev_err(&port->dev, "failed to resubmit interrupt urb: %d\n",
 563                        result);
 564}
 565
 566
 567/* Data comes in via the bulk (data) URB, errors/interrupts via the int URB.
 568 * This means that we cannot be sure which data byte has an associated error
 569 * condition, so we report an error for all data in the next bulk read.
 570 *
 571 * Actually, there might even be a window between the bulk data leaving the
 572 * ark and reading/resetting the lsr in the read_bulk_callback where an
 573 * interrupt for the next data block could come in.
 574 * Without somekind of ordering on the ark, we would have to report the
 575 * error for the next block of data as well...
 576 * For now, let's pretend this can't happen.
 577 */
 578static void ark3116_process_read_urb(struct urb *urb)
 579{
 580        struct usb_serial_port *port = urb->context;
 581        struct ark3116_private *priv = usb_get_serial_port_data(port);
 582        unsigned char *data = urb->transfer_buffer;
 583        char tty_flag = TTY_NORMAL;
 584        unsigned long flags;
 585        __u32 lsr;
 586
 587        /* update line status */
 588        spin_lock_irqsave(&priv->status_lock, flags);
 589        lsr = priv->lsr;
 590        priv->lsr &= ~UART_LSR_BRK_ERROR_BITS;
 591        spin_unlock_irqrestore(&priv->status_lock, flags);
 592
 593        if (!urb->actual_length)
 594                return;
 595
 596        if (lsr & UART_LSR_BRK_ERROR_BITS) {
 597                if (lsr & UART_LSR_BI)
 598                        tty_flag = TTY_BREAK;
 599                else if (lsr & UART_LSR_PE)
 600                        tty_flag = TTY_PARITY;
 601                else if (lsr & UART_LSR_FE)
 602                        tty_flag = TTY_FRAME;
 603
 604                /* overrun is special, not associated with a char */
 605                if (lsr & UART_LSR_OE)
 606                        tty_insert_flip_char(&port->port, 0, TTY_OVERRUN);
 607        }
 608        tty_insert_flip_string_fixed_flag(&port->port, data, tty_flag,
 609                                                        urb->actual_length);
 610        tty_flip_buffer_push(&port->port);
 611}
 612
 613static struct usb_serial_driver ark3116_device = {
 614        .driver = {
 615                .owner =        THIS_MODULE,
 616                .name =         "ark3116",
 617        },
 618        .id_table =             id_table,
 619        .num_ports =            1,
 620        .num_bulk_in =          1,
 621        .num_bulk_out =         1,
 622        .num_interrupt_in =     1,
 623        .port_probe =           ark3116_port_probe,
 624        .port_remove =          ark3116_port_remove,
 625        .set_termios =          ark3116_set_termios,
 626        .tiocmget =             ark3116_tiocmget,
 627        .tiocmset =             ark3116_tiocmset,
 628        .tiocmiwait =           usb_serial_generic_tiocmiwait,
 629        .get_icount =           usb_serial_generic_get_icount,
 630        .open =                 ark3116_open,
 631        .close =                ark3116_close,
 632        .break_ctl =            ark3116_break_ctl,
 633        .read_int_callback =    ark3116_read_int_callback,
 634        .process_read_urb =     ark3116_process_read_urb,
 635};
 636
 637static struct usb_serial_driver * const serial_drivers[] = {
 638        &ark3116_device, NULL
 639};
 640
 641module_usb_serial_driver(serial_drivers, id_table);
 642
 643MODULE_LICENSE("GPL");
 644
 645MODULE_AUTHOR(DRIVER_AUTHOR);
 646MODULE_DESCRIPTION(DRIVER_DESC);
 647
 648/*
 649 * The following describes what I learned from studying the old
 650 * ark3116.c driver, disassembling the windows driver, and some lucky
 651 * guesses. Since I do not have any datasheet or other
 652 * documentation, inaccuracies are almost guaranteed.
 653 *
 654 * Some specs for the ARK3116 can be found here:
 655 * http://web.archive.org/web/20060318000438/
 656 *   www.arkmicro.com/en/products/view.php?id=10
 657 * On that page, 2 GPIO pins are mentioned: I assume these are the
 658 * OUT1 and OUT2 pins of the UART, so I added support for those
 659 * through the MCR. Since the pins are not available on my hardware,
 660 * I could not verify this.
