/*
* 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.
*
* Pipelined operation mode is now supported on both reads and writes.
*
* 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!
*
*
* An overview of the pipelined operation mode.
*
* In the pipelined write mode, we will usually just add requests to our
* pipeline and return immediately, before we even start to service them. The
* user program will then have enough time to prepare the next request while
* we are still busy servicing previous requests. In the pipelined read mode,
* the situation is similar - we add read-ahead requests into the pipeline,
* before the user even requested them.
*
* The pipeline can be viewed as a "safety net" which will be activated when
* the system load is high and prevents the user backup program from keeping up
* with the current tape speed. At this point, the pipeline will get
* shorter and shorter but the tape will still be streaming at the same speed.
* Assuming we have enough pipeline stages, the system load will hopefully
* decrease before the pipeline is completely empty, and the backup program
* will be able to "catch up" and refill the pipeline again.
*
* When using the pipelined mode, it would be best to disable any type of
* buffering done by the user program, as ide-tape already provides all the
* benefits in the kernel, where it can be done in a more efficient way.
* As we will usually not block the user program on a request, the most
* efficient user code will then be a simple read-write-read-... cycle.
* Any additional logic will usually just slow down the backup process.
*
* Using the pipelined mode, I get a constant over 400 KBps throughput,
* which seems to be the maximum throughput supported by my tape.
*
* However, there are some downfalls:
*
* 1. We use memory (for data buffers) in proportional to the number
* of pipeline stages (each stage is about 26 KB with my tape).
* 2. In the pipelined write mode, we cheat and postpone error codes
* to the user task. In read mode, the actual tape position
* will be a bit further than the last requested block.
*
* Concerning (1):
*
* 1. We allocate stages dynamically only when we need them. When
* we don't need them, we don't consume additional memory. In
* case we can't allocate stages, we just manage without them
* (at the expense of decreased throughput) so when Linux is
* tight in memory, we will not pose additional difficulties.
*
* 2. The maximum number of stages (which is, in fact, the maximum
* amount of memory) which we allocate is limited by the compile
* time parameter IDETAPE_MAX_PIPELINE_STAGES.
*
* 3. The maximum number of stages is a controlled parameter - We
* don't start from the user defined maximum number of stages
* but from the lower IDETAPE_MIN_PIPELINE_STAGES (again, we
* will not even allocate this amount of stages if the user
* program can't handle the speed). We then implement a feedback
* loop which checks if the pipeline is empty, and if it is, we
* increase the maximum number of stages as necessary until we
* reach the optimum value which just manages to keep the tape
* busy with minimum allocated memory or until we reach
* IDETAPE_MAX_PIPELINE_STAGES.
*
* Concerning (2):
*
* In pipelined write mode, ide-tape can not return accurate error codes
* to the user program since we usually just add the request to the
* pipeline without waiting for it to be serviced. In case an error
* occurs, I will report it on the next user request.
*
* In the pipelined read mode, subsequent read requests or forward
* filemark spacing will perform correctly, as we preserve all blocks
* and filemarks which we encountered during our excess read-ahead.
*
* For accurate tape positioning and error reporting, disabling
* pipelined mode might be the best option.
*
* You can enable/disable/tune the pipelined operation mode by adjusting
* the compile time parameters below.
*
*
* 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.
*/
