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pass(4)
PASS(4) BSD Kernel Interfaces Manual PASS(4)
NAME
pass — CAM application passthrough driver
SYNOPSIS
device pass
DESCRIPTION
The pass driver provides a way for userland applications to issue CAM
CCBs to the kernel.
Since the pass driver allows direct access to the CAM subsystem, system
administrators should exercise caution when granting access to this
driver. If used improperly, this driver can allow userland applications
to crash a machine or cause data loss.
The pass driver attaches to every SCSI and ATA device found in the sys‐
tem. Since it attaches to every device, it provides a generic means of
accessing SCSI and ATA devices, and allows the user to access devices
which have no "standard" peripheral driver associated with them.
KERNEL CONFIGURATION
It is only necessary to configure one pass device in the kernel; pass
devices are automatically allocated as SCSI and ATA devices are found.
IOCTLS
CAMIOCOMMAND union ccb *
This ioctl takes most kinds of CAM CCBs and passes them through to
the CAM transport layer for action. Note that some CCB types are
not allowed through the passthrough device, and must be sent
through the xpt(4) device instead. Some examples of xpt-only CCBs
are XPT_SCAN_BUS, XPT_DEV_MATCH, XPT_RESET_BUS, XPT_SCAN_LUN,
XPT_ENG_INQ, and XPT_ENG_EXEC. These CCB types have various
attributes that make it illogical or impossible to service them
through the passthrough interface.
If the user would like the kernel to do error recovery, the
CAM_PASS_ERR_RECOVER flag must be set on the CCB, and the
retry_count field set to the number of retries.
CAMGETPASSTHRU union ccb *
This ioctl takes an XPT_GDEVLIST CCB, and returns the passthrough
device corresponding to the device in question. Although this
ioctl is available through the pass driver, it is of limited use,
since the caller must already know that the device in question is
a passthrough device if they are issuing this ioctl. It is proba‐
bly more useful to issue this ioctl through the xpt(4) device.
CAMIOQUEUE union ccb *
Queue a CCB to the pass driver to be executed asynchronously. The
caller may use select(2), poll(2) or kevent(2) to receive notifi‐
cation when the CCB has completed.
This ioctl takes most CAM CCBs, but some CCB types are not allowed
through the pass device, and must be sent through the xpt(4)
device instead. Some examples of xpt-only CCBs are XPT_SCAN_BUS,
XPT_DEV_MATCH, XPT_RESET_BUS, XPT_SCAN_LUN, XPT_ENG_INQ, and
XPT_ENG_EXEC. These CCB types have various attributes that make
it illogical or impossible to service them through the passthrough
interface.
Although the CAMIOQUEUE ioctl is not defined to take an argument,
it does require a pointer to a union ccb. It is not defined to
take an argument to avoid an extra malloc and copy inside the
generic ioctl(2) handler.
The completed CCB will be returned via the CAMIOGET ioctl. An
error will only be returned from the CAMIOQUEUE ioctl if there is
an error allocating memory for the request or copying memory from
userland. All other errors will be reported as standard CAM CCB
status errors. Since the CCB is not copied back to the user
process from the pass driver in the CAMIOQUEUE ioctl, the user's
passed-in CCB will not be modfied. This is the case even with
immediate CCBs. Instead, the completed CCB must be retrieved via
the CAMIOGET ioctl and the status examined.
Multiple CCBs may be queued via the CAMIOQUEUE ioctl at any given
time, and they may complete in a different order than the order
that they were submitted. The caller must take steps to identify
CCBs that are queued and completed. The periph_priv structure
inside struct ccb_hdr is available for userland use with the
CAMIOQUEUE and CAMIOGET ioctls, and will be preserved across
calls. Also, the periph_links linked list pointers inside struct
ccb_hdr are available for userland use with the CAMIOQUEUE and
CAMIOGET ioctls and will be preserved across calls.
If the user would like the kernel to do error recovery, the
CAM_PASS_ERR_RECOVER flag must be set on the CCB, and the
retry_count field set to the number of retries.
CAMIOGET union ccb *
Retrieve completed CAM CCBs queued via the CAMIOQUEUE ioctl. An
error will only be returned from the CAMIOGET ioctl if the pass
driver fails to copy data to the user process or if there are no
completed CCBs available to retrieve. If no CCBs are available to
retrieve, errno will be set to ENOENT.
All other errors will be reported as standard CAM CCB status
errors.
Although the CAMIOGET ioctl is not defined to take an argument, it
does require a pointer to a union ccb. It is not defined to take
an argument to avoid an extra malloc and copy inside the generic
ioctl(2) handler.
The pass driver will report via select(2), poll(2) or kevent(2)
when a CCB has completed. One CCB may be retrieved per CAMIOGET
call. CCBs may be returned in an order different than the order
they were submitted. So the caller should use the periph_priv
area inside the CCB header to store pointers to identifying infor‐
mation.
FILES
/dev/passn Character device nodes for the pass driver. There should be
one of these for each device accessed through the CAM subsys‐
tem.
DIAGNOSTICS
None.
SEE ALSO
kqueue(2), poll(2), select(2), cam(3), cam_cdbparse(3), cam(4), cd(4),
ctl(4), da(4), sa(4), xpt(4), camcontrol(8), camdd(8)
HISTORY
The CAM passthrough driver first appeared in FreeBSD 3.0.
AUTHORS
Kenneth Merry <ken@FreeBSD.org>
BSD May 3, 2017 BSD