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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 system. 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 probably 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 notification 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 information.
FILES
/dev/passn Character device nodes for the pass driver. There should be one of these for each device
accessed through the CAM subsystem.
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>
Debian May 3, 2017 PASS(4)