>> Can we change bdevsw/cdevsw to be struct Xdevsw *Xdevsw[]; ...

>... at least some parts of it are good things to do (and have
>precendent in e.g. BSDI if i recall correctly ...

Just FYI, we have:

 - a combined "devsw" (no separate b-vs-c; a device can be used
   as a block device iff it has a strategy routine)

 - 32-bit dev_t split up as {
	12 bits major
	20 bits minor {
	   10 bits "unit"
	   10 bits "subunit"
	}
   }
   (interpretation of minor is left to the device itself; disks and
   tapes use the subunit while ttys are generally just sequential);
   ls -l shows <major,unit,subunit> (even on ttys).

 - new macros (make the obvious changes to the old ones):

	#define	dv_unit(x)	((int)(((x) >> 10) & 0x3ff))
	#define	dv_subunit(x)	((int)((x) & 0x3ff))
	#define	dv_makedev(x,y,z)	((dev_t)(((x)<<20) | ((y) << 10) | (z)))

 - the devsw struct looks like this [fn params squished out for space]:

	struct devsw {
		struct	cfdriver	*d_cd;
		int	(*d_open)();
		int	(*d_close)();
		int	(*d_read)();
		int	(*d_write)();
		int	(*d_ioctl)();
		int	(*d_select)();
		vm_offset_t (*d_mmap)();
		void	(*d_strategy)();
		int	(*d_dump)();
		int	(*d_psize)();

	#define	D_TAPE	1
	#define	D_DISK	2
	#define	D_TTY	3
		int	d_type;

		void	(*d_stop)();
	};

   (The d_cd field can be left NULL and is not used for all that
   much; the others are what you get when you combine bdevsw and
   cdevsw.)

 - Each port has its own devsw[], in a template file in
   sys/${MACHINE}/conf/ioconf.c.${MACHINE}.  "config" writes
   an appropriate ioconf.c for the configuration, per directives
   that I am pretty sure I described here before (%CONFIG, %DECLSW,
   %DEVSW, %IFSEL).

 - The ioconf.c template contains the iskmemdev and iszerodev
   routines.  (The "securelevel" disk test uses d_cd->cd_class
   to test for disks; maybe should use d_type.)

 - The ioconf.c template also contains the compat code for translating
   old 16-bit dev_t's.  All ("both" :-) ) our existing ports had
   major 0==console and still have 0==console, so a dev_t is "new" if
   (new)major(dev) != 0; the translation from old to new is done
   with `switch' (since old disk and tape dev_t's need extra
   translation for unit+subunit).

Chris