From time to time I am asked how to pass a 32 byte SCSI
command (cdb) through the sg driver. My stock answer is
to refer people to the bsg driver for that purpose.
Since the bsg driver and the sg driver now make essentially
the same calls to the underlying block layer interface
it seems about time to relax the 16 byte cdb restriction
on the sg driver. Obviously the implementation to do this
has been well tested already by the bsg driver so the same
implementation is lifted by this patch into the sg driver.

SPC-4 (draft rev 36i section 3.1.30) states the maximum cdb
size in SCSI is 260 bytes. [Seems to me XCDBs could exceed
that since they can contain multiple XCDB descriptors each
of which is 140 bytes long.] Anyway the sg v3 interface
(sg_io_hdr as used by the sg driver) restricts the cdb length
by the type of its 'unsigned char cmd_len;' field, namely
255. So not quite 260 but better than 16. The bsg driver
uses a uint32_t for its cdb length field and may want to
consider putting an upper limit on that ...

Documentation: This file in the kernel:
   Documentation/scsi/scsi-generic.txt
does not describe the sg v3 interface but instead points to
this page:
   http://sg.danny.cz/sg/p/sg_v3_ho.html
which does describe the sg v3 interface. If this patch is
accepted I will update the description of the cmd_len
field in that web page.


ChangeLog:
     - remove the 16 byte CDB (SCSI command) length limit
       from the sg driver by handling longer CDBs the same
       way as the bsg driver. Remove comment from sg.h
       public interface about the cmd_len field being
       limited to 16 bytes.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>