10.1. Pseudo drivers

SCSI can be viewed as a command set and a set of hardware buses that convey that command set. Those hardware buses can be further divided into those used exclusively for SCSI (e.g. ultra wide), those shared with other protocols (e.g. USB, IEEE 1394) and those buses not defined by the various SCSI standards. In the final category there are several interesting examples including ATAPI CD writers and PC parallel bus ZIP drives. Such devices use the SCSI command set (or something very close to it) over a foreign bus.

This section briefly outlines various pseudo lower level drivers which essentially communicate with other Linux subsystems in order to send the SCSI command set to devices controlled by those other subsystems. This raises some ownership issues that often confuse users and result in many questions to the maintainers.

The preferences of the IDE subsystem can be overridden with one of these kernel boot time parameters (of which the first is most interesting for this subsystem):

• hdx=ide-scsi

• hdx=ide-cdrom

• hdx=ide-floppy

When the driver is running, the device will be accessible using the SCSI device (/dev/sda, /dev/sr0 , etc), and not through the corresponding /dev/hdx device. Still, the /dev/hdx device will be available, but only for configuration.

All the generic IDE configuration parameters (DMA on/off, 32-bit I/O, unmasking irq's, etc) are available by using the /dev/hdx device, for example to enable DMA:

hdparm -d1 /dev/hdx

Using cat /proc/ide/hdx/settings will show the available settings. All the generic IDE driver settings will be available there, as well as the following "ide-scsi specific" settings:

• bios_cyl

• bios_head

• bios_sect

• transform

• log

• bit 0: Enable(1)/Disable(0) transformation for commands not originated from the sg driver.

• bit 1: Enable/Disable transformation for commands issued using the sg driver.

        echo "log:1" > /proc/ide/hdx/settings

The parallel port ZIP drives are actually SCSI devices which tunnel SCSI commands over the parallel port using interfaces called VPI0 (older-style) and VPI2 (newer-style). The ppa driver is the VPI0 host implementation and the imm driver is the VPI2 host implementation.

The way it works is that the HBA is a chip inside the ZIP drive, so that the host adapter and the peripheral are in the same actual case.

Each ppscsi protocol module registers itself with the ppscsi module, passing in a list of entry points for the various things that are common to all protocol drivers.

The plan is that the ppscsi architecture will absorb both the ppa and imm drivers and protocol modules; only vpi0 has been written so far. See www.torque.net/parport/ppscsi.html.

There is also the usb/microtek driver for controlling X6 USB scanners from Microtek. When configured, the SANE application uses the sg driver to send SCSI commands over USB to control this scanner.