CAM(4) FreeBSD Kernel Interfaces Manual CAM(4)
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, NVMe, 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)
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:
CAM_BOOT_DELAY Additional time to wait after the static parts of
the kernel have run to allow for discovery of
additional devices which may take time to connect,
such as USB attached storage.
CAM_IOSCHED_DYNAMIC Enable dynamic decisions in the I/O scheduler
based on hints and the current performance of the
storage devices.
CAM_IO_STATS Enable collection of statistics for periph
devices.
CAM_TEST_FAILURE Enable ability to simulate I/O failures.
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), 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) 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)
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.
ARCHITECTURE
The CAM subsystem glues together the upper layers of the system to the
storage devices. PERIPH devices accept storage requests from GEOM and
other upper layers of the system and translates them into protocol
requests. XPT (transport) dispatches these protocol requests to a SIM
driver. A SIM driver takes protocol requests and translates them into
hardware commands the host adapter understands to transfer the protocol
requests, and data (if any) to the storage device. The CCB transports
these requests around as messages.
CAM
The Common Access Method was a standard defined in the 1990s to talk to
disk drives. FreeBSD is one of the few operating systems to fully
implement this model. The interface between different parts of CAM is
the CCB (or CAM Control Block). Each CCB has a standard header, which
contains the type of request and dispatch information, and a command
specific portion. A CAM Periph generates requests. The XPT layer
dispatches these requests to the appropriate SIM. Some CCBs are sent
directly to the SIM for immediate processing, while others are queued and
complete when the I/O has finished. A SIM takes CCBs and translates them
into hardware specific commands to push the SCSI CDB or other protocol
control block to the peripheral, along with setting up the DMA for the
associated data.
Periph Devices
A periph driver knows how to translate standard requests into protocol
messages that a SIM can deliver to hardware. These requests can come
from any upper layer source, but primarily come in via GEOM as a bio
request. They can also come in directly from character device requests
for tapes and pass through commands.
Disk devices, or direct access (da) in CAM, are one type of peripheral.
These devices present themselves to the kernel a device ending in "da".
Each protocol has a unique device name:
da(4)
SCSI or SAS device, or devices that accept SCSI CDBs for I/O.
ada(4)
ATA or SATA device
nda(4)
NVME device
sdda(4)
An SD or MMC block storage device.
Tape devices are called serial access (sa(4)) in CAM. They interface to
the system via a character device and provide ioctl(2) control for tape
drives.
The pass(4) device will pass through CCB requests from userland to the
SIM directly. The device is used to send commands other than read,
write, trim or flush to a device. The camcontrol(8) command uses this
device.
XPT drivers
The transport driver connects the periph to the SIM. It is not
configured separately. It is also responsible for device discovery for
those SIM drivers that do not enumerate themselves.
SIM driver
SIM used to stand for SCSI Interface Module. Now it is just SIM because
it understands protocols other than SCSI. There are two types of SIM
drivers: virtual and physical. Physical SIMs are typically called host
bus adapters (HBA), but not universally. Virtual SIM drivers are for
communicating with virtual machine hosts.
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 specifies 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 sets 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)
utility, if wanted options built into the kernel. See camcontrol(8) for
details.
SEE ALSO
Commands:
camcontrol(8), camdd(8)
Libraries:
cam(3)
Periph Drivers:
ada(4), da(4), nda(4), pass(4), sa(4)
SIM Devices:
aac(4), aacraid(4), ahc(4), ahci(4), ata(4), aw_mmc(4), ciss(4),
hv_storvsc(4), isci(4), iscsi(4), isp(4), mpr(4), mps(4), mpt(4),
mrsas(4), mvs(4), nvme(4), pms(4), pvscsi(4), sdhci(4), smartpqi(4),
sym(4), tws(4), umass(4), virtio_scsi(4)
Deprecated or Poorly Supported SIM Devices:
ahd(4), amr(4), arcmsr(4), esp(4), hpt27xx(4), hptiop(4), hptmv(4),
hptnr(4), iir(4) mfi(4), sbp(4), twa(4)
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>.
FreeBSD 13.1-RELEASE-p6 June 18, 2020 FreeBSD 13.1-RELEASE-p6
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