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CAM(4)                 FreeBSD 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_COMPILE=CAM_DEBUG_INFO|CAM_DEBUG_CDB|CAM_DEBUG_PROBE
     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
     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.

KERNEL CONFIGURATION
     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 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
     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 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
     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.

FILES
     see other CAM device entries.

DIAGNOSTICS
     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 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.

     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.

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 11.0-PRERELEASE          June 7, 2012          FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | KERNEL CONFIGURATION | ADAPTERS | TARGET MODE | FILES | DIAGNOSTICS | SEE ALSO | HISTORY | AUTHORS

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