66 lines
2.5 KiB
Text
66 lines
2.5 KiB
Text
|
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.
|