The main topics covered in this overview are:
This section frequently refers to both a SCSI device driver and a SCSI adapter device driver. These two distinct device drivers work together in a layered approach to support attachment of a range of SCSI devices. The SCSI adapter device driver is the lower device driver of the pair, and the SCSI device driver is the upper device driver.
The SCSI adapter device driver (the lower layer) is the software interface to the system hardware. This hardware includes the SCSI bus hardware plus any other system I/O hardware required to run an I/O request. The SCSI adapter device driver hides the details of the I/O hardware from the SCSI device driver. The design of the software interface allows a user with limited knowledge of the system hardware to write the upper device driver.
The SCSI adapter device driver manages the SCSI bus but not the SCSI devices. It can send and receive SCSI commands, but it cannot interpret the contents of the command. The lower driver also provides recovery and logging for errors related to the SCSI bus and system I/O hardware. Management of the device specifics is left to the SCSI device driver. The interface of the two drivers allows the upper driver to communicate with different SCSI bus adapters without requiring special code paths for each adapter.
The SCSI device driver (the upper layer) provides the rest of the operating system with the software interface to a given SCSI device or device class. The upper layer recognizes which SCSI commands are required to control a particular SCSI device or device class. The SCSI device driver builds I/O requests containing device SCSI commands and sends them to the SCSI adapter device driver in the sequence needed to operate the device successfully. The SCSI device driver cannot manage adapter resources or give the SCSI command to the adapter. Specifics about the adapter and system hardware are left to the lower layer.
The SCSI device driver also provides recovery and logging for errors related to the SCSI device it controls.
The operating system provides several kernel services allowing the SCSI device driver to communicate with SCSI adapter device driver entry points without having the actual name or address of those entry points. The description contained in "Logical File System Kernel Services" can provide more information.
When two SCSI devices communicate, one assumes the initiator-mode role, and the other assumes the target-mode role. The initiator-mode device generates the SCSI command, which requests an operation, and the target-mode device receives the SCSI command and acts. It is possible for a SCSI device to perform both roles simultaneously.
When writing a new SCSI adapter device driver, the writer must know which mode or modes must be supported to meet the requirements of the SCSI adapter and any interfaced SCSI device drivers. When a SCSI adapter device driver is added so that a new SCSI adapter works with all existing SCSI device drivers, both initiator-mode and target-mode must be supported in the SCSI adapter device driver.
The interface between the SCSI device driver and the SCSI adapter device driver for initiator-mode support (that is, the attached device acts as a target) is accessed through calls to the SCSI adapter device driver open, close, ioctl, and strategy routines. I/O requests are queued to the SCSI adapter device driver through calls to its strategy entry point.
Communication between the SCSI device driver and the SCSI adapter device driver for a particular initiator I/O request is made through the sc_buf structure, which is passed to and from the strategy routine in the same way a standard driver uses a struct buf structure.
The interface between the SCSI device driver and the SCSI adapter device driver for target-mode support (that is, the attached device acts as an initiator) is accessed through calls to the SCSI adapter device driver open, close, and ioctl subroutines. Buffers that contain data received from an attached initiator device are passed from the SCSI adapter device driver to the SCSI device driver, and back again, in tm_buf structures.
Communication between the SCSI adapter device driver and the SCSI device driver for a particular data transfer is made by passing the tm_buf structures by pointer directly to routines whose entry points have been previously registered. This registration occurs as part of the sequence of commands the SCSI device driver executes using calls to the SCSI adapter device driver when the device driver opens a target-mode device instance.
SCSI Target-Mode Overview.
Understanding Target-Mode Data Pacing.
Understanding the SCSI Target-Mode Device Driver Receive Buffer Routine.
Logical File System Kernel Services.
Required SCSI Adapter Device Driver ioctl Commands.
Understanding the Execution of Initiator I/O Requests.
SCSI Error Recovery.
Understanding the sc_buf Structure.