drivers/usb/serial/ark3116.c

   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
  43 static const struct usb_device_id id_table[] = {
  44         { USB_DEVICE(0x6547, 0x0232) },
  45         { USB_DEVICE(0x18ec, 0x3118) },         /* USB to IrDA adapter */
  46         { },
  47 };
  48 MODULE_DEVICE_TABLE(usb, id_table);
  49
  50 static 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
  59 struct 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
  77 static 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
  92 static 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
 114 static 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
 124 static 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
 181 static void 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
 190 static void ark3116_set_termios(struct tty_struct *tty,
 191                                 struct usb_serial_port *port,
 192                                 const struct ktermios *old_termios)
 193 {
 194         struct usb_serial *serial = port->serial;
 195         struct ark3116_private *priv = usb_get_serial_port_data(port);
 196         struct ktermios *termios = &tty->termios;
 197         unsigned int cflag = termios->c_cflag;
 198         int bps = tty_get_baud_rate(tty);
 199         int quot;
 200         __u8 lcr, hcr, eval;
 201
 202         /* set data bit count */
 203         lcr = UART_LCR_WLEN(tty_get_char_size(cflag));
 204
 205         if (cflag & CSTOPB)
 206                 lcr |= UART_LCR_STOP;
 207         if (cflag & PARENB)
 208                 lcr |= UART_LCR_PARITY;
 209         if (!(cflag & PARODD))
 210                 lcr |= UART_LCR_EPAR;
 211         if (cflag & CMSPAR)
 212                 lcr |= UART_LCR_SPAR;
 213
 214         /* handshake control */
 215         hcr = (cflag & CRTSCTS) ? 0x03 : 0x00;
 216
 217         /* calc baudrate */
 218         dev_dbg(&port->dev, "%s - setting bps to %d\n", __func__, bps);
 219         eval = 0;
 220         switch (bps) {
 221         case 0:
 222                 quot = calc_divisor(9600);
 223                 break;
 224         default:
 225                 if ((bps < 75) || (bps > 3000000))
 226                         bps = 9600;
 227                 quot = calc_divisor(bps);
 228                 break;
 229         case 460800:
 230                 eval = 1;
 231                 quot = calc_divisor(bps);
 232                 break;
 233         case 921600:
 234                 eval = 2;
 235                 quot = calc_divisor(bps);
 236                 break;
 237         }
 238
 239         /* Update state: synchronize */
 240         mutex_lock(&priv->hw_lock);
 241
 242         /* keep old LCR_SBC bit */
 243         lcr |= (priv->lcr & UART_LCR_SBC);
 244
 245         dev_dbg(&port->dev, "%s - setting hcr:0x%02x,lcr:0x%02x,quot:%d\n",
 246                 __func__, hcr, lcr, quot);
 247
 248         /* handshake control */
 249         if (priv->hcr != hcr) {
 250                 priv->hcr = hcr;
 251                 ark3116_write_reg(serial, 0x8, hcr);
 252         }
 253
 254         /* baudrate */
 255         if (priv->quot != quot) {
 256                 priv->quot = quot;
 257                 priv->lcr = lcr; /* need to write lcr anyway */
 258
 259                 /* disable DMA since transmit/receive is
 260                  * shadowed by UART_DLL
 261                  */
 262                 ark3116_write_reg(serial, UART_FCR, 0);
 263
 264                 ark3116_write_reg(serial, UART_LCR,
 265                                   lcr|UART_LCR_DLAB);
 266                 ark3116_write_reg(serial, UART_DLL, quot & 0xff);
 267                 ark3116_write_reg(serial, UART_DLM, (quot>>8) & 0xff);
 268
 269                 /* restore lcr */
 270                 ark3116_write_reg(serial, UART_LCR, lcr);
 271                 /* magic baudrate thingy: not sure what it does,
 272                  * but windows does this as well.
