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     CAM — Common Access Method Storage 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 implementation of drivers to
     control various SCSI, ATA, NMVe, and MMC / SD devices, and to utilize different SCSI, ATA,
     NVMe, and MMC / SD host adapters through host adapter drivers.  When the system probes
     buses, 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:

     CAMDEBUG               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.

     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 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 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!

     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
                            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

     All devices and buses 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

     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:


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


     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:


     This assigns CAM bus 0 to the bus 1 instance on ahc1.  Peripheral drivers can be wired to a
     specific bus, target, and lun as so:


     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 together to allow a peripheral device to be wired to any
     particular controller, bus, target, and/or unit instance.

     This also works with nvme(4) drives as well.


     This assigns the NVMe card living at PCI bus 7 to scbus 10 (in PCIe, slot and function are
     rarely used and usually 0).  The target for nda(4) devices is always 1.  The unit is the
     namespace identifier from the drive.  The namespace id 1 is exported as nda10 and namespace
     id 2 is exported as nda11.

     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.


     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, ATA, NVMe,
     or MMC / SD 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


     Some adapters support target mode 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:

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

     CAM_DEBUG_TRACE     This 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 flag enables debugging output internal to various functions.

     CAM_DEBUG_CDB       This flag will cause the kernel to print out all ATA and SCSI commands
                         sent to a particular device or devices.

     CAM_DEBUG_XPT       This flag will enable command scheduler tracing.

     CAM_DEBUG_PERIPH    This flag will enable peripheral drivers messages.

     CAM_DEBUG_PROBE     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:

     CAMDEBUG           This builds into the kernel all possible CAM debugging.

     CAM_DEBUG_COMPILE  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.

     CAM_DEBUG_FLAGS    This allows to set the various debugging flags from a kernel config file.

     CAM_DEBUG_BUS      Specify a bus to debug.  To debug all buses, 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.

     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), ahc(4), ahci(4), ahd(4), ata(4), bt(4), cd(4), ch(4), da(4), nda(4),
     nvme(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 <>.  The CAM NVMe support was added by Warner
     Losh <>.