[SCSI] st: expand ability to write immediate filemarks

The st tape driver recently added the MTWEOFI ioctl, which writes
a tape filemark (EOF), like the MTWEOF ioctl, except that MTWEOFI
returns immediately. This makes certain applications, like backup
software, run much more quickly on buffered tape drives.

Since legacy applications do not know about this new MTWEOFI ioctl,
this patch adds a new ioctl option that tells the st driver to return
immediately when writing an EOF (i.e. a filemark). This new flag
is much like the existing flag that tells the st driver to perform
writes (and certain other IOs) immediately, but this new flag only
applies to writing EOFs.

This new feature is controlled via the MTSETDRVBUFFER ioctl, using
the newly-defined MT_ST_NOWAIT_EOF flag.

Use of this new feature is displayed via the sysfs tape "options"
attribute.

The st documentation was updated to mention this new flag, as well
as the problems that can occur from using it.

Signed-off-by: Lee Duncan <[email protected]>
Acked-by: Kai Makisara <[email protected]>
Signed-off-by: James Bottomley <[email protected]>

Documentation/scsi/st.txt

@@ -390,6 +390,10 @@ MTSETDRVBUFFER
 	     MT_ST_SYSV sets the SYSV semantics (mode)
 	     MT_ST_NOWAIT enables immediate mode (i.e., don't wait for
 	        the command to finish) for some commands (e.g., rewind)
+	     MT_ST_NOWAIT_EOF enables immediate filemark mode (i.e. when
+	        writing a filemark, don't wait for it to complete). Please
+		see the BASICS note about MTWEOFI with respect to the
+		possible dangers of writing immediate filemarks.
 	     MT_ST_SILI enables setting the SILI bit in SCSI commands when
 		reading in variable block mode to enhance performance when
 		reading blocks shorter than the byte count; set this only