--- name: "CAM" section: "4freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/CAM/4freebsd" --- # CAM(4freebsd) Common Access Method Storage subsystem ## NAME **CAM** – Common Access Method Storage 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, ATA, NMVe, and MMC / SD devices, and to utilize different SCSI, ATA, NVMe, and MMC / SD host adapters through host adapter drivers. When the system probes buses, 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 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 buses 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)](/loader/8.md), 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 *ahc1*. 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. This also works with [nvme(4)](/nvme/4.md) drives as well. ``` hint.nvme.4.at="pci7:0:0" hint.scbus.10.at="nvme4" hint.nda.10.at="scbus10" hint.nda.10.target="1" hint.nda.10.unit="12" hint.nda.11.at="scbus10" hint.nda.11.target="1" hint.nda.11.unit="2" ``` This assigns the NVMe card living at PCI bus 7 to scbus 10 (in PCIe, slot and function are rarely used and usually 0). The target for [nda(4)](/nda/4.md) devices is always 1. The unit is the namespace identifier from the drive. The namespace id 1 is exported as nda10 and namespace id 2 is exported as nda11. 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, ATA, NVMe, or MMC / SD 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 buses, 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)](/camcontrol/8.md) utility, if wanted options built into the kernel. See [camcontrol(8)](/camcontrol/8.md) for details. ## SEE ALSO [ada(4)](/ada/4.md), [aha(4)](/aha/4.md), [ahc(4)](/ahc/4.md), [ahci(4)](/ahci/4.md), [ahd(4)](/ahd/4.md), [ata(4)](/ata/4.md), [bt(4)](/bt/4.md), [cd(4)](/cd/4.md), [ch(4)](/ch/4.md), [da(4)](/da/4.md), [nda(4)](/nda/4.md), [nvme(4)](/nvme/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. 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*⟩. The **CAM** NVMe support was added by Warner Losh ⟨*imp@FreeBSD.org*⟩.