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CAM(4) Device Drivers Manual CAM(4)


CAMCommon 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_LUN=-1
options SCSI_DELAY=8000


The CAM subsystem provides a uniform and modular system for the implementation 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 appropriate 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 subsystem:

This option compiles in all the CAM debugging printf code. This will not actually cause any debugging information to be printed out when included by itself. See below for details.
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 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.
This eliminates text descriptions of each SCSI Additional 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 memory space environments. In most cases, though, this should be enabled, since it speeds the interpretation of SCSI error messages. Do not let the "kernel bloat" zealots get to you -- leave the sense descriptions in your kernel!
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.
This is the SCSI "bus settle delay." In CAM, it is specified in , 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 transfer 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 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 so they appear as a particular device unit or 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:"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 driver instance. For controllers 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 . Peripheral drivers can be wired to a specific bus, target, and lun as so:"scbus0""0"

This assigns 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 together to allow a peripheral device to be wired to any particular controller, 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 , then the first non-wired disk shall come on line as .


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 in which the system is capable of operating 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.


An XPT_DEBUG CCB can be used to enable various amounts of tracing information on any specific bus/device from the list of options compiled into the kernel. There are currently seven debugging flags that may be compiled in and used:

This flag enables general informational printfs for the device or devices in question.
This flag enables function-level command flow tracing. i.e. kernel printfs will happen at the entrance and exit of various functions.
This flag enables debugging output internal to various functions.
This flag will cause the kernel to print out all ATA and SCSI commands sent to a particular device or devices.
This flag will enable command scheduler tracing.
This flag will enable peripheral drivers messages.
This flag will enable devices probe process tracing.

Some of these flags, most notably CAM_DEBUG_TRACE and CAM_DEBUG_SUBTRACE, will produce kernel printfs in EXTREME numbers.

Users can enable debugging from their kernel config file, by using the following kernel config options:

This builds into the kernel all possible CAM debugging.
This allows to specify support for which debugging flags described above should be built into the kernel. Flags may be ORed together if the user wishes to see printfs for multiple debugging levels.
This allows to set the various debugging flags from a kernel config file.
Specify a bus to debug. To debug all busses, set this to -1.
Specify a target to debug. To debug all targets, set this to -1.
Specify a lun to debug. To debug all luns, set this to -1.

Users may also enable debugging on the fly by using the camcontrol(8) utility, if wanted options built into the kernel. 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 support 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 <>.

June 7, 2012 Debian