Here are changes to make the cons driver use the EEPROM setting.
I moved this from arch/sun3 to arch/dev just for convenience.

*** arch/sun3/sun3/cons.h	Fri Dec 17 01:57:19 1993
--- arch/sun3/dev/cons.h	Mon Mar 14 01:36:08 1994
***************
*** 39,44 ****
--- 39,45 ----
   *
   *	from: @(#)cons.h	7.3 (Berkeley) 11/2/90
   *	cons.h,v 1.2 1993/05/22 07:57:19 cgd Exp
+  *  gwr: moved and modified
   */
  
  struct consdev {

*** arch/sun3/sun3/cons.c	Tue Mar  1 06:21:51 1994
--- arch/sun3/dev/cons.c	Mon Mar 14 01:35:58 1994
***************
*** 40,113 ****
   *
   *	from: @(#)cons.c	7.6 (Berkeley) 5/4/91
   *	cons.c,v 1.2 1993/05/22 07:57:18 cgd Exp
   */
  
- #include <sys/systm.h>
  #include <sys/param.h>
  #include <sys/proc.h>
  #include <sys/systm.h>
- #include <sys/buf.h>
  #include <sys/ioctl.h>
  #include <sys/tty.h>
  #include <sys/file.h>
  #include <sys/conf.h>
  
! #include "cons.h"
  
  #include "prom.h"
- 
  #include "zs.h"
  
  int promcnprobe(), promcninit(), promcngetc(), promcnputc();
  int zscnprobe(), zscninit(), zscngetc(), zscnputc();
  
! struct	consdev constab[] = {
  #if NPROM
!     {promcnprobe, promcninit, promcngetc, promcnputc},
  #endif
! #if NZS
!     {zscnprobe, zscninit, zscngetc, zscnputc},
  #endif
-     { 0 },
  };
- /* end XXX */
  
  struct	tty *constty = 0;	/* virtual console output device */
  struct	consdev *cn_tab;	/* physical console device info */
  struct	tty *cn_tty;		/* XXX: console tty struct for tprintf */
  
  cninit()
  {
!     struct consdev *cp;
!     cn_tab = NULL;
  
  
! 	/*
! 	 * should add code to use eeprom's console setting   XXX
! 	 * when applicable.
! 	 */
  
  	/*
- 	 * Collect information about all possible consoles
- 	 * and find the one with highest priority
- 	 */
- 	for (cp = constab; cp->cn_probe; cp++) {
- 		(*cp->cn_probe)(cp);
- 		if (cp->cn_pri > CN_DEAD &&
- 		    (cn_tab == NULL || cp->cn_pri > cn_tab->cn_pri))
- 			cn_tab = cp;
- 	}
- 	/*
  	 * No console, we can handle it	 (really??? XXX)
  	 */
! 	if ((cp = cn_tab) == NULL)
  		return;
  	/*
  	 * Turn on console
  	 */
! 	cn_tty = cp->cn_tp;
! 	(*cp->cn_init)(cp);
! 	cn_tty = cp->cn_tp;
  }
  
  cnopen(dev, flag, mode, p)
--- 40,166 ----
   *
   *	from: @(#)cons.c	7.6 (Berkeley) 5/4/91
   *	cons.c,v 1.2 1993/05/22 07:57:18 cgd Exp
+  *  gwr: moved and modified
   */
  
  #include <sys/param.h>
  #include <sys/proc.h>
  #include <sys/systm.h>
  #include <sys/ioctl.h>
  #include <sys/tty.h>
  #include <sys/file.h>
  #include <sys/conf.h>
  
! #include <machine/mon.h>
! #include <machine/obio.h>
! #include <machine/eeprom.h>
  
+ #include "cons.h"
  #include "prom.h"
  #include "zs.h"
  
  int promcnprobe(), promcninit(), promcngetc(), promcnputc();
  int zscnprobe(), zscninit(), zscngetc(), zscnputc();
+ static int cn_probe_dead();
  
