tr -- Device Driver

Driver to read stored error messages
/dev/trace

The device  driver tr is the ``traceback'' driver  for the COHERENT kernel.
It manipulates an internal buffer that holds error messages from the kernel
or another device driver.  It has major number 6.  This driver is extremely
useful to persons who are writing device drivers.

The DDI/DKI  kernel routine  cmn_err() can be  invoked by drivers  to write
formatted  messages.  By  default,  it writes  the messages  both onto  the
system's console and into an internal  buffer in memory that can hold up to
four megabytes of  text.  Messages that begin with a  caret, `^', go to the
console  but  not  the  internal  buffer.   Messages  that  begin  with  an
exclamation point, `!', go to the buffer but not to the console.

The  trace driver  tr reads  this internal  buffer, and  lets you  copy its
contents into  a file for later perusal.  This  offers two major advantages
to persons developing and debugging device drivers:

-> First, copious  diagnostic output will  no longer scroll  off the screen
   and be lost.  The trace buffer holds every error message written through
   cmn_err() until you  read the buffer.  When you install  tr, you can set
   the size of the buffer, up to four megabytes.

-> Second, if messages are written to  the trace buffer only and not to the
   console, system  timing is affected much less than  if the messages were
   written to  the console.  This makes it easier  to catch subtle problems
   in timing.

To add tr to your kernel, do the following:

-> Log in as the superuser root.

-> cd to directory /etc/conf.

-> Execute  script tr/mkdev.  This  will walk  you through  the process  of
   configuring  your  kernel to  use  this driver,  and  create the  device
   /dev/trace.

-> Execute script bin/idmkcoh, to generate a new kernel.

-> Invoke the  script /etc/shutdown to  shutdown system, then  boot the new
   kernel.

To read the contents of the trace buffer, simply use the command

    cp /dev/trace file

where file  is the  file into which  you wish to  copy the contents  of the
trace buffer.

See Also

device drivers
COHERENT Device Driver Kit:
cmn_err()

© 2012 by Stephen A. Ness