xref: /illumos-gate/usr/src/boot/i386/libi386/comconsole.c (revision 201ceb75ab95f9bf1f42ea1dc9ab363b43ba47cf)
1 /*
2  * Copyright (c) 1998 Michael Smith (msmith@freebsd.org)
3  *
4  * Redistribution and use in source and binary forms, with or without
5  * modification, are permitted provided that the following conditions
6  * are met:
7  * 1. Redistributions of source code must retain the above copyright
8  *    notice, this list of conditions and the following disclaimer.
9  * 2. Redistributions in binary form must reproduce the above copyright
10  *    notice, this list of conditions and the following disclaimer in the
11  *    documentation and/or other materials provided with the distribution.
12  *
13  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
14  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
16  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
17  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
18  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
19  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
20  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
21  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
22  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
23  * SUCH DAMAGE.
24  */
25 
26 /*
27  * This code is shared on BIOS and UEFI systems on x86 because
28  * we can access io ports on both platforms and the UEFI Serial IO protocol
29  * is not giving us reliable port order and we see issues with input.
30  */
31 #include <sys/cdefs.h>
32 
33 #include <stand.h>
34 #include <bootstrap.h>
35 #include <stdbool.h>
36 #include <machine/cpufunc.h>
37 #include <dev/ic/ns16550.h>
38 #include <dev/pci/pcireg.h>
39 #include "libi386.h"
40 
41 #define	COMC_TXWAIT	0x40000		/* transmit timeout */
42 #define	COMC_BPS(x)	(115200 / (x))	/* speed to DLAB divisor */
43 #define	COMC_DIV2BPS(x)	(115200 / (x))	/* DLAB divisor to speed */
44 
45 #ifndef	COMSPEED
46 #define	COMSPEED	9600
47 #endif
48 
49 #define	COM_NPORTS	4
50 #define	COM1_IOADDR	0x3f8
51 #define	COM2_IOADDR	0x2f8
52 #define	COM3_IOADDR	0x3e8
53 #define	COM4_IOADDR	0x2e8
54 
55 #define	STOP1		0x00
56 #define	STOP2		0x04
57 
58 #define	PARODD		0x00
59 #define	PAREN		0x08
60 #define	PAREVN		0x10
61 #define	PARMARK		0x20
62 
63 #define	BITS5		0x00	/* 5 bits per char */
64 #define	BITS6		0x01	/* 6 bits per char */
65 #define	BITS7		0x02	/* 7 bits per char */
66 #define	BITS8		0x03	/* 8 bits per char */
67 
68 struct serial {
69     int		speed;		/* baud rate */
70     uint8_t	lcr;		/* line control */
71     uint8_t	ignore_cd;	/* boolean */
72     uint8_t	rtsdtr_off;	/* boolean */
73     int		ioaddr;
74     uint32_t	locator;
75 };
76 
77 static void	comc_probe(struct console *);
78 static int	comc_init(struct console *, int);
79 static void	comc_putchar(struct console *, int);
80 static int	comc_getchar(struct console *);
81 int		comc_getspeed(int);
82 static int	comc_ischar(struct console *);
83 static int	comc_ioctl(struct console *, int, void *);
84 static uint32_t comc_parse_pcidev(const char *);
85 static int	comc_pcidev_set(struct env_var *, int, const void *);
86 static int	comc_pcidev_handle(struct console *, uint32_t);
87 static bool	comc_setup(struct console *);
88 static char	*comc_asprint_mode(struct serial *);
89 static int	comc_parse_mode(struct serial *, const char *);
90 static int	comc_mode_set(struct env_var *, int, const void *);
91 static int	comc_cd_set(struct env_var *, int, const void *);
92 static int	comc_rtsdtr_set(struct env_var *, int, const void *);
93 static void	comc_devinfo(struct console *);
94 
95 static void
96 comc_devinfo(struct console *cp)
97 {
98 	struct serial *port = cp->c_private;
99 
100 	printf("\tport %#x", port->ioaddr);
101 }
102 
103 static bool
104 comc_port_is_present(int ioaddr)
105 {
106 	/*
107 	 * Write byte to scratch register and read it out.
