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 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 switch (cflag & CSIZE) { 204 case CS5: 205 lcr = UART_LCR_WLEN5; 206 break; 207 case CS6: 208 lcr = UART_LCR_WLEN6; 209 break; 210 case CS7: 211 lcr = UART_LCR_WLEN7; 212 break; 213 default: 214 case CS8: 215 lcr = UART_LCR_WLEN8; 216 break; 217 } 218 if (cflag & CSTOPB) 219 lcr |= UART_LCR_STOP; 220 if (cflag & PARENB) 221 lcr |= UART_LCR_PARITY; 222 if (!(cflag & PARODD)) 223 lcr |= UART_LCR_EPAR; 224 #ifdef CMSPAR 225 if (cflag & CMSPAR) 226 lcr |= UART_LCR_SPAR; 227 #endif 228 /* handshake control */ 229 hcr = (cflag & CRTSCTS) ? 0x03 : 0x00; 230 231 /* calc baudrate */ 232 dev_dbg(&port->dev, "%s - setting bps to %d\n", __func__, bps); 233 eval = 0; 234 switch (bps) { 235 case 0: 236 quot = calc_divisor(9600); 237 break; 238 default: 239 if ((bps < 75) || (bps > 3000000)) 240 bps = 9600; 241 quot = calc_divisor(bps); 242 break; 243 case 460800: 244 eval = 1; 245 quot = calc_divisor(bps); 246 break; 247 case 921600: 248 eval = 2; 249 quot = calc_divisor(bps); 250 break; 251 } 252 253 /* Update state: synchronize */ 254 mutex_lock(&priv->hw_lock); 255 256 /* keep old LCR_SBC bit */ 257 lcr |= (priv->lcr & UART_LCR_SBC); 258 259 dev_dbg(&port->dev, "%s - setting hcr:0x%02x,lcr:0x%02x,quot:%d\n", 260 __func__, hcr, lcr, quot); 261 262 /* handshake control */ 263 if (priv->hcr != hcr) { 264 priv->hcr = hcr; 265 ark3116_write_reg(serial, 0x8, hcr); 266 } 267 268 /* baudrate */ 269 if (priv->quot != quot) { 270 priv->quot = quot; 271 priv->lcr = lcr; /* need to write lcr anyway */ 272 273 /* disable DMA since transmit/receive is 274 * shadowed by UART_DLL 275 */ 276 ark3116_write_reg(serial, UART_FCR, 0); 277 278 ark3116_write_reg(serial, UART_LCR, 279 lcr|UART_LCR_DLAB); 280 ark3116_write_reg(serial, UART_DLL, quot & 0xff); 281 ark3116_write_reg(serial, UART_DLM, (quot>>8) & 0xff); 282 283 /* restore lcr */ 284 ark3116_write_reg(serial, UART_LCR, lcr); 285 /* magic baudrate thingy: not sure what it does, 286 * but windows does this as well. 287 */ 288 ark3116_write_reg(serial, 0xe, eval); 289 290 /* enable DMA */ 291 ark3116_write_reg(serial, UART_FCR, UART_FCR_DMA_SELECT); 292 } else if (priv->lcr != lcr) { 293 priv->lcr = lcr; 294 ark3116_write_reg(serial, UART_LCR, lcr); 295 } 296 297 mutex_unlock(&priv->hw_lock); 298 299 /* check for software flow control */ 300 if (I_IXOFF(tty) || I_IXON(tty)) { 301 dev_warn(&port->dev, 302 "software flow control not implemented\n"); 303 } 304 305 /* Don't rewrite B0 */ 306 if (tty_termios_baud_rate(termios)) 307 tty_termios_encode_baud_rate(termios, bps, bps); 308 } 309 310 static void ark3116_close(struct usb_serial_port *port) 311 { 312 struct usb_serial *serial = port->serial; 313 314 /* disable DMA */ 315 ark3116_write_reg(serial, UART_FCR, 0); 316 317 /* deactivate interrupts */ 318 ark3116_write_reg(serial, UART_IER, 0); 319 320 usb_serial_generic_close(port); 321 322 usb_kill_urb(port->interrupt_in_urb); 323 } 324 325 static int ark3116_open(struct tty_struct *tty, struct usb_serial_port *port) 326 { 327 struct ark3116_private *priv = usb_get_serial_port_data(port); 328 struct usb_serial *serial = port->serial; 329 unsigned char *buf; 330 int result; 331 332 buf = kmalloc(1, GFP_KERNEL); 333 if (buf == NULL) 334 return -ENOMEM; 335 336 result = usb_serial_generic_open(tty, port); 337 if (result) { 338 dev_dbg(&port->dev, 339 "%s - usb_serial_generic_open failed: %d\n", 340 __func__, result); 341 goto err_free; 342 } 343 344 /* remove any data still left: also clears error state */ 345 ark3116_read_reg(serial, UART_RX, buf); 346 347 /* read modem status */ 348 result = ark3116_read_reg(serial, UART_MSR, buf); 349 if (result) 350 goto err_close; 351 priv->msr = *buf; 352 353 /* read line status */ 354 result = ark3116_read_reg(serial, UART_LSR, buf); 355 if (result) 356 goto err_close; 357 priv->lsr = *buf; 358 359 result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL); 360 if (result) { 361 dev_err(&port->dev, "submit irq_in urb failed %d\n", 362 result); 363 goto err_close; 364 } 365 366 /* activate interrupts */ 367 ark3116_write_reg(port->serial, UART_IER, UART_IER_MSI|UART_IER_RLSI); 368 369 /* enable DMA */ 370 ark3116_write_reg(port->serial, UART_FCR, UART_FCR_DMA_SELECT); 371 372 /* setup termios */ 373 if (tty) 374 ark3116_set_termios(tty, port, NULL); 375 376 kfree(buf); 377 378 return 0; 379 380 err_close: 381 usb_serial_generic_close(port); 382 err_free: 383 kfree(buf); 384 385 return result; 386 } 387 388 static int ark3116_tiocmget(struct tty_struct *tty) 389 { 390 struct usb_serial_port *port = tty->driver_data; 391 struct ark3116_private *priv = usb_get_serial_port_data(port); 392 __u32 status; 393 __u32 ctrl; 394 unsigned long flags; 395 396 mutex_lock(&priv->hw_lock); 397 ctrl = priv->mcr; 398 mutex_unlock(&priv->hw_lock); 399 400 spin_lock_irqsave(&priv->status_lock, flags); 401 status = priv->msr; 402 spin_unlock_irqrestore(&priv->status_lock, flags); 403 404 return (status & UART_MSR_DSR ? TIOCM_DSR : 0) | 405 (status & UART_MSR_CTS ? TIOCM_CTS : 0) | 406 (status & UART_MSR_RI ? TIOCM_RI : 0) | 407 (status & UART_MSR_DCD ? TIOCM_CD : 0) | 408 (ctrl & UART_MCR_DTR ? TIOCM_DTR : 0) | 409 (ctrl & UART_MCR_RTS ? TIOCM_RTS : 0) | 410 (ctrl & UART_MCR_OUT1 ? TIOCM_OUT1 : 0) | 411 (ctrl & UART_MCR_OUT2 ? TIOCM_OUT2 : 0); 412 } 413 414 static int ark3116_tiocmset(struct tty_struct *tty, 415 unsigned set, unsigned clr) 416 { 417 struct usb_serial_port *port = tty->driver_data; 418 struct ark3116_private *priv = usb_get_serial_port_data(port); 419 420 /* we need to take the mutex here, to make sure that the value 421 * in priv->mcr is actually the one that is in the hardware 422 */ 423 424 mutex_lock(&priv->hw_lock); 425 426 if (set & TIOCM_RTS) 427 priv->mcr |= UART_MCR_RTS; 428 if (set & TIOCM_DTR) 429 priv->mcr |= UART_MCR_DTR; 430 if (set & TIOCM_OUT1) 431 priv->mcr |= UART_MCR_OUT1; 432 if (set & TIOCM_OUT2) 433 priv->mcr |= UART_MCR_OUT2; 434 if (clr & TIOCM_RTS) 435 priv->mcr &= ~UART_MCR_RTS; 436 if (clr & TIOCM_DTR) 437 priv->mcr &= ~UART_MCR_DTR; 438 if (clr & TIOCM_OUT1) 439 priv->mcr &= ~UART_MCR_OUT1; 440 if (clr & TIOCM_OUT2) 441 priv->mcr &= ~UART_MCR_OUT2; 442 443 ark3116_write_reg(port->serial, UART_MCR, priv->mcr); 444 445 mutex_unlock(&priv->hw_lock); 446 447 return 0; 448 } 449 450 static void ark3116_break_ctl(struct tty_struct *tty, int break_state) 451 { 452 struct usb_serial_port *port = tty->driver_data; 453 struct ark3116_private *priv = usb_get_serial_port_data(port); 454 455 /* LCR is also used for other things: protect access */ 456 mutex_lock(&priv->hw_lock); 457 458 if (break_state) 459 priv->lcr |= UART_LCR_SBC; 460 else 461 priv->lcr &= ~UART_LCR_SBC; 462 463 ark3116_write_reg(port->serial, UART_LCR, priv->lcr); 464 465 mutex_unlock(&priv->hw_lock); 466 } 467 468 static void ark3116_update_msr(struct usb_serial_port *port, __u8 msr) 469 { 470 struct ark3116_private *priv = usb_get_serial_port_data(port); 471 unsigned long flags; 472 473 spin_lock_irqsave(&priv->status_lock, flags); 474 priv->msr = msr; 475 spin_unlock_irqrestore(&priv->status_lock, flags); 476 477 if (msr & UART_MSR_ANY_DELTA) { 478 /* update input line counters */ 479 if (msr & UART_MSR_DCTS) 480 port->icount.cts++; 481 if (msr & UART_MSR_DDSR) 482 port->icount.dsr++; 483 if (msr & UART_MSR_DDCD) 484 port->icount.dcd++; 485 if (msr & UART_MSR_TERI) 486 port->icount.rng++; 487 wake_up_interruptible(&port->port.delta_msr_wait); 488 } 489 } 490 491 static void ark3116_update_lsr(struct usb_serial_port *port, __u8 lsr) 492 { 493 struct ark3116_private *priv = usb_get_serial_port_data(port); 494 unsigned long flags; 495 496 spin_lock_irqsave(&priv->status_lock, flags); 497 /* combine bits */ 498 priv->lsr |= lsr; 499 spin_unlock_irqrestore(&priv->status_lock, flags); 500 501 if (lsr&UART_LSR_BRK_ERROR_BITS) { 502 if (lsr & UART_LSR_BI) 503 port->icount.brk++; 504 if (lsr & UART_LSR_FE) 505 port->icount.frame++; 506 if (lsr & UART_LSR_PE) 507 port->icount.parity++; 508 if (lsr & UART_LSR_OE) 509 port->icount.overrun++; 510 } 511 } 512 513 static void ark3116_read_int_callback(struct urb *urb) 514 { 515 struct usb_serial_port *port = urb->context; 516 int status = urb->status; 517 const __u8 *data = urb->transfer_buffer; 518 int result; 519 520 switch (status) { 521 case -ECONNRESET: 522 case -ENOENT: 523 case -ESHUTDOWN: 524 /* this urb is terminated, clean up */ 525 dev_dbg(&port->dev, "%s - urb shutting down with status: %d\n", 526 __func__, status); 527 return; 528 default: 529 dev_dbg(&port->dev, "%s - nonzero urb status received: %d\n", 530 __func__, status); 531 break; 532 case 0: /* success */ 533 /* discovered this by trail and error... */ 534 if ((urb->actual_length == 4) && (data[0] == 0xe8)) { 535 const __u8 id = data[1]&UART_IIR_ID; 536 dev_dbg(&port->dev, "%s: iir=%02x\n", __func__, data[1]); 537 if (id == UART_IIR_MSI) { 538 dev_dbg(&port->dev, "%s: msr=%02x\n", 539 __func__, data[3]); 540 ark3116_update_msr(port, data[3]); 541 break; 542 } else if (id == UART_IIR_RLSI) { 543 dev_dbg(&port->dev, "%s: lsr=%02x\n", 544 __func__, data[2]); 545 ark3116_update_lsr(port, data[2]); 546 break; 547 } 548 } 549 /* 550 * Not sure what this data meant... 551 */ 552 usb_serial_debug_data(&port->dev, __func__, 553 urb->actual_length, 554 urb->transfer_buffer); 555 break; 556 } 557 558 result = usb_submit_urb(urb, GFP_ATOMIC); 559 if (result) 560 dev_err(&port->dev, "failed to resubmit interrupt urb: %d\n", 561 result); 562 } 563 564 565 /* Data comes in via the bulk (data) URB, errors/interrupts via the int URB. 566 * This means that we cannot be sure which data byte has an associated error 567 * condition, so we report an error for all data in the next bulk read. 568 * 569 * Actually, there might even be a window between the bulk data leaving the 570 * ark and reading/resetting the lsr in the read_bulk_callback where an 571 * interrupt for the next data block could come in. 572 * Without somekind of ordering on the ark, we would have to report the 573 * error for the next block of data as well... 574 * For now, let's pretend this can't happen. 