[PATCH] 2.5.22 SCSI tape buffering changes

This contains the following changes to the SCSI tape driver:

- one buffer is used for each tape (no buffer pool)
- buffers allocated when needed and freed when device closed
- common code from read and write moved to a function
- default maximum number of scatter/gather segments increased to 64
- tape status set to "no tape" after succesful unload

drivers/scsi/README.st

@@ -2,7 +2,7 @@ This file contains brief information about the SCSI tape driver.
 The driver is currently maintained by Kai M{kisara (email
 Kai.Makisara@metla.fi)
 
-Last modified: Tue Jan 22 21:08:57 2002 by makisara
+Last modified: Tue Jun 18 18:13:50 2002 by makisara
 
 
 BASICS
@@ -105,15 +105,19 @@ The default is BSD semantics.
 
 BUFFERING
 
-The driver uses tape buffers allocated either at system initialization
-or at run-time when needed. One buffer is used for each open tape
-device. The size of the buffers is selectable at compile and/or boot
-time. The buffers are used to store the data being transferred to/from
-the SCSI adapter. The following buffering options are selectable at
-compile time and/or at run time (via ioctl):
+The driver uses tape buffers allocated at run-time when needed and it
+is freed when the device file is closed. One buffer is used for each
+open tape device. 
+
+The size of the buffers is always at least one tape block. In fixed
+block mode, the minimum buffer size is defined (in 1024 byte units) by
+ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
+several blocks and using one SCSI read or write to transfer all of the
+blocks. Buffering of data across write calls in fixed block mode is
+allowed if ST_BUFFER_WRITES is non-zero. Buffer allocation uses chunks of
+memory having sizes 2^n * (page size). Because of this the actual
+buffer size may be larger than the minimum allowable buffer size.
 
-Buffering of data across write calls in fixed block mode (define
-ST_BUFFER_WRITES).
 
 Asynchronous writing. Writing the buffer contents to the tape is
 started and the write call returns immediately. The status is checked
@@ -128,30 +132,6 @@ attempted even if the user does not want to get all of the data at
 this read command. Should be disabled for those drives that don't like
 a filemark to truncate a read request or that don't like backspacing.
 
-The buffer size is defined (in 1024 byte units) by ST_BUFFER_BLOCKS or
-at boot time. If this size is not large enough, the driver tries to
-temporarily enlarge the buffer. Buffer allocation uses chunks of
-memory having sizes 2^n * (page size). Because of this the actual
-buffer size may be larger than the buffer size specified with
-ST_BUFFER_BLOCKS.
-
-A small number of buffers are allocated at driver initialisation. The
-maximum number of these buffers is defined by ST_MAX_BUFFERS. The
-maximum can be changed with kernel or module startup options. One
-buffer is allocated for each drive detected when the driver is
-initialized up to the maximum.
-
-The driver tries to allocate new buffers at run-time if
-necessary. These buffers are freed after use. If the maximum number of
-initial buffers is set to zero, all buffer allocation is done at
-run-time. The advantage of run-time allocation is that memory is not
-wasted for buffers not being used. The disadvantage is that there may
-not be memory available at the time when a buffer is needed for the
-first time (once a buffer is allocated, it is not released). This risk
-should not be big if the tape drive is connected to a PCI adapter that
-supports scatter/gather (the allocation is not limited to "DMA memory"
-and the buffer can be composed of several fragments).
-
 The threshold for triggering asynchronous write in fixed block mode
 is defined by ST_WRITE_THRESHOLD. This may be optimized for each
 use pattern. The default triggers asynchronous write after three

drivers/scsi/st_options.h

@@ -3,7 +3,7 @@
 
    Copyright 1995-2000 Kai Makisara.
 
-   Last modified: Tue Jan 22 21:52:34 2002 by makisara
+   Last modified: Sun May  5 15:09:56 2002 by makisara
 */
 
 #ifndef _ST_OPTIONS_H
@@ -30,22 +30,17 @@
    SENSE. */
 #define ST_DEFAULT_BLOCK 0
 
-/* The tape driver buffer size in kilobytes. Must be non-zero. */
-#define ST_BUFFER_BLOCKS 32
+/* The minimum tape driver buffer size in kilobytes in fixed block mode.
+   Must be non-zero. */
+#define ST_FIXED_BUFFER_BLOCKS 32
 
 /* The number of kilobytes of data in the buffer that triggers an
    asynchronous write in fixed block mode. See also ST_ASYNC_WRITES
    below. */
 #define ST_WRITE_THRESHOLD_BLOCKS 30
 
-/* The maximum number of tape buffers the driver tries to allocate at 
-   driver initialisation. The number is also constrained by the number
-   of drives detected. If more buffers are needed, they are allocated
-   at run time and freed after use. */
-#define ST_MAX_BUFFERS 4
-
 /* Maximum number of scatter/gather segments */
-#define ST_MAX_SG      16
+#define ST_MAX_SG      64
 
 /* The number of scatter/gather segments to allocate at first try (must be
    smaller or equal to the maximum). */