-/*
- * 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.
- *
- * Pipelined operation mode is now supported on both reads and writes.
- *
- * 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!
- *
- *
- * An overview of the pipelined operation mode.
- *
- * In the pipelined write mode, we will usually just add requests to our
- * pipeline and return immediately, before we even start to service them. The
- * user program will then have enough time to prepare the next request while
- * we are still busy servicing previous requests. In the pipelined read mode,
- * the situation is similar - we add read-ahead requests into the pipeline,
- * before the user even requested them.
- *
- * The pipeline can be viewed as a "safety net" which will be activated when
- * the system load is high and prevents the user backup program from keeping up
- * with the current tape speed. At this point, the pipeline will get
- * shorter and shorter but the tape will still be streaming at the same speed.
- * Assuming we have enough pipeline stages, the system load will hopefully
- * decrease before the pipeline is completely empty, and the backup program
- * will be able to "catch up" and refill the pipeline again.
- *
- * When using the pipelined mode, it would be best to disable any type of
- * buffering done by the user program, as ide-tape already provides all the
- * benefits in the kernel, where it can be done in a more efficient way.
- * As we will usually not block the user program on a request, the most
- * efficient user code will then be a simple read-write-read-... cycle.
- * Any additional logic will usually just slow down the backup process.
- *
- * Using the pipelined mode, I get a constant over 400 KBps throughput,
- * which seems to be the maximum throughput supported by my tape.
- *
- * However, there are some downfalls:
- *
- * 1. We use memory (for data buffers) in proportional to the number
- * of pipeline stages (each stage is about 26 KB with my tape).
- * 2. In the pipelined write mode, we cheat and postpone error codes
- * to the user task. In read mode, the actual tape position
- * will be a bit further than the last requested block.
- *
- * Concerning (1):
- *
- * 1. We allocate stages dynamically only when we need them. When
- * we don't need them, we don't consume additional memory. In
- * case we can't allocate stages, we just manage without them
- * (at the expense of decreased throughput) so when Linux is
- * tight in memory, we will not pose additional difficulties.
- *
- * 2. The maximum number of stages (which is, in fact, the maximum
- * amount of memory) which we allocate is limited by the compile
- * time parameter IDETAPE_MAX_PIPELINE_STAGES.
- *
- * 3. The maximum number of stages is a controlled parameter - We
- * don't start from the user defined maximum number of stages
- * but from the lower IDETAPE_MIN_PIPELINE_STAGES (again, we
- * will not even allocate this amount of stages if the user
- * program can't handle the speed). We then implement a feedback
- * loop which checks if the pipeline is empty, and if it is, we
- * increase the maximum number of stages as necessary until we
- * reach the optimum value which just manages to keep the tape
- * busy with minimum allocated memory or until we reach
- * IDETAPE_MAX_PIPELINE_STAGES.
- *
- * Concerning (2):
- *
- * In pipelined write mode, ide-tape can not return accurate error codes
- * to the user program since we usually just add the request to the
- * pipeline without waiting for it to be serviced. In case an error
- * occurs, I will report it on the next user request.
- *
- * In the pipelined read mode, subsequent read requests or forward
- * filemark spacing will perform correctly, as we preserve all blocks
- * and filemarks which we encountered during our excess read-ahead.
- *
- * For accurate tape positioning and error reporting, disabling
- * pipelined mode might be the best option.
- *
- * You can enable/disable/tune the pipelined operation mode by adjusting
- * the compile time parameters below.
- *
- *
- * 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.
- */
+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.
projects / cascardo / linux.git / blobdiff