Generic SCSI Device Driver

Generic SCSI Device Driver

Generally, non-disk, non-tape SCSI devices are installed in the /dev/scsi directory. Devices so named are controlled by the generic SCSI device driver, which is documented in the ds(7m) reference page.

Unlike most kernel-level device drivers, the generic SCSI driver does not support interrupts, and does not support the read() and write() functions. Instead, it supports a wide variety of ioctl() functions that you can use to issue SCSI commands to a device. In order to invoke these operations you prepare a dsreq structure describing the operation and pass it to the device driver. Operations can include input and output as well as control and diagnostic commands.

The programming interface supported by the generic SCSI driver is quite primitive. A library of higher-level functions makes it easier to use. This library is documented in the dslib(3x) reference page. It is also described in detail in the IRIX Device Driver Programmer's Guide. The most important functions in it are listed below:

dsopen(), which takes a device pathname, opens it for exclusive access, and returns a dsreq structure to be used with other functions.
fillg0cmd(), fillg1cmd(), and filldsreq(), which simplify the task of preparing the many fields of a dsreq structure for a particular command.
doscsireq(), which calls the device driver and checks status afterward.

The dsreq structure for some operations specifies a buffer in memory for data transfer. The generic SCSI driver handles the task of locking the buffer into memory (if necessary) and managing a DMA transfer of data.

When the ioctl() function is called (through doscsireq() or directly), it does not return until the SCSI command is complete. You should only request a SCSI operation from a process that can tolerate being blocked.

Upon the basic dslib functions are built several functions that execute specific SCSI commands, for example, read08() performs a read. However, there are few SCSI commands that are recognized by all devices. Even the read operation has many variations, and the read08() function as supplied is unlikely to work without modification. The dslib library functions are not complete. Instead, you must alter them and extend them with functions tailored to a specific device.

The source for dslib, and some example programs that use dslib, can be found in the 4DGifts distribution, in /usr/people/4Dgifts/examples/devices/devscsi.