108 	 */
109 #define	COMC_TEST	0xbb
110 	outb(ioaddr + com_scr, COMC_TEST);
111 	return (inb(ioaddr + com_scr) == COMC_TEST);
112 }
113 
114 /*
115  * Set up list of possible serial consoles.
116  * This function is run very early, so we do not expect to
117  * run out of memory, and on error, we can not print output.
118  */
119 void
120 comc_ini(void)
121 {
122 	uint_t n = 0, c;
123 	bool ports[COM_NPORTS];
124 	struct console **tmp;
125 	struct console *tty;
126 	struct serial *port;
127 
128 	/*
129 	 * Test the presence of 4 serial devices com1-com4
130 	 */
131 	ports[0] = comc_port_is_present(COM1_IOADDR);
132 	ports[1] = comc_port_is_present(COM2_IOADDR);
133 	ports[2] = comc_port_is_present(COM3_IOADDR);
134 	ports[3] = comc_port_is_present(COM4_IOADDR);
135 
136 	for (uint_t i = 0; i < COM_NPORTS; i++)
137 		if (ports[i])
138 			n++;
139 
140 	if (n == 0)	/* there are no serial ports */
141 		return;
142 
143 	c = cons_array_size();
144 	if (c == 0)
145 		n++;	/* For NULL pointer */
146 
147 	tmp = realloc(consoles, (c + n) * sizeof (*consoles));
148 	if (tmp == NULL)
149 		return;
150 	consoles = tmp;
151 	if (c > 0)
152 		c--;
153 
154 	for (uint_t i = 0; i < COM_NPORTS; i++) {
155 		if (!ports[i])
156 			continue;
157 		tty = malloc(sizeof (*tty));
158 		if (tty == NULL) {
159 			/* Out of memory?! can not continue */
160 			consoles[c] = tty;
161 			return;
162 		}
163 		if (asprintf(&tty->c_name, "tty%c", 'a' + i) < 0) {
164 			free(tty);
165 			consoles[c] = NULL;
166 			return;
167 		}
168 		if (asprintf(&tty->c_desc, "serial port %c", 'a' + i) < 0) {
169 			free(tty->c_name);
170 			free(tty);
171 			consoles[c] = NULL;
172 			return;
173 		}
174 		tty->c_flags = 0;
175 		tty->c_probe = comc_probe;
176 		tty->c_init = comc_init;
177 		tty->c_out = comc_putchar;
178 		tty->c_in = comc_getchar;
179 		tty->c_ready = comc_ischar;
180 		tty->c_ioctl = comc_ioctl;
181 		tty->c_devinfo = comc_devinfo;
182 		port = malloc(sizeof (*port));
183 		if (port == NULL) {
184 			free(tty->c_name);
185 			free(tty->c_desc);
186 			free(tty);
187 			consoles[c] = NULL;
188 			return;
189 		}
190 		port->speed = 0;	/* Leave this for comc_probe */
191 		switch (i) {
192 		case 0:
193 			port->ioaddr = COM1_IOADDR;
194 			break;
195 		case 1:
196 			port->ioaddr = COM2_IOADDR;
197 			break;
198 		case 2:
199 			port->ioaddr = COM3_IOADDR;
200 			break;
201 		case 3:
202 			port->ioaddr = COM4_IOADDR;
203 			break;
204 		}
205 		port->lcr = BITS8;	/* 8,n,1 */
206 		port->ignore_cd = 1;	/* ignore cd */
207 		port->rtsdtr_off = 0;	/* rts-dtr is on */
208 
209 		tty->c_private = port;
210 		consoles[c++] = tty;
211 	}
212 	consoles[c] = NULL;
213 }
214 
215 static void
216 comc_probe(struct console *cp)
217 {
218 	struct serial *port;
219 	char name[20];
220 	char value[20];
221 	char *env;
222 
223 	port = cp->c_private;
224 	if (port->speed != 0)
225 		return;
226 
227 	port->speed = COMSPEED;
228 
229 	/*
230 	 * Assume that the speed was set by an earlier boot loader if
231 	 * comconsole is already the preferred console.
