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21. Error handling

The functions open, ioctl, write and read can report errors. In this case their return value is -1 and the global variable errno is set to the error number. The errno values are defined in /usr/include/errno.h. Possible values are:

Function | Error        | Description
=========|==============|=============================================
open     | ENXIO        | not a valid device
         | EACCES       | access mode is not read/write (O_RDWR)
         | EBUSY        | device was requested for nonblocking access, 
         |              | but is busy now.
         | ERESTARTSYS  | this indicates an internal error. Try to 
         |              | make it reproducible and inform the SCSI 
         |              | channel (for details on bug reporting
         |              | see Drew Eckhardts SCSI-HOWTO).
ioctl    | ENXIO        | not a valid device
read     | EAGAIN       | the device would block. Try again later.
         | ERESTARTSYS  | this indicates an internal error. Try to
         |              | make it reproducible and inform the SCSI
         |              | channel (for details on bug reporting
         |              | see Drew Eckhardts SCSI-HOWTO).
write    | EIO          | the length is too small (smaller than the 
         |              | generic header struct). Caution: Currently 
         |              | there is no overlength checking.
         | EAGAIN       | the device would block. Try again later.
         | ENOMEM       | memory required for this request could not be
         |              | allocated. Try later again unless you
         |              | exceeded the maximum transfer size (see above)
select   |              | none
close    |              | none

For read/write positive return values indicate as usual the amount of bytes that have been successfully transferred. This should equal the amount you requested.

21.1 Error status decoding

Furthermore a detailed reporting is done via the kernels hd_status and the devices sense_buffer (see section sec-sensebuff ) both from the generic header structure.

The meaning of hd_status can be found in drivers/scsi/scsi.h: This unsigned int is composed out of different parts:

  lsb  |    ...    |    ...    | msb
=======|===========|===========|============
status | sense key | host code | driver byte

These macros from drivers/scsi/scsi.h are available, but unfortunately cannot be easily used due to weird header file interdependencies. This has to be cleaned.

        Macro          | Description
=======================|=================================================
status_byte(hd_status) | The SCSI device status. See section Status codes
msg_byte(hd_status)    | From the device. See section SCSI sense keys
host_byte(hd_status)   | From the kernel. See section Hostcodes
driver_byte(hd_status) | From the kernel. See section midlevel codes

21.2 Status codes

The following status codes from the SCSI device (defined in scsi/scsi.h) are available.

Value | Symbol
======|=====================
0x00  | GOOD
0x01  | CHECK_CONDITION
0x02  | CONDITION_GOOD
0x04  | BUSY
0x08  | INTERMEDIATE_GOOD
0x0a  | INTERMEDIATE_C_GOOD
0x0c  | RESERVATION_CONFLICT

Note that these symbol values have been shifted right once. When the status is CHECK_CONDITION, the sense data in the sense buffer is valid (check especially the additional sense code and additional sense code qualifier).

These values carry the meaning from the SCSI-2 specification:


                           Table 27: Status Byte Code
+=================================-==============================+
|       Bits of Status Byte       |  Status                      |
|  7   6   5   4   3   2   1   0  |                              |
|---------------------------------+------------------------------|
|  R   R   0   0   0   0   0   R  |  GOOD                        |
|  R   R   0   0   0   0   1   R  |  CHECK CONDITION             |
|  R   R   0   0   0   1   0   R  |  CONDITION MET               |
|  R   R   0   0   1   0   0   R  |  BUSY                        |
|  R   R   0   1   0   0   0   R  |  INTERMEDIATE                |
|  R   R   0   1   0   1   0   R  |  INTERMEDIATE-CONDITION MET  |
|  R   R   0   1   1   0   0   R  |  RESERVATION CONFLICT        |
|  R   R   1   0   0   0   1   R  |  COMMAND TERMINATED          |
|  R   R   1   0   1   0   0   R  |  QUEUE FULL                  |
|                                 |                              |
|       All Other Codes           |  Reserved                    |
|----------------------------------------------------------------|
|  Key: R = Reserved bit                                         |
+================================================================+

A definition of the status byte codes is given below.

GOOD.  This status indicates that the target has successfully completed the
command.

CHECK CONDITION.  This status indicates that a contingent allegiance condition
has occurred (see 6.6).

CONDITION MET.  This status or INTERMEDIATE-CONDITION MET is returned whenever
the requested operation is satisfied (see the SEARCH DATA and PRE-FETCH
commands).

BUSY.  This status indicates that the target is busy.  This status shall be
returned whenever a target is unable to accept a command from an otherwise
acceptable initiator (i.e., no reservation conflicts).  The recommended
initiator recovery action is to issue the command again at a later time.