 273                  */
 274                 ark3116_write_reg(serial, 0xe, eval);
 275
 276                 /* enable DMA */
 277                 ark3116_write_reg(serial, UART_FCR, UART_FCR_DMA_SELECT);
 278         } else if (priv->lcr != lcr) {
 279                 priv->lcr = lcr;
 280                 ark3116_write_reg(serial, UART_LCR, lcr);
 281         }
 282
 283         mutex_unlock(&priv->hw_lock);
 284
 285         /* check for software flow control */
 286         if (I_IXOFF(tty) || I_IXON(tty)) {
 287                 dev_warn(&port->dev,
 288                                 "software flow control not implemented\n");
 289         }
 290
 291         /* Don't rewrite B0 */
 292         if (tty_termios_baud_rate(termios))
 293                 tty_termios_encode_baud_rate(termios, bps, bps);
 294 }
 295
 296 static void ark3116_close(struct usb_serial_port *port)
 297 {
 298         struct usb_serial *serial = port->serial;
 299
 300         /* disable DMA */
 301         ark3116_write_reg(serial, UART_FCR, 0);
 302
 303         /* deactivate interrupts */
 304         ark3116_write_reg(serial, UART_IER, 0);
 305
 306         usb_serial_generic_close(port);
 307
 308         usb_kill_urb(port->interrupt_in_urb);
 309 }
 310
 311 static int ark3116_open(struct tty_struct *tty, struct usb_serial_port *port)
 312 {
 313         struct ark3116_private *priv = usb_get_serial_port_data(port);
 314         struct usb_serial *serial = port->serial;
 315         unsigned char *buf;
 316         int result;
 317
 318         buf = kmalloc(1, GFP_KERNEL);
 319         if (buf == NULL)
 320                 return -ENOMEM;
 321
 322         result = usb_serial_generic_open(tty, port);
 323         if (result) {
 324                 dev_dbg(&port->dev,
 325                         "%s - usb_serial_generic_open failed: %d\n",
 326                         __func__, result);
 327                 goto err_free;
 328         }
 329
 330         /* remove any data still left: also clears error state */
 331         ark3116_read_reg(serial, UART_RX, buf);
 332
 333         /* read modem status */
 334         result = ark3116_read_reg(serial, UART_MSR, buf);
 335         if (result)
 336                 goto err_close;
 337         priv->msr = *buf;
 338
 339         /* read line status */
 340         result = ark3116_read_reg(serial, UART_LSR, buf);
 341         if (result)
 342                 goto err_close;
 343         priv->lsr = *buf;
 344
 345         result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
 346         if (result) {
 347                 dev_err(&port->dev, "submit irq_in urb failed %d\n",
 348                         result);
 349                 goto err_close;
 350         }
 351
 352         /* activate interrupts */
 353         ark3116_write_reg(port->serial, UART_IER, UART_IER_MSI|UART_IER_RLSI);
 354
 355         /* enable DMA */
 356         ark3116_write_reg(port->serial, UART_FCR, UART_FCR_DMA_SELECT);
 357
 358         /* setup termios */
 359         if (tty)
 360                 ark3116_set_termios(tty, port, NULL);
 361
 362         kfree(buf);
 363
 364         return 0;
 365
 366 err_close:
 367         usb_serial_generic_close(port);
 368 err_free:
 369         kfree(buf);
 370
 371         return result;
 372 }
 373
 374 static int ark3116_tiocmget(struct tty_struct *tty)
 375 {
 376         struct usb_serial_port *port = tty->driver_data;
 377         struct ark3116_private *priv = usb_get_serial_port_data(port);
 378         __u32 status;
 379         __u32 ctrl;
 380         unsigned long flags;
 381
 382         mutex_lock(&priv->hw_lock);
 383         ctrl = priv->mcr;
 384         mutex_unlock(&priv->hw_lock);
 385
 386         spin_lock_irqsave(&priv->status_lock, flags);
 387         status = priv->msr;
 388         spin_unlock_irqrestore(&priv->status_lock, flags);
 389
 390         return  (status & UART_MSR_DSR  ? TIOCM_DSR  : 0) |
 391                 (status & UART_MSR_CTS  ? TIOCM_CTS  : 0) |
 392                 (status & UART_MSR_RI   ? TIOCM_RI   : 0) |
 393                 (status & UART_MSR_DCD  ? TIOCM_CD   : 0) |
 394                 (ctrl   & UART_MCR_DTR  ? TIOCM_DTR  : 0) |
 395                 (ctrl   & UART_MCR_RTS  ? TIOCM_RTS  : 0) |
 396                 (ctrl   & UART_MCR_OUT1 ? TIOCM_OUT1 : 0) |
 397                 (ctrl   & UART_MCR_OUT2 ? TIOCM_OUT2 : 0);
 398 }
 399
 400 static int ark3116_tiocmset(struct tty_struct *tty,
 401                         unsigned set, unsigned clr)
 402 {
 403         struct usb_serial_port *port = tty->driver_data;
 404         struct ark3116_private *priv = usb_get_serial_port_data(port);
 405
 406         /* we need to take the mutex here, to make sure that the value
 407          * in priv->mcr is actually the one that is in the hardware
 408          */
 409
 410         mutex_lock(&priv->hw_lock);
 411
 412         if (set & TIOCM_RTS)
 413                 priv->mcr |= UART_MCR_RTS;
 414         if (set & TIOCM_DTR)
