CAM(4) BSD Kernel Interfaces Manual CAM(4)
CAM -- Common Access Method SCSI/ATA subsystem
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.
There are a number of generic kernel configuration options for the CAM subsystem:
CAMDEBUG This option enables the CAM debugging printf code. This will not 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 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 nec-
essary 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 spe-
cial 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:
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
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:
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 particu-
lar 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.
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.
Some adapters support target mode in which the system is capable of operating as a device, responding to operations initiated by another sys-
tem. 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.
When the kernel is compiled with options CAMDEBUG, an XPT_DEBUG CCB can be used to enable various amounts of tracing information on any spe-
cific 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, and because of that,
they are not especially useful. There are not many things logged at the CAM_DEBUG_INFO level, so it is not especially useful. The most use-
ful 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
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.
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)
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 <mav@FreeBSD.org>.
March 4, 2010 BSD