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