SCSI(4)

NAME
SCSI, CAM - CAM SCSI subsystem

SYNOPSIS
device scbus
device scbus1 at ahci0
device scbus3 at ahci1 bus 0
device scbus2 at ahci1 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_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 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.

KERNEL CONFIGURATION
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 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 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 ahci0 which assigns scsi bus 0 to the first unit using the ahci driver. For controllers supporting more than one bus, the particular bus can be specified as in device scbus3 at ahci1 bus 1 which assigns scbus 1 to the second bus probed on the ahci1 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.

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 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 scsi device entries.

DIAGNOSTICS
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 devices.

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 options:

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 -1.

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.

SEE ALSO
ahci(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.

AUTHORS
-nosplit The CAM SCSI subsystem was written by Justin Gibbs and Kenneth Merry.