detecting AIT compression

[Hal Snyder <hal_(_at_)_vailsys_(_dot_)_com>]

Here are a couple patches that let you query the compression settings
of an AIT tape drive. One is to the st.c driver allowing "unsafe"
ioctls; the other is for the layout of the Data Compression Control
Page (part of Mode Sense and Mode Select).

After rebuilding the kernel with modified st.c, rebooting, and
patching scsi_modes as indicated, the following is possible at the
command line:

# scsi -f /dev/enrst0 -m 0x0F
DCE:  1 
DCC:  1 
DDE:  1 
RED:  0 
Compression Algorithm:  3 
Decompression Algorithm:  3 

Which says:
  data compression is enabled
  tape drive has data compression capability
  data decompression is enabled
  check condition will not be reported on boundaries between
    uncompressed and ALDC1 compressed data
  compression and decompression algorithms are set to ALDC1

Patch 1 - apply from /usr/src/sys
===========
*** scsi/st.c.orig	Wed Oct 25 21:22:37 2000
--- scsi/st.c	Wed Oct 25 21:29:00 2000
***************
*** 1284,1290 ****
  
  	default:
  		if (STMODE(dev) == CTLMODE)
! 			error = scsi_do_safeioctl(st->sc_link, dev,
  			    cmd, arg, flag, p);
  		else
  			error = ENOTTY;
--- 1284,1290 ----
  
  	default:
  		if (STMODE(dev) == CTLMODE)
! 			error = scsi_do_ioctl(st->sc_link, dev,
  			    cmd, arg, flag, p);
  		else
  			error = ENOTTY;
===========

Patch 2. Apply from /usr/share.
===========
*** misc/scsi_modes.orig        Wed Jun 28 12:13:45 2000
--- misc/scsi_modes     Wed Oct 25 15:07:52 2000
***************
*** 234,239 ****
--- 234,255 ----
        {Output Port 3 Volume} i1
  }
  
+ # Data Compression Control Page
+ 0x0F  {
+       {DCE} t1
+       {DCC} t1
+       {Reserved} *t6
+       {DDE} t1
+       {RED} t2
+       {Reserved} *t5
+       {Compression Algorithm} i4
+       {Decompression Algorithm} i4
+       {Reserved} *i1
+       {Reserved} *i1
+       {Reserved} *i1
+       {Reserved} *i1
+ }
+ 
  0x10 {
        {Reserved} *t1
        {Change Active Partition} t1
===========