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Manual page for SG(4)

sg - generic SCSI device driver

SYNOPSIS

Generic SCSI Device as sg[0-3] at sc0

DESCRIPTION

This driver allows a user to pass SCSI commands to any arbitrary device on the SCSI bus. The user specifies an arbitrary SCSI command descriptor block (CDB), an address at which DMA is to occur to or from, a maximum allowed DMA size, and a maximum I/O time in seconds (after which the I/O will timeout and terminate). A status block is returned by the driver, indicating such things as I/O status (e.g., success, bus timeout, bad SCSI status, etc.), the SCSI status byte, the number of bytes transferred in case of Data In/Data Out phases, and so on.

The basic mechanism for performing I/O through the sg driver is as follows:

1. Open /dev/sgn for read/write access.

2. Bind the sg driver to a particular SCSI target and LUN using the following call:


ioctl(fd,SGIOCSTL,&sa)

Where fd is a file descriptor obtained by opening /dev/sgn and sa is a struct scsi_adr (defined in <bsd/dev/scsireg.h>, as are all other structs and constants used by this interface) which contains target and LUN data.

3. To execute a command on the SCSI bus, execute the following call:


ioctl(fd,SGIOCREQ,srp)

Where SGIOCREQ defines the ioctl operation and srp is a pointer to a struct scsi_req.

Steps 2 and/or 3 can be repeated as many times as necessary.

Struct scsi_req contains both the input to the sg driver required for execution of a command (SCSI CDB bytes, address at which DMA will occur, etc.) as well as the status block upon return. The following status is provided:

srp->sr_io_status: Indicates success, I/O timeout, SCSI check status, DMA overrun, etc.
srp->sr_scsi_status: The actual SCSI status byte returned by the target.
srp->sr_esense: Extended sense data, if pertinent (see description of autosense in "Other ioctl calls", below).
srp->sr_dma_xfr: Number of data bytes actually transferred to/from device.
srp->sr_exec_time: Command execution time, with microsecond resolution.

OTHER IOCTL CALLS

SGIOCENAS / SGIOCDAS: There is an optional feature called autosense which can be enabled and disabled by ioctl(SGIOCENAS) and ioctl(SGIOCDAS), respectively. When autosense is enabled, the sg driver will perform a Request Sense command whenever a Check Status is detected upon completion of execution of a user-specified command. If the Request Sense command is successful, the sense data will be in scsi_req.sr_esense. The autosense mode is disabled by default.
SGIOCRST: This causes a SCSI bus reset to occur.

The following restrictions are placed on using the sg driver:

1. You cannot bind the sg driver to a target/lun which is already in use by any SCSI driver unless you are superuser. This is an extremely hazardous operation.

2. You also must be superuser to reset the SCSI bus (via ioctl(SGIOCRST)).

FILES

<bsd/dev/scsireg.h>
/dev/sgn

DIAGNOSTICS

All errors encountered during the processing of SCSI commands by the sg driver are reported by scsi_req.sr_io_status. See <bsd/dev/scsireg.h> for constants used in this field. Note that this means that a SCSI error (like Selection timeout) which is detected by the driver would NOT be reported to the user via EIO or any other errno.