232 	 */
233 	snprintf(name, sizeof (name), "%s-mode", cp->c_name);
234 	env = getenv(name);
235 	if (env != NULL) {
236 		port->speed = comc_getspeed(port->ioaddr);
237 	}
238 	env = comc_asprint_mode(port);
239 
240 	if (env != NULL) {
241 		unsetenv(name);
242 		env_setenv(name, EV_VOLATILE, env, comc_mode_set, env_nounset);
243 		free(env);
244 	}
245 
246 	snprintf(name, sizeof (name), "%s-ignore-cd", cp->c_name);
247 	env = getenv(name);
248 	if (env != NULL) {
249 		if (strcmp(env, "true") == 0)
250 			port->ignore_cd = 1;
251 		else if (strcmp(env, "false") == 0)
252 			port->ignore_cd = 0;
253 	}
254 
255 	snprintf(value, sizeof (value), "%s",
256 	    port->ignore_cd? "true" : "false");
257 	unsetenv(name);
258 	env_setenv(name, EV_VOLATILE, value, comc_cd_set, env_nounset);
259 
260 	snprintf(name, sizeof (name), "%s-rts-dtr-off", cp->c_name);
261 	env = getenv(name);
262 	if (env != NULL) {
263 		if (strcmp(env, "true") == 0)
264 			port->rtsdtr_off = 1;
265 		else if (strcmp(env, "false") == 0)
266 			port->rtsdtr_off = 0;
267 	}
268 
269 	snprintf(value, sizeof (value), "%s",
270 	    port->rtsdtr_off? "true" : "false");
271 	unsetenv(name);
272 	env_setenv(name, EV_VOLATILE, value, comc_rtsdtr_set, env_nounset);
273 
274 	snprintf(name, sizeof (name), "%s-pcidev", cp->c_name);
275 	env = getenv(name);
276 	if (env != NULL) {
277 		port->locator = comc_parse_pcidev(env);
278 		if (port->locator != 0)
279 			comc_pcidev_handle(cp, port->locator);
280 	}
281 
282 	unsetenv(name);
283 	env_setenv(name, EV_VOLATILE, env, comc_pcidev_set, env_nounset);
284 
285 	cp->c_flags = 0;
286 	if (comc_setup(cp))
287 		cp->c_flags = C_PRESENTIN | C_PRESENTOUT;
288 }
289 
290 static int
291 comc_init(struct console *cp, int arg __attribute((unused)))
292 {
293 
294 	if (comc_setup(cp))
295 		return (CMD_OK);
296 
297 	cp->c_flags = 0;
298 	return (CMD_ERROR);
299 }
300 
301 static void
302 comc_putchar(struct console *cp, int c)
303 {
304 	int wait;
305 	struct serial *sp = cp->c_private;
306 
307 	for (wait = COMC_TXWAIT; wait > 0; wait--)
308 		if (inb(sp->ioaddr + com_lsr) & LSR_TXRDY) {
309 			outb(sp->ioaddr + com_data, (uchar_t)c);
310 			break;
311 		}
312 }
313 
314 static int
315 comc_getchar(struct console *cp)
316 {
317 	struct serial *sp = cp->c_private;
318 	return (comc_ischar(cp) ? inb(sp->ioaddr + com_data) : -1);
319 }
320 
321 static int
322 comc_ischar(struct console *cp)
323 {
324 	struct serial *sp = cp->c_private;
325 	return (inb(sp->ioaddr + com_lsr) & LSR_RXRDY);
326 }
327 
328 static int
329 comc_ioctl(struct console *cp __unused, int cmd __unused, void *data __unused)
330 {
331 	return (ENOTTY);
332 }
333 
334 static char *
335 comc_asprint_mode(struct serial *sp)
336 {
337 	char par, *buf;
338 
339 	if (sp == NULL)
340 		return (NULL);
341 
342 	if ((sp->lcr & (PAREN|PAREVN)) == (PAREN|PAREVN))
343 		par = 'e';
344 	else if ((sp->lcr & PAREN) == PAREN)
345 		par = 'o';
346 	else
347 		par = 'n';
348 
349 	asprintf(&buf, "%d,%d,%c,%d,-", sp->speed,
350 	    (sp->lcr & BITS8) == BITS8? 8:7,
351 	    par, (sp->lcr & STOP2) == STOP2? 2:1);
352 	return (buf);
