scsi: st: Fix input/output error on empty drive reset

A previous change was introduced to prevent data loss during a power-on
reset when a tape is present inside the drive. This commit set the
"pos_unknown" flag to true to avoid operations that could compromise data
by performing actions from an untracked position. The relevant change is
commit 9604eea ("scsi: st: Add third party poweron reset handling")

As a consequence of this change, a new issue has surfaced: the driver now
returns an "Input/output error" even for empty drives when the drive, host,
or bus is reset. This issue stems from the "flush_buffer" function, which
first checks whether the "pos_unknown" flag is set. If the flag is set, the
user will encounter an "Input/output error" until the tape position is
known again. This behavior differs from the previous implementation, where
empty drives were not affected at system start up time, allowing tape
software to send commands to the driver to retrieve the drive's status and
other information.

The current behavior prioritizes the "pos_unknown" flag over the
"ST_NO_TAPE" status, leading to issues for software that detects drives
during system startup. This software will receive an "Input/output error"
until a tape is loaded and its position is known.

To resolve this, the "ST_NO_TAPE" status should take priority when the
drive is empty, allowing communication with the drive following a power-on
reset. At the same time, the change should continue to protect data by
maintaining the "pos_unknown" flag when the drive contains a tape and its
position is unknown.

Signed-off-by: Rafael Rocha <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Fixes: 9604eea ("scsi: st: Add third party poweron reset handling")
Acked-by: Kai Mäkisara <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

Author: RafaARV

drivers/scsi/st.c

@@ -834,15 +834,16 @@ static int flush_buffer(struct scsi_tape *STp, int seek_next)
 	int backspace, result;
 	struct st_partstat *STps;
 
+	if (STp->ready != ST_READY)
+		return 0;
+
 	/*
 	 * If there was a bus reset, block further access
 	 * to this device.
 	 */
 	if (STp->pos_unknown)
 		return (-EIO);
 
-	if (STp->ready != ST_READY)
-		return 0;
 	STps = &(STp->ps[STp->partition]);
 	if (STps->rw == ST_WRITING)	/* Writing */
 		return st_flush_write_buffer(STp);