 415                 priv->mcr |= UART_MCR_DTR;
 416         if (set & TIOCM_OUT1)
 417                 priv->mcr |= UART_MCR_OUT1;
 418         if (set & TIOCM_OUT2)
 419                 priv->mcr |= UART_MCR_OUT2;
 420         if (clr & TIOCM_RTS)
 421                 priv->mcr &= ~UART_MCR_RTS;
 422         if (clr & TIOCM_DTR)
 423                 priv->mcr &= ~UART_MCR_DTR;
 424         if (clr & TIOCM_OUT1)
 425                 priv->mcr &= ~UART_MCR_OUT1;
 426         if (clr & TIOCM_OUT2)
 427                 priv->mcr &= ~UART_MCR_OUT2;
 428
 429         ark3116_write_reg(port->serial, UART_MCR, priv->mcr);
 430
 431         mutex_unlock(&priv->hw_lock);
 432
 433         return 0;
 434 }
 435
 436 static int ark3116_break_ctl(struct tty_struct *tty, int break_state)
 437 {
 438         struct usb_serial_port *port = tty->driver_data;
 439         struct ark3116_private *priv = usb_get_serial_port_data(port);
 440         int ret;
 441
 442         /* LCR is also used for other things: protect access */
 443         mutex_lock(&priv->hw_lock);
 444
 445         if (break_state)
 446                 priv->lcr |= UART_LCR_SBC;
 447         else
 448                 priv->lcr &= ~UART_LCR_SBC;
 449
 450         ret = ark3116_write_reg(port->serial, UART_LCR, priv->lcr);
 451
 452         mutex_unlock(&priv->hw_lock);
 453
 454         return ret;
 455 }
 456
 457 static void ark3116_update_msr(struct usb_serial_port *port, __u8 msr)
 458 {
 459         struct ark3116_private *priv = usb_get_serial_port_data(port);
 460         unsigned long flags;
 461
 462         spin_lock_irqsave(&priv->status_lock, flags);
 463         priv->msr = msr;
 464         spin_unlock_irqrestore(&priv->status_lock, flags);
 465
 466         if (msr & UART_MSR_ANY_DELTA) {
 467                 /* update input line counters */
 468                 if (msr & UART_MSR_DCTS)
 469                         port->icount.cts++;
 470                 if (msr & UART_MSR_DDSR)
 471                         port->icount.dsr++;
 472                 if (msr & UART_MSR_DDCD)
 473                         port->icount.dcd++;
 474                 if (msr & UART_MSR_TERI)
 475                         port->icount.rng++;
 476                 wake_up_interruptible(&port->port.delta_msr_wait);
 477         }
 478 }
 479
 480 static void ark3116_update_lsr(struct usb_serial_port *port, __u8 lsr)
 481 {
 482         struct ark3116_private *priv = usb_get_serial_port_data(port);
 483         unsigned long flags;
 484
 485         spin_lock_irqsave(&priv->status_lock, flags);
 486         /* combine bits */
 487         priv->lsr |= lsr;
 488         spin_unlock_irqrestore(&priv->status_lock, flags);
 489
 490         if (lsr&UART_LSR_BRK_ERROR_BITS) {
 491                 if (lsr & UART_LSR_BI)
 492                         port->icount.brk++;
 493                 if (lsr & UART_LSR_FE)
 494                         port->icount.frame++;
 495                 if (lsr & UART_LSR_PE)
 496                         port->icount.parity++;
 497                 if (lsr & UART_LSR_OE)
 498                         port->icount.overrun++;
 499         }
 500 }
 501
 502 static void ark3116_read_int_callback(struct urb *urb)
 503 {
 504         struct usb_serial_port *port = urb->context;
 505         int status = urb->status;
 506         const __u8 *data = urb->transfer_buffer;
 507         int result;
 508
 509         switch (status) {
 510         case -ECONNRESET:
 511         case -ENOENT:
 512         case -ESHUTDOWN:
 513                 /* this urb is terminated, clean up */
 514                 dev_dbg(&port->dev, "%s - urb shutting down with status: %d\n",
 515                         __func__, status);
 516                 return;
 517         default:
 518                 dev_dbg(&port->dev, "%s - nonzero urb status received: %d\n",
 519                         __func__, status);
 520                 break;
 521         case 0: /* success */
 522                 /* discovered this by trail and error... */
 523                 if ((urb->actual_length == 4) && (data[0] == 0xe8)) {
 524                         const __u8 id = data[1]&UART_IIR_ID;
 525                         dev_dbg(&port->dev, "%s: iir=%02x\n", __func__, data[1]);
 526                         if (id == UART_IIR_MSI) {
 527                                 dev_dbg(&port->dev, "%s: msr=%02x\n",
 528                                         __func__, data[3]);
 529                                 ark3116_update_msr(port, data[3]);
 530                                 break;
 531                         } else if (id == UART_IIR_RLSI) {
 532                                 dev_dbg(&port->dev, "%s: lsr=%02x\n",
 533                                         __func__, data[2]);
 534                                 ark3116_update_lsr(port, data[2]);
 535                                 break;
 536                         }
 537                 }
 538                 /*
 539                  * Not sure what this data meant...
