[PATCH 0/3] ata: fix deferred QC handling for port multipliers

[Niklas Cassel <cassel@kernel.org>]

Hello all,

Tommy Kelly reported a regression with PMP that use CBS:
https://lore.kernel.org/linux-ide/ce09cc21-a8e9-4845-b205-35411e22fba9@tkel.ly/

To me, this appears to be a problem in ata_pmp_qc_defer_cmd_switch() that
has been there since this function was introduced. ap->excl_link did not
always get set. This is fixed in patch 1/3.

While looking at the code, when using a PMP with CBS, it turns out that we
are incorrectly using the deferred_qc feature that issues a QC via a
workqueue, even for non-NCQ commands that are issued to another link
(a link that is not the active link). This workqueue feature was meant to
avoid dealing with non-NCQ vs NCQ commands for a single drive, not to
duplicate the existing ap->excl_link logic. This is fixed in patch 2/3.

While looking at the code, it turns out that the deferred qc issuing via
workqueue is misdesigned. It assumed that we can't mix NCQ and non-NCQ
commands on the same port. The limitation is that you can not mix NCQ and
non-NCQ commands on the same drive. However, with a PMP with FBS, you can
issue (mixed NCQ and non-NCQ commands) to the different drives. Thus, move
the saved deferred QC from struct ata_port to struct ata_link. This is
fixed in patch 3/3.

Tommy, it would be nice if you could apply this series and see if it solves
your problem. Right now the series is compile tested only.


Niklas Cassel (3):
  ata: libata-pmp: fix ata_pmp_qc_defer_cmd_switch()
  ata: libata-scsi: do not use the deferred QC feature on PMPs with CBS
  ata: libata-scsi: do not needlessly defer commands when using PMP with
    FBS

 drivers/ata/libata-core.c | 16 +++++++---
 drivers/ata/libata-eh.c   |  8 ++---
 drivers/ata/libata-pmp.c  | 13 +++++---
 drivers/ata/libata-scsi.c | 66 ++++++++++++++++++++++-----------------
 include/linux/libata.h    |  6 ++--
 5 files changed, 64 insertions(+), 45 deletions(-)

-- 
2.54.0