CH(4) BSD Kernel Interfaces Manual CH(4)
ch -- 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 configura-
It is only necessary to explicitly configure one ch device; data structures 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
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 desti-
nation 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 specified 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 operation, respec-
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 address.
CHIOGPARAMS (struct changer_params) Return the configuration parameters for the media changer. This ioctl fills the changer_params struc-
ture passed by the user with at least the following fields:
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 dimensions of the jukebox before using the CHIGSTATUS ioctl to query the
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 structures.
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 fields:
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 applica-
tion 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) information 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 only.
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 operator (and not by a picker).
CESTATUS_EXCEPT The element is in an exceptional state (e.g. invalid barcode label, barcode not yet scanned).
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, bar-
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 flipping, import/export element handling, multiple picker operation
and other things.
/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.
cam(4), chio(1), cd(4), da(4), sa(4)
The ch driver appeared in 386BSD 0.1.
The ch driver was written by Jason R. Thorpe <email@example.com> for And Communications, http://www.and.com/. It was added to the system by
Stefan Grefen <firstname.lastname@example.org> 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 <email@example.com>.
May 14, 1998 BSD