575 */ 576 static void ark3116_process_read_urb(struct urb *urb) 577 { 578 struct usb_serial_port *port = urb->context; 579 struct ark3116_private *priv = usb_get_serial_port_data(port); 580 unsigned char *data = urb->transfer_buffer; 581 char tty_flag = TTY_NORMAL; 582 unsigned long flags; 583 __u32 lsr; 584 585 /* update line status */ 586 spin_lock_irqsave(&priv->status_lock, flags); 587 lsr = priv->lsr; 588 priv->lsr &= ~UART_LSR_BRK_ERROR_BITS; 589 spin_unlock_irqrestore(&priv->status_lock, flags); 590 591 if (!urb->actual_length) 592 return; 593 594 if (lsr & UART_LSR_BRK_ERROR_BITS) { 595 if (lsr & UART_LSR_BI) 596 tty_flag = TTY_BREAK; 597 else if (lsr & UART_LSR_PE) 598 tty_flag = TTY_PARITY; 599 else if (lsr & UART_LSR_FE) 600 tty_flag = TTY_FRAME; 601 602 /* overrun is special, not associated with a char */ 603 if (lsr & UART_LSR_OE) 604 tty_insert_flip_char(&port->port, 0, TTY_OVERRUN); 605 } 606 tty_insert_flip_string_fixed_flag(&port->port, data, tty_flag, 607 urb->actual_length); 608 tty_flip_buffer_push(&port->port); 609 } 610 611 static struct usb_serial_driver ark3116_device = { 612 .driver = { 613 .owner = THIS_MODULE, 614 .name = "ark3116", 615 }, 616 .id_table = id_table, 617 .num_ports = 1, 618 .num_bulk_in = 1, 619 .num_bulk_out = 1, 620 .num_interrupt_in = 1, 621 .port_probe = ark3116_port_probe, 622 .port_remove = ark3116_port_remove, 623 .set_termios = ark3116_set_termios, 624 .tiocmget = ark3116_tiocmget, 625 .tiocmset = ark3116_tiocmset, 626 .tiocmiwait = usb_serial_generic_tiocmiwait, 627 .get_icount = usb_serial_generic_get_icount, 628 .open = ark3116_open, 629 .close = ark3116_close, 630 .break_ctl = ark3116_break_ctl, 631 .read_int_callback = ark3116_read_int_callback, 632 .process_read_urb = ark3116_process_read_urb, 633 }; 634 635 static struct usb_serial_driver * const serial_drivers[] = { 636 &ark3116_device, NULL 637 }; 638 639 module_usb_serial_driver(serial_drivers, id_table); 640 641 MODULE_LICENSE("GPL"); 642 643 MODULE_AUTHOR(DRIVER_AUTHOR); 644 MODULE_DESCRIPTION(DRIVER_DESC); 645 646 /* 647 * The following describes what I learned from studying the old 648 * ark3116.c driver, disassembling the windows driver, and some lucky 649 * guesses. Since I do not have any datasheet or other 650 * documentation, inaccuracies are almost guaranteed. 651 * 652 * Some specs for the ARK3116 can be found here: 653 * http://web.archive.org/web/20060318000438/ 654 * www.arkmicro.com/en/products/view.php?id=10 655 * On that page, 2 GPIO pins are mentioned: I assume these are the 656 * OUT1 and OUT2 pins of the UART, so I added support for those 657 * through the MCR. Since the pins are not available on my hardware, 658 * I could not verify this. 659 * Also, it states there is "on-chip hardware flow control". I have 660 * discovered how to enable that. Unfortunately, I do not know how to 661 * enable XON/XOFF (software) flow control, which would need support 662 * from the chip as well to work. Because of the wording on the web 663 * page there is a real possibility the chip simply does not support 664 * software flow control. 