353 }
354 
355 static int
356 comc_parse_mode(struct serial *sp, const char *value)
357 {
358 	unsigned long n;
359 	int speed;
360 	int lcr;
361 	char *ep;
362 
363 	if (value == NULL || *value == '\0')
364 		return (CMD_ERROR);
365 
366 	errno = 0;
367 	n = strtoul(value, &ep, 10);
368 	if (errno != 0 || *ep != ',')
369 		return (CMD_ERROR);
370 	speed = n;
371 
372 	ep++;
373 	errno = 0;
374 	n = strtoul(ep, &ep, 10);
375 	if (errno != 0 || *ep != ',')
376 		return (CMD_ERROR);
377 
378 	switch (n) {
379 	case 7: lcr = BITS7;
380 		break;
381 	case 8: lcr = BITS8;
382 		break;
383 	default:
384 		return (CMD_ERROR);
385 	}
386 
387 	ep++;
388 	switch (*ep++) {
389 	case 'n':
390 		break;
391 	case 'e': lcr |= PAREN|PAREVN;
392 		break;
393 	case 'o': lcr |= PAREN|PARODD;
394 		break;
395 	default:
396 		return (CMD_ERROR);
397 	}
398 
399 	if (*ep == ',')
400 		ep++;
401 	else
402 		return (CMD_ERROR);
403 
404 	switch (*ep++) {
405 	case '1':
406 		break;
407 	case '2': lcr |= STOP2;
408 		break;
409 	default:
410 		return (CMD_ERROR);
411 	}
412 
413 	/* handshake is ignored, but we check syntax anyhow */
414 	if (*ep == ',')
415 		ep++;
416 	else
417 		return (CMD_ERROR);
418 
419 	switch (*ep++) {
420 	case '-':
421 	case 'h':
422 	case 's':
423 		break;
424 	default:
425 		return (CMD_ERROR);
426 	}
427 
428 	if (*ep != '\0')
429 		return (CMD_ERROR);
430 
431 	sp->speed = speed;
432 	sp->lcr = lcr;
433 	return (CMD_OK);
434 }
435 
436 static struct console *
437 get_console(const char *name)
438 {
439 	char port[5];
440 
441 	(void) strlcpy(port, name, sizeof (port));
442 	for (uint_t i = 0; consoles[i] != NULL; i++) {
443 		if (strcmp(port, consoles[i]->c_name) == 0)
444 			return (consoles[i]);
445 	}
446 
447 	printf("No such port: %s\n", port);
448 	return (NULL);
449 }
450 
451 /*
452  * CMD_ERROR will cause set/setenv/setprop command to fail,
453  * when used in loader scripts (forth), this will cause processing
454  * of boot scripts to fail, rendering bootloading impossible.
455  * To prevent such unfortunate situation, we return CMD_OK when
456  * there is no such port, or there is invalid value in mode line.
457  */
458 static int
459 comc_mode_set(struct env_var *ev, int flags, const void *value)
460 {
461 	struct console *cp;
462 	char name[15];
463 
464 	if (value == NULL)
465 		return (CMD_ERROR);
466 
467 	if ((cp = get_console(ev->ev_name)) == NULL)
468 		return (CMD_OK);
469 
470 	/* Do not override serial setup from SPCR */
471 	snprintf(name, sizeof (name), "%s-spcr-mode", cp->c_name);
472 	if (getenv(name) == NULL) {
473 		if (comc_parse_mode(cp->c_private, value) == CMD_ERROR) {
474 			printf("%s: invalid mode: %s\n", ev->ev_name,
475 			    (char *)value);
476 			return (CMD_OK);
477 		}
478 		(void) comc_setup(cp);
479 		env_setenv(ev->ev_name, flags | EV_NOHOOK, value, NULL, NULL);
480 	}
481 
482 	return (CMD_OK);
483 }
484 
485 /*
486  * CMD_ERROR will cause set/setenv/setprop command to fail,
487  * when used in loader scripts (forth), this will cause processing
488  * of boot scripts to fail, rendering bootloading impossible.
