[PATCH 0/2] scsi: st: Fix drive settings and position after reset recovery with MTLOAD/MTRETEN

[Laurence Oberman <loberman@redhat.com>]

After a device reset st blocks tape access until one of MTREW, MTOFFL,
MTLOAD, MTRETEN, MTERASE, MTSEEK or MTEOM clears the reset condition.
Testing the reset handling on scsi_debug and on IBM LTO drives found two
problems when the condition is cleared with MTLOAD or MTRETEN.

1. Changed block size and density are silently lost

   A reset returns the drive's block size and density to their defaults.
   Commit 7081dc75df79 ("scsi: st: Restore some drive settings after
   reset") re-applies the values set by the user, but only for MTREW,
   MTSEEK and MTEOM.  Because check_tape() reads the drive's block size
   with MODE SENSE on every open, the driver follows the drive back to
   its default after the reset.  An application that set fixed-block
   mode with MTSETBLK and recovers with MTLOAD or MTRETEN keeps writing
   without any error, but in variable-length blocks.

   Patch 1 restores the changed settings for MTLOAD and MTRETEN as well.
   Both may make the drive report a new medium, and the new session
   started by check_tape() clears the "changed" flags and applies the
   mode defaults.  The values are therefore saved before the operation
   and re-applied after check_tape() has run.  scsi_debug reports a new
   medium after MTRETEN and MTLOAD; the IBM drives do not.  The series
   was tested against both behaviours.

2. MTIOCGET reports file/block -1 at BOT after reset + MTLOAD

   Commit 0b120edb37dc ("scsi: st: Add MTIOCGET and MTLOAD to ioctls
   allowed after device reset") allows MTLOAD because the tape location
   is known after it, but the position is not recorded:
   reset_state() sets -1/-1, and do_load_unload() leaves it there unless
   check_tape() starts a new session.  With drives that do not report a
   new medium for an already loaded cartridge (IBM LTO), MTIOCGET keeps
   reporting -1/-1 and no BOT.

   Patch 2 records file/block 0 after a successful load when the current
   partition is 0.

MTOFFL is deliberately not changed: the medium is unloaded, and a
different one may be loaded next, for which the mode defaults should
apply.

Immediate mode: with MT_ST_NOWAIT, MTRETEN returns before the
retension completes, so patch 1 does not restore the settings there
rather than waiting for the drive.

Testing (v7.3-rc2, scsi_debug and IBM LTO drives on FC):

                            R03.load       R04.load      R04.retension  R04.offline
                            (pos. after    (MTSETBLK     (MTSETBLK      (MTSETBLK
                             reset+LOAD)    after LOAD)   after RETEN)   after OFFL)
unpatched:
  scsi_debug                ok (0:0)       discarded     discarded      discarded
  IBM ULTRIUM-TD5 (LTO-5)   -1:-1          discarded     discarded      discarded
  IBM ULT3580-TDA           -1:-1          discarded     discarded      discarded
patched:
  scsi_debug (2 hosts)      ok             restored      restored       discarded (*)
  IBM ULTRIUM-TD5 (LTO-5)   ok             restored      restored       discarded (*)
  IBM ULT3580-TDA           ok             restored      restored       discarded (*)

(*) unchanged by design, see above.

For the position test, "ok" on patched kernels means that MTIOCPOS
returns 0 and the first block read without repositioning is block 0 of
file 0; it is not based on MTIOCGET alone.  The restore after
MTREW/MTSEEK/MTEOM from 7081dc75df79 is unaffected on all devices.
Every test ends with a byte-exact read-back of the whole tape.

The tests are part of a tape validation harness (reset detection, the
blocked-operation matrix, recovery operations, interrupted I/O, SCSI EH
via scsi_debug error injection):
https://gitlab.com/loberman/tapetest (developed with AI assistance)

Laurence Oberman (2):
  scsi: st: Restore changed drive settings after reset also for MTLOAD and MTRETEN
  scsi: st: Record the tape position after a successful MTLOAD

 drivers/scsi/st.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)