 540                  */
 541                 usb_serial_debug_data(&port->dev, __func__,
 542                                       urb->actual_length,
 543                                       urb->transfer_buffer);
 544                 break;
 545         }
 546
 547         result = usb_submit_urb(urb, GFP_ATOMIC);
 548         if (result)
 549                 dev_err(&port->dev, "failed to resubmit interrupt urb: %d\n",
 550                         result);
 551 }
 552
 553
 554 /* Data comes in via the bulk (data) URB, errors/interrupts via the int URB.
 555  * This means that we cannot be sure which data byte has an associated error
 556  * condition, so we report an error for all data in the next bulk read.
 557  *
 558  * Actually, there might even be a window between the bulk data leaving the
 559  * ark and reading/resetting the lsr in the read_bulk_callback where an
 560  * interrupt for the next data block could come in.
 561  * Without somekind of ordering on the ark, we would have to report the
 562  * error for the next block of data as well...
 563  * For now, let's pretend this can't happen.
 564  */
 565 static void ark3116_process_read_urb(struct urb *urb)
 566 {
 567         struct usb_serial_port *port = urb->context;
 568         struct ark3116_private *priv = usb_get_serial_port_data(port);
 569         unsigned char *data = urb->transfer_buffer;
 570         char tty_flag = TTY_NORMAL;
 571         unsigned long flags;
 572         __u32 lsr;
 573
 574         /* update line status */
 575         spin_lock_irqsave(&priv->status_lock, flags);
 576         lsr = priv->lsr;
 577         priv->lsr &= ~UART_LSR_BRK_ERROR_BITS;
 578         spin_unlock_irqrestore(&priv->status_lock, flags);
 579
 580         if (!urb->actual_length)
 581                 return;
 582
 583         if (lsr & UART_LSR_BRK_ERROR_BITS) {
 584                 if (lsr & UART_LSR_BI)
 585                         tty_flag = TTY_BREAK;
 586                 else if (lsr & UART_LSR_PE)
 587                         tty_flag = TTY_PARITY;
 588                 else if (lsr & UART_LSR_FE)
 589                         tty_flag = TTY_FRAME;
 590
 591                 /* overrun is special, not associated with a char */
 592                 if (lsr & UART_LSR_OE)
 593                         tty_insert_flip_char(&port->port, 0, TTY_OVERRUN);
 594         }
 595         tty_insert_flip_string_fixed_flag(&port->port, data, tty_flag,
 596                                                         urb->actual_length);
 597         tty_flip_buffer_push(&port->port);
 598 }
 599
 600 static struct usb_serial_driver ark3116_device = {
 601         .driver = {
 602                 .name =         "ark3116",
 603         },
 604         .id_table =             id_table,
 605         .num_ports =            1,
 606         .num_bulk_in =          1,
 607         .num_bulk_out =         1,
 608         .num_interrupt_in =     1,
 609         .port_probe =           ark3116_port_probe,
 610         .port_remove =          ark3116_port_remove,
 611         .set_termios =          ark3116_set_termios,
 612         .tiocmget =             ark3116_tiocmget,
 613         .tiocmset =             ark3116_tiocmset,
 614         .tiocmiwait =           usb_serial_generic_tiocmiwait,
 615         .get_icount =           usb_serial_generic_get_icount,
 616         .open =                 ark3116_open,
 617         .close =                ark3116_close,
 618         .break_ctl =            ark3116_break_ctl,
 619         .read_int_callback =    ark3116_read_int_callback,
 620         .process_read_urb =     ark3116_process_read_urb,
 621 };
 622
 623 static struct usb_serial_driver * const serial_drivers[] = {
 624         &ark3116_device, NULL
 625 };
 626
 627 module_usb_serial_driver(serial_drivers, id_table);
 628
 629 MODULE_LICENSE("GPL");
 630
 631 MODULE_AUTHOR(DRIVER_AUTHOR);
 632 MODULE_DESCRIPTION(DRIVER_DESC);
 633
 634 /*
 635  * The following describes what I learned from studying the old
 636  * ark3116.c driver, disassembling the windows driver, and some lucky
 637  * guesses. Since I do not have any datasheet or other
 638  * documentation, inaccuracies are almost guaranteed.
