--- name: "SCSI" section: "4" platform: "linux" package: "freebsd-manpages" version: "6.0-5" release: "Debian 4.0 (Etch)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/SCSI" --- # SCSI(4) CAM SCSI subsystem ## NAME **SCSI**, **CAM** – CAM SCSI subsystem ## SYNOPSIS **device scbus** **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\_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)](/pass/4.md) 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 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 (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. 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 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. Units are wired down by setting kernel environment hints. This is usually done either interactively from the [loader(8)](/loader/8.md), or automatically via the */boot/device.hints* file. The basic syntax is: ``` hint.device.unit.property="value" ``` Individual **SCSI** bus numbers can be wired down to specific controllers with a config line similar to the following: ``` hint.scbus.0.at="ahd1" ``` This assigns **SCSI** 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 **SCSI** 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* 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, 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 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)](/camcontrol/8.md). - 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)](/camcontrol/8.md) utility. See [camcontrol(8)](/camcontrol/8.md) for details. ## SEE ALSO [aha(4)](/aha/4.md), [ahb(4)](/ahb/4.md), [ahc(4)](/ahc/4.md), [bt(4)](/bt/4.md), [cd(4)](/cd/4.md), [ch(4)](/ch/4.md), [da(4)](/da/4.md), [pass(4)](/pass/4.md), [pt(4)](/pt/4.md), [sa(4)](/sa/4.md), [xpt(4)](/xpt/4.md), [camcontrol(8)](/camcontrol/8.md) ## HISTORY The CAM SCSI subsystem first appeared in FreeBSD 3.0. ## AUTHORS The CAM SCSI subsystem was written by Justin Gibbs and Kenneth Merry.