489  * To prevent such unfortunate situation, we return CMD_OK when
490  * there is no such port or invalid value was used.
491  */
492 static int
493 comc_cd_set(struct env_var *ev, int flags, const void *value)
494 {
495 	struct console *cp;
496 	struct serial *sp;
497 
498 	if (value == NULL)
499 		return (CMD_ERROR);
500 
501 	if ((cp = get_console(ev->ev_name)) == NULL)
502 		return (CMD_OK);
503 
504 	sp = cp->c_private;
505 	if (strcmp(value, "true") == 0) {
506 		sp->ignore_cd = 1;
507 	} else if (strcmp(value, "false") == 0) {
508 		sp->ignore_cd = 0;
509 	} else {
510 		printf("%s: invalid value: %s\n", ev->ev_name,
511 		    (char *)value);
512 		return (CMD_ERROR);
513 	}
514 
515 	(void) comc_setup(cp);
516 
517 	env_setenv(ev->ev_name, flags | EV_NOHOOK, value, NULL, NULL);
518 
519 	return (CMD_OK);
520 }
521 
522 /*
523  * CMD_ERROR will cause set/setenv/setprop command to fail,
524  * when used in loader scripts (forth), this will cause processing
525  * of boot scripts to fail, rendering bootloading impossible.
526  * To prevent such unfortunate situation, we return CMD_OK when
527  * there is no such port, or invalid value was used.
528  */
529 static int
530 comc_rtsdtr_set(struct env_var *ev, int flags, const void *value)
531 {
532 	struct console *cp;
533 	struct serial *sp;
534 
535 	if (value == NULL)
536 		return (CMD_ERROR);
537 
538 	if ((cp = get_console(ev->ev_name)) == NULL)
539 		return (CMD_OK);
540 
541 	sp = cp->c_private;
542 	if (strcmp(value, "true") == 0) {
543 		sp->rtsdtr_off = 1;
544 	} else if (strcmp(value, "false") == 0) {
545 		sp->rtsdtr_off = 0;
546 	} else {
547 		printf("%s: invalid value: %s\n", ev->ev_name,
548 		    (char *)value);
549 		return (CMD_ERROR);
550 	}
551 
552 	(void) comc_setup(cp);
553 
554 	env_setenv(ev->ev_name, flags | EV_NOHOOK, value, NULL, NULL);
555 
556 	return (CMD_OK);
557 }
558 
559 /*
560  * Input: bus:dev:func[:bar]. If bar is not specified, it is 0x10.
561  * Output: bar[24:16] bus[15:8] dev[7:3] func[2:0]
562  */
563 static uint32_t
564 comc_parse_pcidev(const char *string)
565 {
566 #ifdef EFI
567 	(void) string;
568 	return (0);
569 #else
570 	char *p, *p1;
571 	uint8_t bus, dev, func, bar;
572 	uint32_t locator;
573 	int pres;
574 
575 	errno = 0;
576 	pres = strtoul(string, &p, 10);
577 	if (errno != 0 || p == string || *p != ':' || pres < 0)
578 		return (0);
579 	bus = pres;
580 	p1 = ++p;
581 
582 	pres = strtoul(p1, &p, 10);
583 	if (errno != 0 || p == string || *p != ':' || pres < 0)
584 		return (0);
585 	dev = pres;
586 	p1 = ++p;
587 
588 	pres = strtoul(p1, &p, 10);
589 	if (errno != 0 || p == string || (*p != ':' && *p != '\0') || pres < 0)
590 		return (0);
591 	func = pres;
592 
593 	if (*p == ':') {
594 		p1 = ++p;
595 		pres = strtoul(p1, &p, 10);
596 		if (errno != 0 || p == string || *p != '\0' || pres <= 0)
597 			return (0);
598 		bar = pres;
599 	} else
600 		bar = 0x10;
601 
