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CAM(4)			 BSD Kernel Interfaces Manual			CAM(4)

     CAM -- Common Access Method SCSI/ATA subsystem

     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_MAX_HIGHPOWER=4
     options SCSI_NO_OP_STRINGS
     options SCSI_DELAY=8000

     The CAM subsystem provides	a uniform and modular system for the implemen-
     tation 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 appro-
     priate drivers.  The pass(4) driver, if it	is configured in the kernel,
     will attach to all	devices.

     There are a number	of generic kernel configuration	options	for the	CAM

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

     CAM_MAX_HIGHPOWER=4    This sets the maximum allowable number of concur-
			    rent "high power" commands.	 A "high power"	com-
			    mand 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 significantly	more electrical	power than
			    normal operation.  This option allows the user to
			    specify how	many concurrent	high power commands
			    may	be outstanding without overloading the power
			    supply on his computer.

     SCSI_NO_SENSE_STRINGS  This eliminates text descriptions of each SCSI Ad-
			    ditional Sense Code	and Additional Sense Code
			    Qualifier pair.  Since this	is a fairly large text
			    database, eliminating it reduces the size of the
			    kernel somewhat.  This is primarily	necessary for
			    boot floppies and other low	disk space or low mem-
			    ory	space environments.  In	most cases, though,
			    this should	be enabled, since it speeds the	inter-
			    pretation of SCSI error messages.  Do not let the
			    "kernel bloat" zealots get to you -- leave the
			    sense descriptions in your kernel!

     SCSI_NO_OP_STRINGS	    This disables text descriptions of each SCSI op-
			    code.  This	option,	like the sense string option
			    above, is primarily	useful for environments	like a
			    boot floppy	where kernel size is critical.	En-
			    abling this	option for normal use is not recom-
			    mended, 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
			    device to reset itself to a	default	set of trans-
			    fer	negotiations and other settings.  Most SCSI
			    devices need some amount of	time to	recover	from a
			    bus	reset.	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 seconds.  The	minimum	allowable
			    value for SCSI_DELAY is "100", or 100ms.  One spe-
			    cial 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 re-
			    set	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	usu-
     ally done either interactively from the loader(8),	or automatically via
     the /boot/device.hints file.  The basic syntax is:"value"

     Individual	CAM bus	numbers	can be wired down to specific controllers with
     a config line similar to the following:"ahd1"

     This assigns CAM bus number 0 to the ahd1 driver instance.	 For con-
     trollers supporting more than one bus, a particular bus can be assigned
     as	follows:"ahc1"

     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:"scbus0""0"

     This assigns da0 to target	0, unit	(lun) 0	of scbus 0.  Omitting the tar-
     get or unit hints will instruct CAM to treat them as wildcards and	use
     the first respective counted instances.  These examples can be combined
     together 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	count-
     ing 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.

     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.

     Some adapters support target mode in which	the system is capable of oper-
     ating 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.

     see other CAM device entries.

     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
     device.  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
			 entrance 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

     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
     debugging flag is the CAM_DEBUG_CDB flag.	Users can enable debugging
     from their	kernel config file, by using the following kernel config op-

     CAMDEBUG	       This enables CAM	debugging.  Without this option, users
		       will not	even be	able to	turn on	debugging from user-
		       land 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
		       multiple	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

     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 op-
     tion is their config file,	by using the camcontrol(8) utility.  See
     camcontrol(8) for details.

     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)

     The CAM SCSI subsystem first appeared in FreeBSD 3.0.  The	CAM ATA	sup-
     port was added in FreeBSD 8.0.

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

BSD				 March 4, 2010				   BSD


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