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32 .Nd Common Access Method Storage subsystem
42 .Cd "options CAMDEBUG"
43 .Cd "options CAM_DEBUG_BUS=-1"
44 .Cd "options CAM_DEBUG_TARGET=-1"
45 .Cd "options CAM_DEBUG_LUN=-1"
46 .Cd "options CAM_DEBUG_COMPILE=CAM_DEBUG_INFO|CAM_DEBUG_CDB|CAM_DEBUG_PROBE"
47 .Cd "options CAM_DEBUG_FLAGS=CAM_DEBUG_INFO|CAM_DEBUG_CDB"
48 .Cd "options CAM_MAX_HIGHPOWER=4"
49 .Cd "options SCSI_NO_SENSE_STRINGS"
50 .Cd "options SCSI_NO_OP_STRINGS"
51 .Cd "options SCSI_DELAY=8000"
55 subsystem provides a uniform and modular system for the implementation
56 of drivers to control various
62 devices, and to utilize different
68 host adapters through host adapter drivers.
69 When the system probes buses, it attaches any devices it finds to the
73 driver, if it is configured in the kernel, will attach to all devices.
74 .Sh KERNEL CONFIGURATION
75 There are a number of generic kernel configuration options for the
78 .Bl -tag -width SCSI_NO_SENSE_STRINGS
80 Additional time to wait after the static parts of the kernel have run to allow
81 for discovery of additional devices which may take time to connect,
82 such as USB attached storage.
83 .It Dv CAM_IOSCHED_DYNAMIC
84 Enable dynamic decisions in the I/O scheduler based on hints and the current
85 performance of the storage devices.
87 Enable collection of statistics for periph devices.
88 .It Dv CAM_TEST_FAILURE
89 Enable ability to simulate I/O failures.
91 This option compiles in all the
93 debugging printf code.
94 This will not actually
95 cause any debugging information to be printed out when included by itself.
96 See below for details.
97 .It Dv "CAM_MAX_HIGHPOWER=4"
98 This sets the maximum allowable number of concurrent "high power" commands.
99 A "high power" command is a command that takes more electrical power than
101 An example of this is the
104 Starting a disk often takes significantly more electrical power than normal
106 This option allows the
107 user to specify how many concurrent high power commands may be outstanding
108 without overloading the power supply on his computer.
109 .It Dv SCSI_NO_SENSE_STRINGS
110 This eliminates text descriptions of each
112 Additional Sense Code and Additional Sense Code Qualifier pair.
114 is a fairly large text database, eliminating it reduces the size of the
116 This is primarily necessary for boot floppies and other
117 low disk space or low memory space environments.
118 In most cases, though,
119 this should be enabled, since it speeds the interpretation of
122 Do not let the "kernel bloat" zealots get to you -- leave
123 the sense descriptions in your kernel!
124 .It Dv SCSI_NO_OP_STRINGS
125 This disables text descriptions of each
128 This option, like the sense string option above, is primarily
129 useful for environments like a boot floppy where kernel size is critical.
130 Enabling this option for normal use is not recommended, since it slows
134 .It Dv SCSI_DELAY=8000
142 not seconds like the old
145 When the kernel boots, it sends a bus reset to each
147 bus to tell each device to reset itself to a default set of transfer
148 negotiations and other settings.
151 devices need some amount of time to recover from a bus reset.
153 may need as little as 100ms, while old, slow devices may need much longer.
156 is not specified, it defaults to 2 seconds.
157 The minimum allowable value for
160 One special case is that if the
162 is set to 0, that will be taken to mean the "lowest possible value."
165 will be reset to 100ms.
168 All devices and buses support dynamic allocation so that
169 an upper number of devices and controllers does not need to be configured;
171 will suffice for any number of disk drivers.
173 The devices are either
175 so they appear as a particular device unit or
177 so that they appear as the next available unused unit.
179 Units are wired down by setting kernel environment hints.
180 This is usually done either interactively from the
182 or automatically via the
183 .Pa /boot/device.hints
186 .Bd -literal -offset indent
187 hint.device.unit.property="value"
192 bus numbers can be wired down to specific controllers with
193 a config line similar to the following:
194 .Bd -literal -offset indent
195 hint.scbus.0.at="ahd1"
203 For controllers supporting more than one bus, a particular bus can be assigned
205 .Bd -literal -offset indent
206 hint.scbus.0.at="ahc1"
212 bus 0 to the bus 1 instance on
214 Peripheral drivers can be wired to a specific bus, target, and lun as so:
215 .Bd -literal -offset indent
216 hint.da.0.at="scbus0"
223 to target 0, unit (lun) 0 of scbus 0.
224 Omitting the target or unit hints will instruct
226 to treat them as wildcards
227 and use the first respective counted instances.
228 These examples can be combined together to allow a peripheral device to be
229 wired to any particular controller, bus, target, and/or unit instance.
234 .Bd -literal -offset indent
235 hint.nvme.4.at="pci7:0:0"
236 hint.scbus.10.at="nvme4"
237 hint.nda.10.at="scbus10"
238 hint.nda.10.target="1"
239 hint.nda.10.unit="12"
240 hint.nda.11.at="scbus10"
241 hint.nda.11.target="1"
245 This assigns the NVMe card living at PCI bus 7 to scbus 10 (in PCIe,
246 slot and function are rarely used and usually 0).
