home | help 
CAM(4)			     Kernel Interfaces Manual			  CAM(4)

NAME
     CAM -- Common Access Method SCSI/ATA subsystem

SYNOPSIS
     device scbus
     device ada
     device cd
     device ch
     device da
     device pass
     device pt
     device sa
     options CAMDEBUG
     options CAM_DEBUG_BUS=-1
     options CAM_DEBUG_TARGET=-1
     options CAM_DEBUG_LUN=-1
     options CAM_DEBUG_FLAGS=CAM_DEBUG_INFO|CAM_DEBUG_CDB
     options CAM_MAX_HIGHPOWER=4
     options SCSI_NO_SENSE_STRINGS
     options SCSI_NO_OP_STRINGS
     options SCSI_DELAY=8000

DESCRIPTION
     The CAM subsystem provides a uniform and modular system for the implementa-
     tion  of  drivers	to  control various SCSI and ATA devices, and to utilize
     different SCSI and ATA host adapters through host	adapter  drivers.   When
     the system probes busses, it attaches any devices it finds to the appropri-
     ate  drivers.   The pass(4) driver, if it is configured in the kernel, will
     attach to all devices.

KERNEL CONFIGURATION
     There are a number of generic kernel configuration options for the CAM sub-
     system:

     CAMDEBUG		    This option enables the CAM debugging  printf  code.
			    This  will not actually cause any debugging informa-
			    tion to be printed out when included by itself.  En-
			    abling printouts requires additional  configuration.
			    See below for details.

     CAM_MAX_HIGHPOWER=4    This sets the maximum allowable number of concurrent
			    "high  power" commands.  A "high power" command is a
			    command that takes more electrical power  than  most
			    to	complete.   An example of this is the SCSI START
			    UNIT command.  Starting a disk often takes	signifi-
			    cantly  more electrical power than normal operation.
			    This option allows the user to specify how many con-
			    current high power commands may be outstanding with-
			    out overloading the power supply on his computer.

     SCSI_NO_SENSE_STRINGS  This eliminates text descriptions of each SCSI Addi-
			    tional Sense Code and Additional Sense  Code  Quali-
			    fier  pair.  Since this is a fairly large text data-
			    base, eliminating it reduces the size of the  kernel
			    somewhat.	This  is  primarily  necessary	for boot
			    floppies and other low  disk  space  or  low  memory
			    space  environments.   In  most  cases, though, this
			    should be enabled, since it speeds	the  interpreta-
			    tion of SCSI error messages.  Do not let the "kernel
			    bloat"  zealots  get  to  you -- leave the sense de-
			    scriptions in your kernel!

     SCSI_NO_OP_STRINGS     This disables text descriptions of each SCSI opcode.
			    This option, like the sense string option above,  is
			    primarily useful for environments like a boot floppy
			    where kernel size is critical.  Enabling this option
			    for  normal  use  is not recommended, since it slows
			    debugging of SCSI problems.

     SCSI_DELAY=8000	    This is the SCSI "bus settle delay." In CAM,  it  is
			    specified  in milliseconds, not seconds like the old
			    SCSI layer used to do.  When the  kernel  boots,  it
			    sends  a bus reset to each SCSI bus to tell each de-
			    vice to reset itself to a default  set  of	transfer
			    negotiations  and other settings.  Most SCSI devices
			    need some amount of time to recover from a	bus  re-
			    set.  Newer disks may need as little as 100ms, while
			    old,  slow	devices  may  need  much longer.  If the
			    SCSI_DELAY is not specified, it defaults to  2  sec-
			    onds.  The minimum allowable value for SCSI_DELAY is
			    "100",  or	100ms.	 One special case is that if the
			    SCSI_DELAY is set to 0, that will be taken	to  mean
			    the  "lowest  possible  value."  In  that  case, the
			    SCSI_DELAY will be reset to 100ms.

     All devices and busses support dynamic allocation so that an  upper  number
     of  devices  and controllers does not need to be configured; device da will
     suffice for any number of disk drivers.

     The devices are either wired so they appear as a particular device unit  or
     counted so that they appear as the next available unused unit.

