[deliverable/linux.git] / Documentation / ide / ide-tape.txt

/*
 * IDE ATAPI streaming tape driver.
 *
 * This driver is a part of the Linux ide driver.
 *
 * The driver, in co-operation with ide.c, basically traverses the
 * request-list for the block device interface. The character device
 * interface, on the other hand, creates new requests, adds them
 * to the request-list of the block device, and waits for their completion.
 *
 * The block device major and minor numbers are determined from the
 * tape's relative position in the ide interfaces, as explained in ide.c.
 *
 * The character device interface consists of the following devices:
 *
 * ht0		major 37, minor 0	first  IDE tape, rewind on close.
 * ht1		major 37, minor 1	second IDE tape, rewind on close.
 * ...
 * nht0		major 37, minor 128	first  IDE tape, no rewind on close.
 * nht1		major 37, minor 129	second IDE tape, no rewind on close.
 * ...
 *
 * The general magnetic tape commands compatible interface, as defined by
 * include/linux/mtio.h, is accessible through the character device.
 *
 * General ide driver configuration options, such as the interrupt-unmask
 * flag, can be configured by issuing an ioctl to the block device interface,
 * as any other ide device.
 *
 * Our own ide-tape ioctl's can be issued to either the block device or
 * the character device interface.
 *
 * Maximal throughput with minimal bus load will usually be achieved in the
 * following scenario:
 *
 *	1.	ide-tape is operating in the pipelined operation mode.
 *	2.	No buffering is performed by the user backup program.
 *
 * Testing was done with a 2 GB CONNER CTMA 4000 IDE ATAPI Streaming Tape Drive.
 *
 * Here are some words from the first releases of hd.c, which are quoted
 * in ide.c and apply here as well:
 *
 * | Special care is recommended.  Have Fun!
 *
 *	Possible improvements.
 *
 *	1.	Support for the ATAPI overlap protocol.
 *
 *		In order to maximize bus throughput, we currently use the DSC
 *		overlap method which enables ide.c to service requests from the
 *		other device while the tape is busy executing a command. The
 *		DSC overlap method involves polling the tape's status register
 *		for the DSC bit, and servicing the other device while the tape
 *		isn't ready.
 *
 *		In the current QIC development standard (December 1995),
 *		it is recommended that new tape drives will *in addition*
 *		implement the ATAPI overlap protocol, which is used for the
 *		same purpose - efficient use of the IDE bus, but is interrupt
 *		driven and thus has much less CPU overhead.
 *
 *		ATAPI overlap is likely to be supported in most new ATAPI
 *		devices, including new ATAPI cdroms, and thus provides us
 *		a method by which we can achieve higher throughput when
 *		sharing a (fast) ATA-2 disk with any (slow) new ATAPI device.
 */
Commit	Line	Data
5ce78af4 BP	1	/*
	2	* IDE ATAPI streaming tape driver.
	3	*
	4	* This driver is a part of the Linux ide driver.
	5	*
	6	* The driver, in co-operation with ide.c, basically traverses the
	7	* request-list for the block device interface. The character device
	8	* interface, on the other hand, creates new requests, adds them
	9	* to the request-list of the block device, and waits for their completion.
	10	*
5ce78af4 BP	11	* The block device major and minor numbers are determined from the
	12	* tape's relative position in the ide interfaces, as explained in ide.c.
	13	*
	14	* The character device interface consists of the following devices:
	15	*
	16	* ht0 major 37, minor 0 first IDE tape, rewind on close.
	17	* ht1 major 37, minor 1 second IDE tape, rewind on close.
	18	* ...
	19	* nht0 major 37, minor 128 first IDE tape, no rewind on close.
	20	* nht1 major 37, minor 129 second IDE tape, no rewind on close.
	21	* ...
	22	*
	23	* The general magnetic tape commands compatible interface, as defined by
	24	* include/linux/mtio.h, is accessible through the character device.
	25	*
	26	* General ide driver configuration options, such as the interrupt-unmask
	27	* flag, can be configured by issuing an ioctl to the block device interface,
	28	* as any other ide device.
	29	*
	30	* Our own ide-tape ioctl's can be issued to either the block device or
	31	* the character device interface.
	32	*
	33	* Maximal throughput with minimal bus load will usually be achieved in the
	34	* following scenario:
	35	*
	36	* 1. ide-tape is operating in the pipelined operation mode.
	37	* 2. No buffering is performed by the user backup program.
	38	*
	39	* Testing was done with a 2 GB CONNER CTMA 4000 IDE ATAPI Streaming Tape Drive.
	40	*
	41	* Here are some words from the first releases of hd.c, which are quoted
	42	* in ide.c and apply here as well:
	43	*
	44	* \| Special care is recommended. Have Fun!
	45	*
5ce78af4 BP	46	* Possible improvements.
	47	*
	48	* 1. Support for the ATAPI overlap protocol.
	49	*
	50	* In order to maximize bus throughput, we currently use the DSC
	51	* overlap method which enables ide.c to service requests from the
	52	* other device while the tape is busy executing a command. The
	53	* DSC overlap method involves polling the tape's status register
	54	* for the DSC bit, and servicing the other device while the tape
	55	* isn't ready.
	56	*
	57	* In the current QIC development standard (December 1995),
	58	* it is recommended that new tape drives will in addition
	59	* implement the ATAPI overlap protocol, which is used for the
	60	* same purpose - efficient use of the IDE bus, but is interrupt
	61	* driven and thus has much less CPU overhead.
	62	*
	63	* ATAPI overlap is likely to be supported in most new ATAPI
	64	* devices, including new ATAPI cdroms, and thus provides us
	65	* a method by which we can achieve higher throughput when
	66	* sharing a (fast) ATA-2 disk with any (slow) new ATAPI device.
	67	*/