The isp(4) driver was changing the tag type for REQUEST SENSE
commands to Head of Queue, when the CAM CCB flag
CAM_TAG_ACTION_VALID was NOT set. CAM_TAG_ACTION_VALID is set
when the tag action in the XPT_SCSI_IO is not CAM_TAG_ACTION_NONE
and when the target has tagged queueing turned on.
In most cases when CAM_TAG_ACTION_VALID is not set, it is because
the target is not doing tagged queueing. In those cases, trying to
send a Head of Queue tag may cause problems. Instead, default to
sending a simple tag.
IBM tape drives claim to support tagged queueing in their standard
Inquiry data, but have the DQue bit set in the control mode page
(mode page 10). CAM correctly detects that these drives do not
support tagged queueing, and clears the CAM_TAG_ACTION_VALID flag
on CCBs sent down to the drives.
This caused the isp(4) driver to go down the path of setting the
tag action to a default value, and for Request Sense commands only,
set the tag action to Head of Queue.
If an IBM tape drive does get a Head of Queue tag, it rejects it with
Invalid Message Error (0x49,0x00). (The Qlogic firmware translates that
to a Transport Error, which the driver translates to an Unrecoverable
HBA Error, or CAM_UNREC_HBA_ERROR.) So, by default, it wasn't possible
to get a good response from a REQUEST SENSE to an FC-attached IBM
tape drive with the isp(4) driver.
IBM tape drives (tested on an LTO-5 with G9N1 firmware and a TS1150
with 4470 firmware) also have a bug in that sending a command with a
non-simple tag attribute breaks the tape drive's Command Reference
Number (CRN) accounting and causes it to ignore all subsequent
commands because it and the initiator disagree about the next
expected CRN. The drives do reject the initial command with a head
of queue tag with an Invalid Message Error (0x49,0x00), but after that
they ignore any subsequent commands. IBM confirmed that it is a bug,
and sent me test firmware that fixes the bug. However tape drives in
the field will still exhibit the bug until they are upgraded.
Request Sense is not often sent to targets because most errors are
reported automatically through autosense in Fibre Channel and other
modern transports. ("Modern" meaning post SCSI-2.) So this is not
an error that would crop up frequently. But Request Sense is useful on
tape devices to report status information, aside from error reporting.
This problem is less serious without FC-Tape features turned on,
specifically precise delivery of commands (which enables Command
Reference Numbers), enabled on the target and initiator. Without
FC-Tape features turned on, the target would return an error and
things would continue on.
And it also does not cause problems for targets that do tagged
queueing, because in those cases the isp(4) driver just uses the
tag type that is specified in the CCB, assuming the
CAM_TAG_ACTION_VALID flag is set, and defaults to sending a Simple
tag action if it isn't an ordered or head of queue tag.
sys/dev/isp/isp.c:
In isp_start(), don't try to send Request Sense commands
with the Head of Queue tag attribute if the CCB doesn't
have a valid tag action. The tag action likely isn't valid
because the target doesn't support tagged queueing.
Sponsored by: Spectra Logic
MFC after: 3 days
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