cohtune -- Command

Set a variable within a device driver
cohtune driver tagfield "tagfield = value"

The  command cohtune  sets the  tagfield to value  within the  given device
driver driver. You  can then use the command idmkcoh  to build a new kernel
that incorporates your changes.  When you boot the new kernel, your changes
will have been made.

cohtune works by modifying the  file Space.c for driver. Each device driver
has  such a  file, that  sets user-definable  dimensions of  its operation.
When  you  invoke the  command  idmkcoh  to build  a  new kernel,  COHERENT
automatically checks  whether a Space.c module  that have changed, compiles
it,  and links  it into  the newly built  kernel.  idmkcoh  also recompiles
every Space.c whenever you change a tunable variable in the kernel, just to
ensure that all drivers are synchronized with changes in the kernel.

For  example,  the   file  /etc/conf/hai/Space.c  gives  the  user-settable
variable for  the driver hai, which  is COHERENT's host-adapter-independent
SCSI driver.  This file contains, among others, the variable HAI_TAPE. This
variable is a bit-map; bit n is turned on if there is a SCSI tape device at
SCSI ID  n. If you have  installed a SCSI tape as SCSI  device 3, then type
the following command:

    cohtune hai "HAI_TAPE" "int HAI_TAPE = 0x08"

The value 0x08  turns on bit 3.  As you  can see, cohtune finds the line in
/etc/conf/hai/Space.c  that  contains   the  string  HAI_TAPE  and  is  not
commented out of the source, and replaces it with the line

    int HAI_TAPE = 0x08

You can read  a driver's Space.c to see how  you can configure it.  Space.c
also  gives some useful  clues as  to how  the driver works  and how  it is
currently configured.

You should never modify a Space.c  by hand.  If you do so, you run the risk
of building a kernel that does not boot, or trashes your file system.

See Also

commands,
device drivers,
idenable,
idmkcoh,
idtune

Notes

cohtune cannot be used with STREAMS drivers.

Note that cohtune peforms no checks whatsoever on the content of the string
with which you  replace tagfield It should only be  used by people familiar
with C  programming, because setting  invalid values may  cause errors that
are difficult to diagnose.  Caveat utilitor.

Because of the primitive nature of cohtune, we recommend that users not use
it  directly, but  work  instead through  the  configuration shell  scripts
supplied  by the  driver's  developer (which  typically  live in  directory
/etc/conf/driver) that  can interactively generate the  correct sequence of
cohtune commands.

© 2012 by Stephen A. Ness