! static struct consdev cons_prom = {
  #if NPROM
!     promcnprobe, promcninit, promcngetc, promcnputc,
! #else
! 	cn_probe_dead,
  #endif
! };
! 
! static struct consdev cons_zs = {
! #if	NZS
! 	zscnprobe, zscninit, zscngetc, zscnputc,
! #else
! 	cn_probe_dead,
  #endif
  };
  
+ static struct consdev cons_kbd_scr = {
+ #if	0 /* XXX - not yet */
+ 	kbd_cnprobe, kbd_cninit, promcngetc, promcnputc,
+ #else
+ 	cn_probe_dead,
+ #endif
+ };
+ 
  struct	tty *constty = 0;	/* virtual console output device */
  struct	consdev *cn_tab;	/* physical console device info */
  struct	tty *cn_tty;		/* XXX: console tty struct for tprintf */
  
+ /*
+  * kern/init_main.c:main() calls consinit() early.
+  * ../sun3/machdep.c:consinit() just calls cninit()
+  */
+ 
  cninit()
  {
!     struct consdev *cn = NULL;
! 	u_char eeCons = 0xFF;	/* XXX - need an "illegal" value */
  
+ 	/* Use EEPROM console selector. */
+ 	if (eeprom_va == NULL) {
+ 		mon_printf("cninit: eeprom not yet mapped!\n");
+ 	} else {
+ 		eeCons = ((struct eeprom *)eeprom_va)->eeConsole;
+ 		mon_printf("cninit: eeConsole=0x%x\n", eeCons);
+ 	}
+ 	switch (eeCons) {
  
! 	case EE_CONS_COLOR:
! 	case EE_CONS_BW:
! 		cn = &cons_kbd_scr;
! 		(*cn->cn_probe)(cn);
! 		if (cn->cn_pri <= CN_DEAD) {
! 			mon_printf("cons: keyboard/screen probe failed\n");
! 			goto probe_failed;
! 		}
! 		break;
  
+ 	case EE_CONS_TTYA:
+ 	case EE_CONS_TTYB:
+ 		cn = &cons_zs;
+ 		/* Use minor to indicate A or B (major is unused). */
+ 		cn->cn_dev = makedev(0, (eeCons == EE_CONS_TTYA) ? 0 : 1);
+ 		(*cn->cn_probe)(cn);
+ 		if (cn->cn_pri <= CN_DEAD) {
+ 			mon_printf("cons: tty A or B probe failed\n");
+ 			goto probe_failed;
+ 		}
+ 		break;
+ 
+ 	default:
+ 		mon_printf("cons: unknown EEPROM console code 0x%x\n", eeCons);
+ 		/* fall through */
+ 	probe_failed:
+ 		cn = &cons_prom;
+ 		(*cn->cn_probe)(cn);
+ 		if (cn->cn_pri > CN_DEAD) {
+ 			mon_printf("cons: using PROM console\n");
+ 		} else {
+ 			mon_printf("cons: prom console probe failed\n");
+ 			cn = NULL;
+ 		}
+ 		break;
+ 
+ 	} /* switch eeCons */
+ 
  	/*
  	 * No console, we can handle it	 (really??? XXX)
  	 */
! 	if (cn == NULL)
  		return;
+ 	cn_tab = cn;
+ 
  	/*
  	 * Turn on console
  	 */
! 	cn_tty = cn->cn_tp;		/* XXX - Why both before... */
! 	(*cn->cn_init)(cn);
! 	cn_tty = cn->cn_tp;		/* XXX  ... and after? -gwr */
  }
  
  cnopen(dev, flag, mode, p)
***************
*** 204,207 ****
--- 257,266 ----
  		if (c == '\n')
  			(*cn_tab->cn_putc)(cn_tab->cn_dev, '\r');
  	}
+ }
+ 
+ static int
+ cn_probe_dead(struct consdev *cn)
+ {
+ 	cn->cn_pri = CN_DEAD;
  }

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