147 lines
6.5 KiB
Plaintext
147 lines
6.5 KiB
Plaintext
/*
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* IDE ATAPI streaming tape driver.
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*
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* This driver is a part of the Linux ide driver.
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*
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* The driver, in co-operation with ide.c, basically traverses the
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* request-list for the block device interface. The character device
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* interface, on the other hand, creates new requests, adds them
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* to the request-list of the block device, and waits for their completion.
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*
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* Pipelined operation mode is now supported on both reads and writes.
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*
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* The block device major and minor numbers are determined from the
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* tape's relative position in the ide interfaces, as explained in ide.c.
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*
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* The character device interface consists of the following devices:
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*
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* ht0 major 37, minor 0 first IDE tape, rewind on close.
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* ht1 major 37, minor 1 second IDE tape, rewind on close.
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* ...
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* nht0 major 37, minor 128 first IDE tape, no rewind on close.
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* nht1 major 37, minor 129 second IDE tape, no rewind on close.
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* ...
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*
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* The general magnetic tape commands compatible interface, as defined by
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* include/linux/mtio.h, is accessible through the character device.
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*
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* General ide driver configuration options, such as the interrupt-unmask
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* flag, can be configured by issuing an ioctl to the block device interface,
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* as any other ide device.
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*
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* Our own ide-tape ioctl's can be issued to either the block device or
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* the character device interface.
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*
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* Maximal throughput with minimal bus load will usually be achieved in the
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* following scenario:
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*
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* 1. ide-tape is operating in the pipelined operation mode.
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* 2. No buffering is performed by the user backup program.
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*
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* Testing was done with a 2 GB CONNER CTMA 4000 IDE ATAPI Streaming Tape Drive.
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*
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* Here are some words from the first releases of hd.c, which are quoted
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* in ide.c and apply here as well:
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*
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* | Special care is recommended. Have Fun!
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*
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*
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* An overview of the pipelined operation mode.
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*
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* In the pipelined write mode, we will usually just add requests to our
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* pipeline and return immediately, before we even start to service them. The
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* user program will then have enough time to prepare the next request while
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* we are still busy servicing previous requests. In the pipelined read mode,
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* the situation is similar - we add read-ahead requests into the pipeline,
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* before the user even requested them.
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*
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* The pipeline can be viewed as a "safety net" which will be activated when
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* the system load is high and prevents the user backup program from keeping up
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* with the current tape speed. At this point, the pipeline will get
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* shorter and shorter but the tape will still be streaming at the same speed.
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* Assuming we have enough pipeline stages, the system load will hopefully
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* decrease before the pipeline is completely empty, and the backup program
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* will be able to "catch up" and refill the pipeline again.
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*
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* When using the pipelined mode, it would be best to disable any type of
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* buffering done by the user program, as ide-tape already provides all the
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* benefits in the kernel, where it can be done in a more efficient way.
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* As we will usually not block the user program on a request, the most
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* efficient user code will then be a simple read-write-read-... cycle.
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* Any additional logic will usually just slow down the backup process.
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*
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* Using the pipelined mode, I get a constant over 400 KBps throughput,
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* which seems to be the maximum throughput supported by my tape.
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*
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* However, there are some downfalls:
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*
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* 1. We use memory (for data buffers) in proportional to the number
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* of pipeline stages (each stage is about 26 KB with my tape).
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* 2. In the pipelined write mode, we cheat and postpone error codes
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* to the user task. In read mode, the actual tape position
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* will be a bit further than the last requested block.
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*
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* Concerning (1):
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*
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* 1. We allocate stages dynamically only when we need them. When
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* we don't need them, we don't consume additional memory. In
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* case we can't allocate stages, we just manage without them
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* (at the expense of decreased throughput) so when Linux is
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* tight in memory, we will not pose additional difficulties.
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*
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* 2. The maximum number of stages (which is, in fact, the maximum
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* amount of memory) which we allocate is limited by the compile
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* time parameter IDETAPE_MAX_PIPELINE_STAGES.
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*
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* 3. The maximum number of stages is a controlled parameter - We
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* don't start from the user defined maximum number of stages
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* but from the lower IDETAPE_MIN_PIPELINE_STAGES (again, we
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* will not even allocate this amount of stages if the user
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* program can't handle the speed). We then implement a feedback
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* loop which checks if the pipeline is empty, and if it is, we
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* increase the maximum number of stages as necessary until we
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* reach the optimum value which just manages to keep the tape
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* busy with minimum allocated memory or until we reach
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* IDETAPE_MAX_PIPELINE_STAGES.
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*
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* Concerning (2):
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*
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* In pipelined write mode, ide-tape can not return accurate error codes
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* to the user program since we usually just add the request to the
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* pipeline without waiting for it to be serviced. In case an error
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* occurs, I will report it on the next user request.
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*
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* In the pipelined read mode, subsequent read requests or forward
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* filemark spacing will perform correctly, as we preserve all blocks
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* and filemarks which we encountered during our excess read-ahead.
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*
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* For accurate tape positioning and error reporting, disabling
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* pipelined mode might be the best option.
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*
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* You can enable/disable/tune the pipelined operation mode by adjusting
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* the compile time parameters below.
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*
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*
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* Possible improvements.
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*
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* 1. Support for the ATAPI overlap protocol.
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*
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* In order to maximize bus throughput, we currently use the DSC
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* overlap method which enables ide.c to service requests from the
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* other device while the tape is busy executing a command. The
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* DSC overlap method involves polling the tape's status register
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* for the DSC bit, and servicing the other device while the tape
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* isn't ready.
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*
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* In the current QIC development standard (December 1995),
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* it is recommended that new tape drives will *in addition*
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* implement the ATAPI overlap protocol, which is used for the
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* same purpose - efficient use of the IDE bus, but is interrupt
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* driven and thus has much less CPU overhead.
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*
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* ATAPI overlap is likely to be supported in most new ATAPI
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* devices, including new ATAPI cdroms, and thus provides us
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* a method by which we can achieve higher throughput when
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* sharing a (fast) ATA-2 disk with any (slow) new ATAPI device.
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*/
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