Common Access Method SCSI/ATA subsystem
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
- This option compiles in all the
debugging printf code. This will not actually cause any debugging
information to be printed out when included by itself. See below for
- 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.
- 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!
- 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.
- 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;
will suffice for any number of
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)
automatically via the /boot/device.hints
file. The basic syntax is:
bus numbers can be wired down
to specific controllers with a config line similar to the following:
bus number 0 to the
driver instance. For controllers supporting
more than one bus, a particular bus can be assigned as follows:
bus 0 to the bus 1 instance
. 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
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
the first non-wired disk shall come on line as
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 system. Target mode is supported for some adapters, but is not yet
complete for this version of the
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:
- This flag enables general informational printfs for the device or devices
- This flag enables function-level command flow tracing. i.e. kernel printfs
will happen at the entrance and exit of various functions.
- This flag enables debugging output internal to various functions.
- This flag will cause the kernel to print out all ATA and SCSI commands
sent to a particular device or devices.
- This flag will enable command scheduler tracing.
- This flag will enable peripheral drivers messages.
- This flag will enable devices probe process tracing.
Some of these flags, most notably
, will produce kernel
printfs in EXTREME numbers.
Users can enable debugging from their kernel config file, by using the following
kernel config options:
- This builds into the kernel all possible
- 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.
- This allows to set the various debugging flags from a kernel config
- Specify a bus to debug. To debug all busses, set this to -1.
- Specify a target to debug. To debug all targets, set this to -1.
- Specify a lun to debug. To debug all luns, set this to -1.
Users may also enable debugging on the fly by using the
utility, if wanted options built
into the kernel. See camcontrol(8)
SCSI subsystem first appeared in
ATA support was added in
SCSI subsystem was written by
ATA support was added by