INTERMEDIATE.  This status or INTERMEDIATE-CONDITION MET shall be returned for
every successfully completed command in a series of linked commands (except
the last command), unless the command is terminated with CHECK CONDITION,
RESERVATION CONFLICT, or COMMAND TERMINATED status.  If INTERMEDIATE or
INTERMEDIATE-CONDITION MET status is not returned, the series of linked
commands is terminated and the I/O process is ended.

INTERMEDIATE-CONDITION MET.  This status is the combination of the CONDITION
MET and INTERMEDIATE statuses.

RESERVATION CONFLICT.  This status shall be returned whenever an initiator
attempts to access a logical unit or an extent within a logical unit that is
reserved with a conflicting reservation type for another SCSI device (see the
RESERVE and RESERVE UNIT commands).  The recommended initiator recovery action
is to issue the command again at a later time.

COMMAND TERMINATED.  This status shall be returned whenever the target
terminates the current I/O process after receiving a TERMINATE I/O PROCESS
message (see 5.6.22).  This status also indicates that a contingent allegiance
condition has occurred (see 6.6).

QUEUE FULL.  This status shall be implemented if tagged queuing is
implemented.  This status is returned when a SIMPLE QUEUE TAG, ORDERED QUEUE
TAG, or HEAD OF QUEUE TAG message is received and the command queue is full.
The I/O process is not placed in the command queue.

21.3 SCSI Sense Keys

These kernel symbols (from scsi/scsi.h) are predefined:

Value | Symbol
======|================
0x00  | NO_SENSE
0x01  | RECOVERED_ERROR
0x02  | NOT_READY
0x03  | MEDIUM_ERROR
0x04  | HARDWARE_ERROR
0x05  | ILLEGAL_REQUEST
0x06  | UNIT_ATTENTION
0x07  | DATA_PROTECT
0x08  | BLANK_CHECK
0x0a  | COPY_ABORTED
0x0b  | ABORTED_COMMAND
0x0d  | VOLUME_OVERFLOW
0x0e  | MISCOMPARE

A verbatim list from the SCSI-2 doc follows (from section 7.2.14.3):

                    Table 69: Sense Key (0h-7h) Descriptions
+========-====================================================================+
| Sense  |  Description                                                       |
|  Key   |                                                                    |
|--------+--------------------------------------------------------------------|
|   0h   |  NO SENSE.  Indicates that there is no specific sense key          |
|        |  information to be reported for the designated logical unit.  This |
|        |  would be the case for a successful command or a command that      |
|        |  received CHECK CONDITION or COMMAND TERMINATED status because one |
|        |  of the filemark, EOM, or ILI bits is set to one.                  |
|--------+--------------------------------------------------------------------|
|   1h   |  RECOVERED ERROR.  Indicates that the last command completed       |
|        |  successfully with some recovery action performed by the target.   |
|        |  Details may be determinable by examining the additional sense     |
|        |  bytes and the information field.  When multiple recovered errors  |
|        |  occur during one command, the choice of which error to report     |
|        |  (first, last, most severe, etc.) is device specific.              |
|--------+--------------------------------------------------------------------|
|   2h   |  NOT READY.  Indicates that the logical unit addressed cannot be   |
|        |  accessed.  Operator intervention may be required to correct this  |
|        |  condition.                                                        |
|--------+--------------------------------------------------------------------|
|   3h   |  MEDIUM ERROR.  Indicates that the command terminated with a non-  |
|        |  recovered error condition that was probably caused by a flaw in   |
|        |  the medium or an error in the recorded data.  This sense key may  |
|        |  also be returned if the target is unable to distinguish between a |
|        |  flaw in the medium and a specific hardware failure (sense key 4h).|
|--------+--------------------------------------------------------------------|
|   4h   |  HARDWARE ERROR.  Indicates that the target detected a non-        |
|        |  recoverable hardware failure (for example, controller failure,    |
|        |  device failure, parity error, etc.) while performing the command  |
|        |  or during a self test.                                            |
|--------+--------------------------------------------------------------------|
|   5h   |  ILLEGAL REQUEST.  Indicates that there was an illegal parameter in|
|        |  the command descriptor block or in the additional parameters      |
|        |  supplied as data for some commands (FORMAT UNIT, SEARCH DATA,     |
|        |  etc.).  If the target detects an invalid parameter in the command |
|        |  descriptor block, then it shall terminate the command without     |
|        |  altering the medium.  If the target detects an invalid parameter  |
|        |  in the additional parameters supplied as data, then the target may|
|        |  have already altered the medium.  This sense key may also indicate|
|        |  that an invalid IDENTIFY message was received (5.6.7).            |
|--------+--------------------------------------------------------------------|
|   6h   |  UNIT ATTENTION.  Indicates that the removable medium may have been|
|        |  changed or the target has been reset.  See 6.9 for more detailed  |
|        |  information about the unit attention condition.                   |
|--------+--------------------------------------------------------------------|
|   7h   |  DATA PROTECT.  Indicates that a command that reads or writes the  |
|        |  medium was attempted on a block that is protected from this       |
|        |  operation.  The read or write operation is not performed.         |
+=============================================================================+

