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d7b461c5 MCC |
1 | =============================== |
2 | IDE ATAPI streaming tape driver | |
3 | =============================== | |
10569713 BP |
4 | |
5 | This driver is a part of the Linux ide driver. | |
6 | ||
7 | The driver, in co-operation with ide.c, basically traverses the | |
8 | request-list for the block device interface. The character device | |
9 | interface, on the other hand, creates new requests, adds them | |
10 | to the request-list of the block device, and waits for their completion. | |
11 | ||
12 | The block device major and minor numbers are determined from the | |
13 | tape's relative position in the ide interfaces, as explained in ide.c. | |
14 | ||
d7b461c5 | 15 | The character device interface consists of the following devices:: |
10569713 | 16 | |
d7b461c5 MCC |
17 | ht0 major 37, minor 0 first IDE tape, rewind on close. |
18 | ht1 major 37, minor 1 second IDE tape, rewind on close. | |
19 | ... | |
20 | nht0 major 37, minor 128 first IDE tape, no rewind on close. | |
21 | nht1 major 37, minor 129 second IDE tape, no rewind on close. | |
22 | ... | |
10569713 BP |
23 | |
24 | The general magnetic tape commands compatible interface, as defined by | |
25 | include/linux/mtio.h, is accessible through the character device. | |
26 | ||
27 | General ide driver configuration options, such as the interrupt-unmask | |
28 | flag, can be configured by issuing an ioctl to the block device interface, | |
29 | as any other ide device. | |
30 | ||
31 | Our own ide-tape ioctl's can be issued to either the block device or | |
32 | the character device interface. | |
33 | ||
34 | Maximal throughput with minimal bus load will usually be achieved in the | |
35 | following scenario: | |
36 | ||
37 | 1. ide-tape is operating in the pipelined operation mode. | |
38 | 2. No buffering is performed by the user backup program. | |
39 | ||
40 | Testing was done with a 2 GB CONNER CTMA 4000 IDE ATAPI Streaming Tape Drive. | |
41 | ||
42 | Here are some words from the first releases of hd.c, which are quoted | |
43 | in ide.c and apply here as well: | |
44 | ||
d7b461c5 | 45 | * Special care is recommended. Have Fun! |
10569713 | 46 | |
d7b461c5 MCC |
47 | Possible improvements |
48 | ===================== | |
10569713 BP |
49 | |
50 | 1. Support for the ATAPI overlap protocol. | |
51 | ||
52 | In order to maximize bus throughput, we currently use the DSC | |
53 | overlap method which enables ide.c to service requests from the | |
54 | other device while the tape is busy executing a command. The | |
55 | DSC overlap method involves polling the tape's status register | |
56 | for the DSC bit, and servicing the other device while the tape | |
57 | isn't ready. | |
58 | ||
59 | In the current QIC development standard (December 1995), | |
60 | it is recommended that new tape drives will *in addition* | |
61 | implement the ATAPI overlap protocol, which is used for the | |
62 | same purpose - efficient use of the IDE bus, but is interrupt | |
63 | driven and thus has much less CPU overhead. | |
64 | ||
65 | ATAPI overlap is likely to be supported in most new ATAPI | |
66 | devices, including new ATAPI cdroms, and thus provides us | |
67 | a method by which we can achieve higher throughput when | |
68 | sharing a (fast) ATA-2 disk with any (slow) new ATAPI device. |