Parameter Rounding; Programmable Operating Definition - Seagate SCSI Interface Product Manual

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The Control Mode page specifies alternative queue management algorithms with additional rules on the order
of execution of commands (see Table 96).
A task received from an initiator without a task queue tag message while there are any tagged I/O commands
in the command queue from that initiator is an incorrect initiator connection, unless there is a contingent alle-
giance condition. A task received from an initiator with a task queue tag message while there is an untagged
command in the command queue from that initiator is also an incorrect initiator connection. In either of these
cases, the drive removes all commands in the queue from that initiator, aborts the command in process if it is
from that initiator, and sets the Sense Key to Aborted Command and the Sense Code to Overlapped Com-
mands Attempted.
The Reserve and Release commands should be sent with an Ordered Task Queue Tag message. Use of the
Head of Task Queue Tag message with these commands could result in reservation conflicts with previously
issued commands.
The Test Unit Ready and Inquiry commands are often sent with a Head of Task Queue Tag message, since the
information returned is either available or has no effect on the condition of the drive.
The drive recovery option, is to continue execution of commands in the queue after the contingent allegiance
condition has cleared. The drive returns Busy status to all other initiators while the contingent allegiance condi-
tion exists. During this time all commands in the queue are suspended. All commands used for recovery oper-
ations are untagged commands.
Deferred errors are normally related to a command that has already completed. As such, there is no attempt to
return the queue tag value assigned to the original command.
4.8

Parameter rounding

Certain parameters sent to a target with various commands contain a range of values. Targets may choose to
implement only selected values from this range. When the target receives a value that it does not support, it
either rejects the command (Check Condition status with Illegal Request sense key) or it rounds the value
received to a supported value. The target shall reject unsupported values unless rounding is permitted in the
description of the parameter.
Rounding of parameter values, when permitted (Rounding is enabled by Mode Select command, page code
00h, byte 2, bit 2) shall be performed as follows:
A target that receives a parameter value that is not an exact supported value shall adjust the value to one that
it supports and shall return Check Condition status with a sense key of Recovered Error. The additional sense
code shall be set to Rounded Parameter. The initiator is responsible to issue an appropriate command to learn
what value the target has selected.
Implementors Note: Generally, the target should adjust maximum-value fields down to the next lower supported
value than the one specified by the initiator. Minimum-value fields should be rounded up to the next higher sup-
ported value than the one specified by the initiator. In some cases, the type of rounding (up or down) is explic-
itly specified in the description of the parameter.
4.9

Programmable operating definition

Some applications require that the operating definition of a logical unit be modified to meet the special require-
ments of a particular initiator. The program-controlled modification of the operating definition is provided to
allow operating systems to change the operating definition of a more recently developed target to one which is
more compatible with the operating system. This ability requires that the system comply with the low-level
hardware definitions of SCSI-2.
The parameters that can be changed by modifying the operating definition of a logical unit include the vendor
identification, the device type, the device model, the SCSI compliance level, the SCSI specification level, the
command set, and other parameters. The low-level hardware parameters including signal timing and parity def-
initions cannot be changed by modifying the operating definition. The present operating definition of a logical
unit with respect to an initiator can be determined at any time by execution of an Inquiry command. In some
vendor-specific cases, it may also be necessary to perform other commands including Mode Sense and Read
Capacity.
SCSI Interface Product Manual, Rev. J