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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>.
Debian June 7, 2012 CAM(4)