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 therequest-list for the block device interface. The character deviceinterface, on the other hand, creates new requests, adds themto the request-list of the block device, and waits for their completion.

The block device major and minor numbers are determined from thetape’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 byinclude/linux/mtio.h, is accessible through the character device.

General ide driver configuration options, such as the interrupt-unmaskflag, 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 orthe character device interface.

Maximal throughput with minimal bus load will usually be achieved in thefollowing 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 quotedin 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 DSCoverlap method which enables ide.c to service requests from theother device while the tape is busy executing a command. TheDSC overlap method involves polling the tape’s status registerfor the DSC bit, and servicing the other device while the tapeisn’t ready.

In the current QIC development standard (December 1995),it is recommended that new tape drives willin additionimplement the ATAPI overlap protocol, which is used for thesame purpose - efficient use of the IDE bus, but is interruptdriven and thus has much less CPU overhead.

ATAPI overlap is likely to be supported in most new ATAPIdevices, including new ATAPI cdroms, and thus provides usa method by which we can achieve higher throughput whensharing a (fast) ATA-2 disk with any (slow) new ATAPI device.