602 	locator = (bar << 16) | biospci_locator(bus, dev, func);
603 	return (locator);
604 #endif
605 }
606 
607 static int
608 comc_pcidev_handle(struct console *cp, uint32_t locator)
609 {
610 #ifdef EFI
611 	(void) cp;
612 	(void) locator;
613 	return (CMD_ERROR);
614 #else
615 	struct serial *sp = cp->c_private;
616 	uint32_t port;
617 
618 	if (biospci_read_config(locator & 0xffff,
619 	    (locator & 0xff0000) >> 16, 2, &port) == -1) {
620 		printf("Cannot read bar at 0x%x\n", locator);
621 		return (CMD_ERROR);
622 	}
623 	if (!PCI_BAR_IO(port)) {
624 		printf("Memory bar at 0x%x\n", locator);
625 		return (CMD_ERROR);
626 	}
627 	port &= PCIM_BAR_IO_BASE;
628 
629 	(void) comc_setup(cp);
630 
631 	sp->locator = locator;
632 
633 	return (CMD_OK);
634 #endif
635 }
636 
637 static int
638 comc_pcidev_set(struct env_var *ev, int flags, const void *value)
639 {
640 	struct console *cp;
641 	struct serial *sp;
642 	uint32_t locator;
643 	int error;
644 
645 	if ((cp = get_console(ev->ev_name)) == NULL)
646 		return (CMD_ERROR);
647 	sp = cp->c_private;
648 
649 	if (value == NULL || (locator = comc_parse_pcidev(value)) <= 0) {
650 		printf("Invalid pcidev\n");
651 		return (CMD_ERROR);
652 	}
653 	if ((cp->c_flags & (C_ACTIVEIN | C_ACTIVEOUT)) != 0 &&
654 	    sp->locator != locator) {
655 		error = comc_pcidev_handle(cp, locator);
656 		if (error != CMD_OK)
657 			return (error);
658 	}
659 	env_setenv(ev->ev_name, flags | EV_NOHOOK, value, NULL, NULL);
660 	return (CMD_OK);
661 }
662 
663 /*
664  * In case of error, we also reset ACTIVE flags, so the console
665  * framefork will try alternate consoles.
666  */
667 static bool
668 comc_setup(struct console *cp)
669 {
670 	struct serial *sp = cp->c_private;
671 	static int TRY_COUNT = 1000000;
672 	int tries;
673 
674 	outb(sp->ioaddr + com_cfcr, CFCR_DLAB | sp->lcr);
675 	outb(sp->ioaddr + com_dlbl, COMC_BPS(sp->speed) & 0xff);
676 	outb(sp->ioaddr + com_dlbh, COMC_BPS(sp->speed) >> 8);
677 	outb(sp->ioaddr + com_cfcr, sp->lcr);
678 	outb(sp->ioaddr + com_mcr,
679 	    sp->rtsdtr_off? ~(MCR_RTS | MCR_DTR) : MCR_RTS | MCR_DTR);
680 
681 	tries = 0;
682 	do {
683 		inb(sp->ioaddr + com_data);
684 	} while (inb(sp->ioaddr + com_lsr) & LSR_RXRDY && ++tries < TRY_COUNT);
685 
686 	if (tries == TRY_COUNT)
687 		return (false);
688 	/* Mark this port usable. */
689 	cp->c_flags |= (C_PRESENTIN | C_PRESENTOUT);
690 	return (true);
691 }
692 
693 int
694 comc_getspeed(int ioaddr)
695 {
696 	uint_t	divisor;
697 	uchar_t	dlbh;
698 	uchar_t	dlbl;
699 	uchar_t	cfcr;
700 
701 	cfcr = inb(ioaddr + com_cfcr);
702 	outb(ioaddr + com_cfcr, CFCR_DLAB | cfcr);
703 
704 	dlbl = inb(ioaddr + com_dlbl);
705 	dlbh = inb(ioaddr + com_dlbh);
706 
707 	outb(ioaddr + com_cfcr, cfcr);
708 
709 	divisor = dlbh << 8 | dlbl;
710 
711 	/* XXX there should be more sanity checking. */
712 	if (divisor == 0)
713 		return (COMSPEED);
714 	return (COMC_DIV2BPS(divisor));
715 }
716