665 * 666 * I got my ark3116 as part of a mobile phone adapter cable. On the 667 * PCB, the following numbered contacts are present: 668 * 669 * 1:- +5V 670 * 2:o DTR 671 * 3:i RX 672 * 4:i DCD 673 * 5:o RTS 674 * 6:o TX 675 * 7:i RI 676 * 8:i DSR 677 * 10:- 0V 678 * 11:i CTS 679 * 680 * On my chip, all signals seem to be 3.3V, but 5V tolerant. But that 681 * may be different for the one you have ;-). 682 * 683 * The windows driver limits the registers to 0-F, so I assume there 684 * are actually 16 present on the device. 685 * 686 * On an UART interrupt, 4 bytes of data come in on the interrupt 687 * endpoint. The bytes are 0xe8 IIR LSR MSR. 688 * 689 * The baudrate seems to be generated from the 12MHz crystal, using 690 * 4-times subsampling. So quot=12e6/(4*baud). Also see description 691 * of register E. 692 * 693 * Registers 0-7: 694 * These seem to be the same as for a regular 16450. The FCR is set 695 * to UART_FCR_DMA_SELECT (0x8), I guess to enable transfers between 696 * the UART and the USB bridge/DMA engine. 697 * 698 * Register 8: 699 * By trial and error, I found out that bit 0 enables hardware CTS, 700 * stopping TX when CTS is +5V. Bit 1 does the same for RTS, making 701 * RTS +5V when the 3116 cannot transfer the data to the USB bus 702 * (verified by disabling the reading URB). Note that as far as I can 703 * tell, the windows driver does NOT use this, so there might be some 704 * hardware bug or something. 705 * 706 * According to a patch provided here 707 * https://lore.kernel.org/lkml/200907261419.50702.linux@rainbow-software.org 708 * the ARK3116 can also be used as an IrDA dongle. Since I do not have 709 * such a thing, I could not investigate that aspect. However, I can 710 * speculate ;-). 711 * 712 * - IrDA encodes data differently than RS232. Most likely, one of 713 * the bits in registers 9..E enables the IR ENDEC (encoder/decoder). 714 * - Depending on the IR transceiver, the input and output need to be 715 * inverted, so there are probably bits for that as well. 716 * - IrDA is half-duplex, so there should be a bit for selecting that. 717 * 718 * This still leaves at least two registers unaccounted for. Perhaps 719 * The chip can do XON/XOFF or CRC in HW? 720 * 721 * Register 9: 722 * Set to 0x00 for IrDA, when the baudrate is initialised. 723 * 724 * Register A: 725 * Set to 0x01 for IrDA, at init. 726 * 727 * Register B: 728 * Set to 0x01 for IrDA, 0x00 for RS232, at init. 729 * 730 * Register C: 731 * Set to 00 for IrDA, at init. 732 * 733 * Register D: 734 * Set to 0x41 for IrDA, at init. 735 * 736 * Register E: 737 * Somekind of baudrate override. The windows driver seems to set 738 * this to 0x00 for normal baudrates, 0x01 for 460800, 0x02 for 921600. 739 * Since 460800 and 921600 cannot be obtained by dividing 3MHz by an integer, 740 * it could be somekind of subdivisor thingy. 741 * However,it does not seem to do anything: selecting 921600 (divisor 3, 742 * reg E=2), still gets 1 MHz. I also checked if registers 9, C or F would 743 * work, but they don't. 744 * 745 * Register F: unknown 746 */ 747