cciss: auto engage SCSI mid layer at driver load time

A long time ago, probably in 2002, one of the distros, or maybe more than
one, loaded block drivers prior to loading the SCSI mid layer. This meant
that the cciss driver, being a block driver, could not engage the SCSI mid
layer at init time without panicking, and relied on being poked by a
userland program after the system was up (and the SCSI mid layer was
therefore present) to engage the SCSI mid layer.

This is no longer the case, and cciss can safely rely on the SCSI mid
layer being present at init time and engage the SCSI mid layer straight
away. This means that users will see their tape drives and medium
changers at driver load time without need for a script in /etc/rc.d that
does this:

for x in /proc/driver/cciss/cciss*
do
echo "engage scsi" > $x
done

However, if no tape drives or medium changers are detected, the SCSI mid
layer will not be engaged. If a tape drive or medium change is later
hot-added to the system it will then be necessary to use the above script
or similar for the device(s) to be acceesible.

Signed-off-by: Stephen M. Cameron <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/blockdev/cciss.txt

···
 "SCSI support" in your kernel configuration to be able to use SCSI
 tape drives with your Smart Array 5xxx controller.
 
-Additionally, note that the driver will not engage the SCSI core at init 
-time.  The driver must be directed to dynamically engage the SCSI core via 
-the /proc filesystem entry which the "block" side of the driver creates as 
-/proc/driver/cciss/cciss* at runtime.  This is because at driver init time, 
-the SCSI core may not yet be initialized (because the driver is a block 
-driver) and attempting to register it with the SCSI core in such a case 
-would cause a hang.  This is best done via an initialization script 
-(typically in /etc/init.d, but could vary depending on distribution). 
+Additionally, note that the driver will engage the SCSI core at init
+time if any tape drives or medium changers are detected.  The driver may
+also be directed to dynamically engage the SCSI core via the /proc filesystem
+entry which the "block" side of the driver creates as
+/proc/driver/cciss/cciss* at runtime.  This is best done via a script.
+
 For example:
 
 	for x in /proc/driver/cciss/cciss[0-9]*

drivers/block/cciss.c

···
 	h->cciss_max_sectors = 8192;
 
 	rebuild_lun_table(h, 1, 0);
+	cciss_engage_scsi(h);
 	h->busy_initializing = 0;
 	return 1;
 

drivers/block/cciss_scsi.c

···
 /* If no tape support, then these become defined out of existence */
 
 #define cciss_scsi_setup(cntl_num)
+#define cciss_engage_scsi(h)
 
 #endif /* CONFIG_CISS_SCSI_TAPE */