                    Table 70: Sense Key (8h-Fh) Descriptions
+========-====================================================================+
| Sense  |  Description                                                       |
|  Key   |                                                                    |
|--------+--------------------------------------------------------------------|
|   8h   |  BLANK CHECK.  Indicates that a write-once device or a sequential- |
|        |  access device encountered blank medium or format-defined end-of-  |
|        |  data indication while reading or a write-once device encountered a|
|        |  non-blank medium while writing.                                   |
|--------+--------------------------------------------------------------------|
|   9h   |  Vendor Specific.  This sense key is available for reporting vendor|
|        |  specific conditions.                                              |
|--------+--------------------------------------------------------------------|
|   Ah   |  COPY ABORTED.  Indicates a COPY, COMPARE, or COPY AND VERIFY      |
|        |  command was aborted due to an error condition on the source       |
|        |  device, the destination device, or both.  (See 7.2.3.2 for        |
|        |  additional information about this sense key.)                     |
|--------+--------------------------------------------------------------------|
|   Bh   |  ABORTED COMMAND.  Indicates that the target aborted the command.  |
|        |  The initiator may be able to recover by trying the command again. |
|--------+--------------------------------------------------------------------|
|   Ch   |  EQUAL.  Indicates a SEARCH DATA command has satisfied an equal    |
|        |  comparison.                                                       |
|--------+--------------------------------------------------------------------|
|   Dh   |  VOLUME OVERFLOW.  Indicates that a buffered peripheral device has |
|        |  reached the end-of-partition and data may remain in the buffer    |
|        |  that has not been written to the medium.  A RECOVER BUFFERED DATA |
|        |  command(s) may be issued to read the unwritten data from the      |
|        |  buffer.                                                           |
|--------+--------------------------------------------------------------------|
|   Eh   |  MISCOMPARE.  Indicates that the source data did not match the data|
|        |  read from the medium.                                             |
|--------+--------------------------------------------------------------------|
|   Fh   |  RESERVED.                                                         |
+=============================================================================+

21.4 Host codes

The following host codes are defined in drivers/scsi/scsi.h. They are set by the kernel driver.

Value | Symbol         | Description
======|================|========================================
0x00  | DID_OK         | No error
0x01  | DID_NO_CONNECT | Couldn't connect before timeout period
0x02  | DID_BUS_BUSY   | BUS stayed busy through time out period
0x03  | DID_TIME_OUT   | TIMED OUT for other reason
0x04  | DID_BAD_TARGET | BAD target
0x05  | DID_ABORT      | Told to abort for some other reason
0x06  | DID_PARITY     | Parity error
0x07  | DID_ERROR      | internal error
0x08  | DID_RESET      | Reset by somebody
0x09  | DID_BAD_INTR   | Got an interrupt we weren't expecting

21.5 Driver codes

The midlevel driver categorizes the returned status from the lowlevel driver based on the sense key from the device. It suggests some actions to be taken such as retry, abort or remap. The routine scsi_done from scsi.c does a very differentiated handling based on host_byte(), status_byte(), msg_byte() and the suggestion. It then sets the driver byte to show what it has done. The driver byte is composed out of two nibbles: the driver status and the suggestion. Each half is composed from the below values being 'or'ed together (found in scsi.h).

Value | Symbol         | Description of Driver status
======|================|========================================
0x00  | DRIVER_OK      | No error
0x01  | DRIVER_BUSY    | not used
0x02  | DRIVER_SOFT    | not used
0x03  | DRIVER_MEDIA   | not used
0x04  | DRIVER_ERROR   | internal driver error
0x05  | DRIVER_INVALID | finished (DID_BAD_TARGET or DID_ABORT)
0x06  | DRIVER_TIMEOUT | finished with timeout
0x07  | DRIVER_HARD    | finished with fatal error
0x08  | DRIVER_SENSE   | had sense information available

Value | Symbol         | Description of suggestion
======|================|========================================
0x10  | SUGGEST_RETRY  | retry the SCSI request
0x20  | SUGGEST_ABORT  | abort the request
0x30  | SUGGEST_REMAP  | remap the block (not yet implemented)
0x40  | SUGGEST_DIE    | let the kernel panic
0x80  | SUGGEST_SENSE  | get sense information from the device
0xff  | SUGGEST_IS_OK  | nothing to be done


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