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From: Peter Dufault <dufault@hda.com>
Message-Id: <199505011049.GAA20745@hda.com>
Subject: SLICES and bits in the device numbers
To: bde@zeta.org.au (Bruce Evans)
Date: Mon, 1 May 1995 06:49:44 -0400 (EDT)
Cc: hackers@FreeBSD.org, julian@FreeBSD.org
In-Reply-To: <199505010448.OAA16733@godzilla.zeta.org.au> from "Bruce Evans" at May 1, 95 02:48:51 pm
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Bruce Evans writes:

> >  Modified:    sys/sys   disklabel.h
> >  Log:
> >  making a reservation for more bits before people start using them
> >  in the minor number, (this stuff HAS to go!)
> >  The change I want is:
> >  /*
> >         3                   2                   1                   0
> >       1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
> >      _________________________________________________________________
> >      | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
> >      -----------------------------------------------------------------
> >      |      TYPE           | SLICE   |  MAJOR?       |  UNIT   |PART | <-now
> >      -----------------------------------------------------------------
> >      |      TYPE     |PART2| SLICE   |  MAJOR?       |  UNIT   |PART | <-soon
> >      -----------------------------------------------------------------
> >  
> >          I want 3 more part bits (taken from 'TYPE' (useless as it is) (JRE)
> >  */
> 
> I already use the low 6 bits of the TYPE for FDTYPE() in isa/fdc.h :-].
> 
> Don't forget to update all the places that decode or encode the bits, e.g.,
> dkminor() in MAKEDEV.

Even if I remove the fixed SCSI device (BUS-ID-LUN embedded in
major number) that can no longer work for disks, I still need a
control device for the disk that bypasses all normal processing
and only permits the ioctl (for formatting disks and fixing broken
tapes).  It appears that for a disk drive, the only field that
belongs to the driver is the UNIT field.

While you're fixing macros should you perhaps add something like
a "This is NOT a slice" bit so that most of the TYPE and SLICE
fields revert to the driver?  Or will someone please propose a
better idea?  Joerg proposes that I use "O_NONBLOCK" to specify a
control device, but I don't like it because the changes are then
spread throughout the SCSI system more than if I intercept a control
device in scsi_driver.c

(As I bounced off Bruce once before: it would be real nice if we
could divorce the actual device from the disk aspects of it, so
that once you had a reliable block transfer mechanism you could
turn it into a full disk driver without modifying it.  You'd have
two devices - a pseudo device that was the disk driver and the
actual device that was the block transfer method.  But not now.)

I need this now, BTW, so I welcome suggestions.

-- 
Peter Dufault               Real Time Machine Control and Simulation
HD Associates, Inc.         Voice: 508 433 6936
dufault@hda.com             Fax:   508 433 5267