 661 * Also, it states there is "on-chip hardware flow control". I have
 662 * discovered how to enable that. Unfortunately, I do not know how to
 663 * enable XON/XOFF (software) flow control, which would need support
 664 * from the chip as well to work. Because of the wording on the web
 665 * page there is a real possibility the chip simply does not support
 666 * software flow control.
 667 *
 668 * I got my ark3116 as part of a mobile phone adapter cable. On the
 669 * PCB, the following numbered contacts are present:
 670 *
 671 *  1:- +5V
 672 *  2:o DTR
 673 *  3:i RX
 674 *  4:i DCD
 675 *  5:o RTS
 676 *  6:o TX
 677 *  7:i RI
 678 *  8:i DSR
 679 * 10:- 0V
 680 * 11:i CTS
 681 *
 682 * On my chip, all signals seem to be 3.3V, but 5V tolerant. But that
 683 * may be different for the one you have ;-).
 684 *
 685 * The windows driver limits the registers to 0-F, so I assume there
 686 * are actually 16 present on the device.
 687 *
 688 * On an UART interrupt, 4 bytes of data come in on the interrupt
 689 * endpoint. The bytes are 0xe8 IIR LSR MSR.
 690 *
 691 * The baudrate seems to be generated from the 12MHz crystal, using
 692 * 4-times subsampling. So quot=12e6/(4*baud). Also see description
 693 * of register E.
 694 *
 695 * Registers 0-7:
 696 * These seem to be the same as for a regular 16450. The FCR is set
 697 * to UART_FCR_DMA_SELECT (0x8), I guess to enable transfers between
 698 * the UART and the USB bridge/DMA engine.
 699 *
 700 * Register 8:
 701 * By trial and error, I found out that bit 0 enables hardware CTS,
 702 * stopping TX when CTS is +5V. Bit 1 does the same for RTS, making
 703 * RTS +5V when the 3116 cannot transfer the data to the USB bus
 704 * (verified by disabling the reading URB). Note that as far as I can
 705 * tell, the windows driver does NOT use this, so there might be some
 706 * hardware bug or something.
 707 *
 708 * According to a patch provided here
 709 * https://lore.kernel.org/lkml/200907261419.50702.linux@rainbow-software.org
 710 * the ARK3116 can also be used as an IrDA dongle. Since I do not have
 711 * such a thing, I could not investigate that aspect. However, I can
 712 * speculate ;-).
 713 *
 714 * - IrDA encodes data differently than RS232. Most likely, one of
 715 *   the bits in registers 9..E enables the IR ENDEC (encoder/decoder).
 716 * - Depending on the IR transceiver, the input and output need to be
 717 *   inverted, so there are probably bits for that as well.
 718 * - IrDA is half-duplex, so there should be a bit for selecting that.
 719 *
 720 * This still leaves at least two registers unaccounted for. Perhaps
 721 * The chip can do XON/XOFF or CRC in HW?
 722 *
 723 * Register 9:
 724 * Set to 0x00 for IrDA, when the baudrate is initialised.
 725 *
 726 * Register A:
 727 * Set to 0x01 for IrDA, at init.
 728 *
 729 * Register B:
 730 * Set to 0x01 for IrDA, 0x00 for RS232, at init.
 731 *
 732 * Register C:
 733 * Set to 00 for IrDA, at init.
 734 *
 735 * Register D:
 736 * Set to 0x41 for IrDA, at init.
 737 *
 738 * Register E:
 739 * Somekind of baudrate override. The windows driver seems to set
 740 * this to 0x00 for normal baudrates, 0x01 for 460800, 0x02 for 921600.
 741 * Since 460800 and 921600 cannot be obtained by dividing 3MHz by an integer,
 742 * it could be somekind of subdivisor thingy.
 743 * However,it does not seem to do anything: selecting 921600 (divisor 3,
 744 * reg E=2), still gets 1 MHz. I also checked if registers 9, C or F would
 745 * work, but they don't.
 746 *
 747 * Register F: unknown
 748 */
 749