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SCSI(4)                FreeBSD Kernel Interfaces Manual                SCSI(4)

     SCSI, CAM - CAM SCSI subsystem

     device scbus
     device scbus1 at ahc0
     device scbus3 at ahc1 bus 0
     device scbus2 at ahc1 bus 1
     device cd
     device ch
     device da
     device pass
     device pt
     device sa
     device ch1 at scbus0 target 4 unit 0
     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 SCSI subsystem provides a uniform and modular system for the
     implementation of drivers to control various SCSI devices, and to utilize
     different SCSI host adapters through host adapter drivers.  When the
     system probes the SCSI 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 SCSI devices.

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

     CAMDEBUG               This option enables the CAM debugging printf code.
                            This won't actually cause any debugging
                            information to be printed out when included by
                            itself.  Enabling 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
                            (and the only command currently tagged as "high
                            power") is the SCSI START UNIT command.  Starting
                            a SCSI disk often takes significantly more
                            electrical power than normal operation of the
                            disk.  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.  Don't 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 isn't
                            recommended, since it slows debugging of SCSI

     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 isn't 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 the SCSI busses support boot time allocation so that an
     upper number of devices and controllers does not need to be configured;
     device da0 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.

     To configure a driver in the kernel without wiring down the device use a
     config line similar to device ch0 to include the changer driver.

     To wire down a unit use a config line similar to device ch1 at scbus0
     target 4 unit 0 to assign changer 1 as the changer with SCSI ID 4, SCSI
     logical unit 0 on SCSI bus 0.  Individual scbuses can be wired down to
     specific controllers with a config line similar to device scbus0 at ahc0
     which assigns scsi bus 0 to the first unit using the ahc driver.  For
     controllers supporting more than one bus, the particular bus can be
     specified as in device scbus3 at ahc1 bus 1 which assigns scbus 1 to the
     second bus probed on the ahc1 device.

     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 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
     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 scsi 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.  Because of that, they
     aren't especially useful.  There aren't many things logged at the
     CAM_DEBUG_INFO level, so it isn't 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

     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
                       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
     enable 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
     camcontrol(8) for details.

     aha(4), ahb(4), ahc(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 SCSI subsystem was written by Justin Gibbs and Kenneth Merry.

FreeBSD 11.0-PRERELEASE        October 15, 1998        FreeBSD 11.0-PRERELEASE


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