250 The unit is the namespace identifier from the drive.
251 The namespace id 1 is exported as
253 and namespace id 2 is exported as
256 When you have a mixture of wired down and counted devices then the
257 counting begins with the first non-wired down unit for a particular
259 That is, if you have a disk wired down as
261 then the first non-wired disk shall come on line as
264 The system allows common device drivers to work through many different
266 The adapters take requests from the upper layers and do
274 The maximum size of a transfer is governed by the
276 Most adapters can transfer 64KB in a single operation, however
277 many can transfer larger amounts.
279 Some adapters support
281 in which the system is capable of operating as a device, responding to
282 operations initiated by another system.
283 Target mode is supported for
284 some adapters, but is not yet complete for this version of the
291 subsystem glues together the upper layers of the system to the storage devices.
292 PERIPH devices accept storage requests from GEOM and other upper layers of the
293 system and translates them into protocol requests.
294 XPT (transport) dispatches these protocol requests to a SIM driver.
295 A SIM driver takes protocol requests and translates them into hardware commands
296 the host adapter understands to transfer the protocol requests, and data (if
297 any) to the storage device.
298 The CCB transports these requests around as messages.
300 The Common Access Method was a standard defined in the 1990s to talk to disk
303 is one of the few operating systems to fully implement this model.
304 The interface between different parts of CAM is the CCB (or CAM Control Block).
305 Each CCB has a standard header, which contains the type of request and dispatch
306 information, and a command specific portion.
307 A CAM Periph generates requests.
308 The XPT layer dispatches these requests to the appropriate SIM.
309 Some CCBs are sent directly to the SIM for immediate processing, while others
310 are queued and complete when the I/O has finished.
311 A SIM takes CCBs and translates them into hardware specific commands to push the
312 SCSI CDB or other protocol control block to the peripheral, along with setting
313 up the DMA for the associated data.
315 A periph driver knows how to translate standard requests into protocol messages
316 that a SIM can deliver to hardware.
317 These requests can come from any upper layer source, but primarily come in via
318 GEOM as a bio request.
319 They can also come in directly from character device requests for tapes and pass
322 Disk devices, or direct access (da) in CAM, are one type of peripheral.
323 These devices present themselves to the kernel a device ending in
325 Each protocol has a unique device name:
328 SCSI or SAS device, or devices that accept SCSI CDBs for I/O.
334 An SD or MMC block storage device.
337 Tape devices are called serial access
342 They interface to the system via a character device and provide
344 control for tape drives.
348 device will pass through CCB requests from userland to the SIM directly.
349 The device is used to send commands other than read, write, trim or flush to a
353 command uses this device.
355 The transport driver connects the periph to the SIM.
356 It is not configured separately.
357 It is also responsible for device discovery for those SIM drivers that do not
358 enumerate themselves.
360 SIM used to stand for SCSI Interface Module.
361 Now it is just SIM because it understands protocols other than SCSI.
362 There are two types of SIM drivers: virtual and physical.
363 Physical SIMs are typically called host bus adapters (HBA), but not universally.
364 Virtual SIM drivers are for communicating with virtual machine hosts.
370 An XPT_DEBUG CCB can be used to enable various amounts of tracing information
371 on any specific bus/device from the list of options compiled into the kernel.
372 There are currently seven debugging flags that may be compiled in and used:
373 .Bl -tag -width CAM_DEBUG_SUBTRACE
374 .It Dv CAM_DEBUG_INFO
375 This flag enables general informational printfs for the device
376 or devices in question.
377 .It Dv CAM_DEBUG_TRACE
378 This flag enables function-level command flow tracing i.e.,
379 kernel printfs will happen at the entrance and exit of various functions.
380 .It Dv CAM_DEBUG_SUBTRACE
381 This flag enables debugging output internal to various functions.
383 This flag will cause the kernel to print out all
387 commands sent to a particular device or devices.
389 This flag will enable command scheduler tracing.
390 .It Dv CAM_DEBUG_PERIPH
391 This flag will enable peripheral drivers messages.
392 .It Dv CAM_DEBUG_PROBE
393 This flag will enable devices probe process tracing.
396 Some of these flags, most notably
399 .Dv CAM_DEBUG_SUBTRACE ,
400 will produce kernel printfs in EXTREME numbers.
402 Users can enable debugging from their kernel config file, by using
403 the following kernel config options:
404 .Bl -tag -width CAM_DEBUG_COMPILE
406 This builds into the kernel all possible
409 .It Dv CAM_DEBUG_COMPILE
410 This specifies support for which debugging flags described above
411 should be built into the kernel.
412 Flags may be ORed together if the user wishes to
413 see printfs for multiple debugging levels.
414 .It Dv CAM_DEBUG_FLAGS
415 This sets the various debugging flags from a kernel config file.
417 Specify a bus to debug.
418 To debug all buses, set this to -1.
419 .It Dv CAM_DEBUG_TARGET
420 Specify a target to debug.
421 To debug all targets, set this to -1.
423 Specify a lun to debug.
424 To debug all luns, set this to -1.
427 Users may also enable debugging on the fly by using the
429 utility, if wanted options built into the kernel.
440 .It Sy Periph Drivers:
475 .It Sy Deprecated or Poorly Supported SIM Devices:
497 subsystem first appeared in
501 ATA support was added in
508 subsystem was written by
516 .An Alexander Motin Aq Mt mav@FreeBSD.org .
521 .An Warner Losh Aq Mt imp@FreeBSD.org .