 639  *
 640  * Some specs for the ARK3116 can be found here:
 641  * http://web.archive.org/web/20060318000438/
 642  *   www.arkmicro.com/en/products/view.php?id=10
 643  * On that page, 2 GPIO pins are mentioned: I assume these are the
 644  * OUT1 and OUT2 pins of the UART, so I added support for those
 645  * through the MCR. Since the pins are not available on my hardware,
 646  * I could not verify this.
 647  * Also, it states there is "on-chip hardware flow control". I have
 648  * discovered how to enable that. Unfortunately, I do not know how to
 649  * enable XON/XOFF (software) flow control, which would need support
 650  * from the chip as well to work. Because of the wording on the web
 651  * page there is a real possibility the chip simply does not support
 652  * software flow control.
 653  *
 654  * I got my ark3116 as part of a mobile phone adapter cable. On the
 655  * PCB, the following numbered contacts are present:
 656  *
 657  *  1:- +5V
 658  *  2:o DTR
 659  *  3:i RX
 660  *  4:i DCD
 661  *  5:o RTS
 662  *  6:o TX
 663  *  7:i RI
 664  *  8:i DSR
 665  * 10:- 0V
 666  * 11:i CTS
 667  *
 668  * On my chip, all signals seem to be 3.3V, but 5V tolerant. But that
 669  * may be different for the one you have ;-).
 670  *
 671  * The windows driver limits the registers to 0-F, so I assume there
 672  * are actually 16 present on the device.
 673  *
 674  * On an UART interrupt, 4 bytes of data come in on the interrupt
 675  * endpoint. The bytes are 0xe8 IIR LSR MSR.
 676  *
 677  * The baudrate seems to be generated from the 12MHz crystal, using
 678  * 4-times subsampling. So quot=12e6/(4*baud). Also see description
 679  * of register E.
 680  *
 681  * Registers 0-7:
 682  * These seem to be the same as for a regular 16450. The FCR is set
 683  * to UART_FCR_DMA_SELECT (0x8), I guess to enable transfers between
 684  * the UART and the USB bridge/DMA engine.
 685  *
 686  * Register 8:
 687  * By trial and error, I found out that bit 0 enables hardware CTS,
 688  * stopping TX when CTS is +5V. Bit 1 does the same for RTS, making
 689  * RTS +5V when the 3116 cannot transfer the data to the USB bus
 690  * (verified by disabling the reading URB). Note that as far as I can
 691  * tell, the windows driver does NOT use this, so there might be some
 692  * hardware bug or something.
 693  *
 694  * According to a patch provided here
 695  * https://lore.kernel.org/lkml/200907261419.50702.linux@rainbow-software.org
 696  * the ARK3116 can also be used as an IrDA dongle. Since I do not have
 697  * such a thing, I could not investigate that aspect. However, I can
 698  * speculate ;-).
 699  *
 700  * - IrDA encodes data differently than RS232. Most likely, one of
 701  *   the bits in registers 9..E enables the IR ENDEC (encoder/decoder).
 702  * - Depending on the IR transceiver, the input and output need to be
 703  *   inverted, so there are probably bits for that as well.
 704  * - IrDA is half-duplex, so there should be a bit for selecting that.
 705  *
 706  * This still leaves at least two registers unaccounted for. Perhaps
 707  * The chip can do XON/XOFF or CRC in HW?
 708  *
 709  * Register 9:
 710  * Set to 0x00 for IrDA, when the baudrate is initialised.
 711  *
 712  * Register A:
 713  * Set to 0x01 for IrDA, at init.
 714  *
 715  * Register B:
 716  * Set to 0x01 for IrDA, 0x00 for RS232, at init.
 717  *
 718  * Register C:
 719  * Set to 00 for IrDA, at init.
 720  *
 721  * Register D:
 722  * Set to 0x41 for IrDA, at init.
 723  *
 724  * Register E:
 725  * Somekind of baudrate override. The windows driver seems to set
 726  * this to 0x00 for normal baudrates, 0x01 for 460800, 0x02 for 921600.
 727  * Since 460800 and 921600 cannot be obtained by dividing 3MHz by an integer,
 728  * it could be somekind of subdivisor thingy.
 729  * However,it does not seem to do anything: selecting 921600 (divisor 3,
 730  * reg E=2), still gets 1 MHz. I also checked if registers 9, C or F would
 731  * work, but they don't.
 732  *
 733  * Register F: unknown
 734  */