     Units  are wired down by setting kernel environment hints.  This is usually
     done either interactively from the  loader(8),  or  automatically	via  the
     /boot/device.hints file.  The basic syntax is:

	   hint.device.unit.property="value"

     Individual CAM bus numbers can be wired down to specific controllers with a
     config line similar to the following:

	   hint.scbus.0.at="ahd1"

     This assigns CAM bus number 0 to the ahd1 driver instance.  For controllers
     supporting more than one bus, a particular bus can be assigned as follows:

	   hint.scbus.0.at="ahc1"
	   hint.scbus.0.bus="1"

     This  assigns  CAM bus 0 to the bus 1 instance on ahc0.  Peripheral drivers
     can be wired to a specific bus, target, and lun as so:

	   hint.da.0.at="scbus0"
	   hint.da.0.target="0"
	   hint.da.0.unit="0"

     This assigns da0 to target 0, unit (lun) 0 of scbus 0.  Omitting the target
     or unit hints will instruct CAM to treat them  as	wildcards  and	use  the
     first  respective	counted  instances.   These examples can be combined to-
     gether to allow a peripheral device to be	wired  to  any	particular  con-
     troller, bus, target, and/or unit instance.

     When you have a mixture of wired down and counted devices then the counting
     begins  with the first non-wired down unit for a particular type.	That is,
     if you have a disk wired down as device da1, then the first non-wired  disk
     shall come on line as da2.

ADAPTERS
     The  system  allows  common  device  drivers to work through many different
     types of adapters.  The adapters take requests from the upper layers and do
     all IO between the SCSI or ATA bus and the system.  The maximum size  of  a
     transfer  is governed by the adapter.  Most adapters can transfer 64KB in a
     single operation, however many can transfer larger amounts.

TARGET MODE
     Some adapters support target mode in which the system is capable of operat-
     ing as a device, responding to  operations  initiated  by	another  system.
     Target  mode  is  supported  for some adapters, but is not yet complete for
     this version of the CAM SCSI subsystem.

FILES
     see other CAM device entries.

DIAGNOSTICS
     When the kernel is compiled with options CAMDEBUG, an XPT_DEBUG CCB can  be
     used  to  enable various amounts of tracing information on any specific de-
     vice.  Devices not being traced will not produce trace information.   There
     are currently four debugging flags that may be turned on:

     CAM_DEBUG_INFO	 This	debugging  flag  enables  general  informational
			 printfs for the device or devices in question.

     CAM_DEBUG_TRACE	 This debugging flag enables function-level command flow
			 tracing.  i.e. kernel printfs will happen  at	the  en-
			 trance and exit of various functions.

     CAM_DEBUG_SUBTRACE  This  debugging  flag enables debugging output internal
			 to various functions.

     CAM_DEBUG_CDB	 This debugging flag will cause the kernel to print  out
			 all  SCSI  commands  sent to a particular device or de-
			 vices.

     Some of these flags, most notably	CAM_DEBUG_TRACE  and  CAM_DEBUG_SUBTRACE
     will  produce  kernel printfs in EXTREME numbers, and because of that, they
     are not especially useful.   There  are  not  many  things  logged  at  the
     CAM_DEBUG_INFO  level, so it is not especially useful.  The most useful de-
     bugging flag is the CAM_DEBUG_CDB flag.  Users can  enable  debugging  from
     their kernel config file, by using the following kernel config options:

     CAMDEBUG	       This  enables  CAM debugging.  Without this option, users
		       will not even be able to turn on debugging from	userland
		       via camcontrol(8).

     CAM_DEBUG_FLAGS   This  allows  the user to set the various debugging flags
		       described above in a kernel config file.   Flags  may  be
		       ORed  together if the user wishes to see printfs for mul-
		       tiple debugging levels.

     CAM_DEBUG_BUS     Specify a bus to debug.	To debug all busses, set this to
		       -1.

     CAM_DEBUG_TARGET  Specify a target to debug.  To  debug  all  targets,  set
		       this to -1.

     CAM_DEBUG_LUN     Specify	a  lun to debug.  To debug all luns, set this to
		       -1.

     When specifying a bus, target or lun to debug, you MUST specify  all  three
     bus/target/lun  options  above.  Using wildcards, you should be able to en-
     able debugging on most anything.

     Users may also enable debugging printfs on the fly, if the CAMDEBUG  option
     is  their	config	file,  by  using the camcontrol(8) utility.  See camcon-
     trol(8) for details.

SEE ALSO
     ada(4), aha(4), ahb(4),  ahc(4),  ahci(4),  ata(4),  bt(4),  cd(4),  ch(4),
     da(4), pass(4), pt(4), sa(4), xpt(4), camcontrol(8)

HISTORY
     The  CAM SCSI subsystem first appeared in FreeBSD 3.0.  The CAM ATA support
     was added in FreeBSD 8.0.

AUTHORS
     The CAM SCSI subsystem was written by Justin Gibbs and Kenneth Merry.   The
     CAM ATA support was added by Alexander Motin <mav@FreeBSD.org>.

FreeBSD 8.1			  March 4, 2010 			  CAM(4)
home | help