CH(4) BSD Kernel Interfaces Manual CH(4)NAMEch — SCSI media-changer (juke box) driver
The ch driver provides support for a SCSI media changer. It allows many
slots of media to be multiplexed between a number of drives. The changer
device may optionally be equipped with a bar code reader, which reads
label information attached to the media.
A SCSI adapter must also be separately configured into the system before
a SCSI changer can be configured.
As the SCSI adapter is probed during boot, the SCSI bus is scanned for
devices. Any devices found which answer as 'Changer' type devices will
be 'attached' to the ch driver. In FreeBSD releases prior to 2.1, the
first found will be attached as ch0 and the next, ch1 etc. Beginning in
2.1 it is possible to specify what ch unit a device should come on line
as; refer to scsi(4) for details on kernel configuration.
It is only necessary to explicitly configure one ch device; data struc‐
tures are dynamically allocated as media changes are found on the SCSI
User mode programs communicate with the changer driver through a number
of ioctls which are described below. Changer element addresses used in
the communication between the kernel and the changer device are mapped to
zero-based logical addresses. Element types are specified as follows:
CHET_MT Medium transport element (picker).
CHET_ST Storage element (slot).
CHET_IE Import/export element (portal).
CHET_DT Data transfer element (drive).
The following ioctl(2) calls apply to the changer. They are defined in
the header file <sys/chio.h>.
CHIOMOVE (struct changer_move) Move a medium from one element to
another (MOVE MEDIUM) using the current picker. The source
and destination elements are specified in a changer_move
structure, which includes at least the following fields:
u_int cm_fromtype; /* element type to move from */
u_int cm_fromunit; /* logical unit of from element */
u_int cm_totype; /* element type to move to */
u_int cm_tounit; /* logical unit of to element */
u_int cm_flags; /* misc. flags */
If the CM_INVERT in the cm_flags field is set, the medium
changer is instructed to flip the medium while moving it.
CHIOEXCHANGE (struct changer_exchange) Move the medium located in the
source element to the first destination element, and move
the medium that had been in the first destination element
to the second destination element. In case of a simple
exchange, the source and second destination elements should
be the same. The current picker is used to perform the
operation. The addresses of the affected elements is spec‐
ified to the ioctl in a changer_exchange structure which
includes at least the following fields:
u_int ce_srctype; /* element type of source */
u_int ce_srcunit; /* logical unit of source */
u_int ce_fdsttype; /* element type of first destination */
u_int ce_fdstunit; /* logical unit of first destination */
u_int ce_sdsttype; /* element type of second destination */
u_int ce_sdstunit; /* logical unit of second destination */
u_int ce_flags; /* misc. flags */
In ce_flags, CM_INVERT1 and/or CM_INVERT2 may be set to
flip the first or second medium during the exchange opera‐
This operation is untested.
CHIOPOSITION (struct changer_position) Position the current picker in
front of the specified element. The element is specified
with a changer_position structure, which includes at least
the following elements:
u_int cp_type; /* element type */
u_int cp_unit; /* logical unit of element */
u_int cp_flags; /* misc. flags */
The cp_flags field may be set to CP_INVERT to invert the
picker during the operation.
CHIOGPICKER (int) Return the logical address of the current picker.
CHIOSPICKER (int) Select the picker specified by the given logical
CHIOGPARAMS (struct changer_params) Return the configuration parameters
for the media changer. This ioctl fills the changer_params
structure passed by the user with at least the following
u_int cp_npickers; /* number of pickers */
u_int cp_nslots; /* number of slots */
u_int cp_nportals; /* number of import/export portals */
u_int cp_ndrives; /* number of drives */
This call can be used by applications to query the dimen‐
sions of the jukebox before using the CHIGSTATUS ioctl to
query the jukebox' status.
CHIOIELEM Perform the INITIALIZE ELEMENT STATUS call on the media
changer device. This forces the media changer to update
its internal status information with respect to loaded
media. It also scans any barcode labels provided that it
has a label reader. The ch driver's status is not affected
by this call.
CHIOGSTATUS (struct changer_element_status_request) Perform the READ
ELEMENT STATUS call on the media changer device. This call
reads the element status information of the media changer
and converts it to an array of changer_element_status
With each call to CHIOGSTATUS, the status of one or more
elements of one type may be queried.
The application passes a changer_element_status_request
structure to the ch driver which contains the following
struct changer_element_status *cesr_element_status;
This structure is read by the driver to determine the type,
logical base address and number of elements for which
information is to be returned in the array of
changer_element_status structures pointed to by the
cesr_element_status field. The application must allocate
enough memory for cesr_element_count status structures (see
below). The cesr_flags can optionally be set to
CESR_VOLTAGS to indicate that volume tag (bar code) infor‐
mation is to be read from the jukebox and returned.
The cesr_element_base and cesr_element_count fields must be
valid with respect to the physical configuration of the
changer. If they are not, the CHIOGSTATUS ioctl returns
the EINVAL error code.
The information about the elements is returned in an array
of changer_element_status structures. This structure
include at least the following fields:
u_int ces_addr; /* element address in media changer */
u_char ces_flags; /* see CESTATUS definitions below */
u_char ces_sensecode; /* additional sense code for element */
u_char ces_sensequal; /* additional sense code qualifier */
u_char ces_invert; /* invert bit */
u_char ces_svalid; /* source address (ces_source) valid */
u_short ces_source; /* source address of medium */
changer_voltag_t ces_pvoltag; /* primary volume tag */
changer_voltag_t ces_avoltag; /* alternate volume tag */
u_char ces_idvalid; /* ces_scsi_id is valid */
u_char ces_scsi_id; /* SCSI id of element (if ces_idvalid is nonzero) */
u_char ces_lunvalid; /* ces_scsi_lun is valid */
u_char ces_scsi_lun; /* SCSI lun of element (if ces_lunvalid is nonzero) */
The ces_addr field contains the address of the element in
the coordinate system of the media changer. It is not used
by the driver, and should be used for diagnostic purposes
The following flags are defined for the ces_flags field:
CESTATUS_FULL A medium is present.
CESTATUS_IMPEXP The medium has been deposited by the oper‐
ator (and not by a picker).
CESTATUS_EXCEPT The element is in an exceptional state
(e.g. invalid barcode label, barcode not
CESTATUS_ACCESS The element is accessible by the picker.
CESTATUS_EXENAB The element supports medium export.
CESTATUS_INENAB The element supports medium import.
Note that not all flags are valid for all element types.
This version of the ch driver has been tested with a DEC TZ875 (5 slot,
one DLT drive) and a Breece Hill Q47 (60 slot, four DLT drives, barcode
Many of the features the ch driver supports are not thoroughly tested due
to the fact that the devices available for testing do not support the
necessary commands. This is true for alternate volume tags, media flip‐
ping, import/export element handling, multiple picker operation and other
/dev/ch[0-9] device entries
If the media changer does not support features requested by the ch
driver, it will produce both console error messages and failure return
codes to the ioctls described here.
SEE ALSOcam(4), chio(1), cd(4), da(4), sa(4)HISTORY
The ch driver appeared in 386BSD 0.1.
The ch driver was written by Jason R. Thorpe ⟨firstname.lastname@example.org⟩ for And
Communications, http://www.and.com/. It was added to the system by
Stefan Grefen ⟨email@example.com⟩ who apparently had such a
device. It was ported to CAM by Kenneth Merry ⟨ken@FreeBSD.org⟩. It was
updated to support volume tags by Hans Huebner ⟨firstname.lastname@example.org⟩.
BSD May 14, 1998 BSD