AHC(4)

NAME
ahc - Adaptec VL/EISA/PCI SCSI host adapter driver

SYNOPSIS
For one or more VL/EISA cards:
controller eisa0
controller ahc0

For one or more PCI cards:
controller pci0
controller ahc0

To enable SCB paging:
options AHC_SCBPAGING_ENABLE

To enable tagged queueing:
options AHC_TAGENABLE

To allow PCI adapters to use memory mapped I/O if enabled:
options AHC_ALLOW_MEMIO

For one or more SCSI busses:
controller scbus0 at ahc0

DESCRIPTION
This driver provides access to the SCSI bus(es) connected to Adaptec 274x, 284x, 2940, 3940, or controllers based on the AIC7770, AIC7850, AIC7860, AIC7870, or AIC7880 host adapter chips. Features include support for twin and wide busses, ultra SCSI, two active commands at a time per non-tagged queueing target, tagged queuing, and SCB paging.

The number of concurrent transactions allowed is chip dependent and ranges from 3 to 16. On PCI adapters, this number can be increased with the SCB paging option. SCB paging implements an algorithm to 'page-out' transactions that are in the disconnected state so that the freed space in the controller's memory can be used to start additional transactions. On the aic7880 and aic7870, this increases the maximum number of outstanding transactions from 16 to 255. On the aic7850 and aic7860 controllers, this maximum rises from 3 to 8. During the hardware probe, a diagnostic showing the ratio of hardware supported 'slots' to number of transactions is printed. SCB paging is enabled with the “AHC_SCBPAGING_ENABLE” configuration option. This option will likely be removed and become the default behavior for adapters that support it, in the near future.

Tagged queueing is enabled with the “AHC_TAGENABLE” configuration option. Tagged queueing allows multiple transactions to be queued at the device level instead of the host level, allowing the device to re-order I/O to minimize seeks, seek distance, and to increase throughput. Tagged queueing can have a significant impact on performance for seek bound applications and should be enabled for most configurations. Unfortunantly, some devices that claim to support tagged queueing fail miserable when it is used. The only reason tagged queueing remains as a controller option is as a stop gap measure until a mechanism to detect these broken devices and to control this feature on a per device basis is in place.

Memory mapped I/O can be enabled with the “AHC_ALLOW_MEMIO” configuration option. Memory mapped I/O is more efficient than the alternative, programmed I/O. Most PCI BIOSes will map devices so that either technique for communicating with the card is availible. In some cases, usually when the PCI device is sitting behind a PCI->PCI bridge, the BIOS fails to properly initialize the chip for memory mapped I/O. The symptom of this problem is usually a system hang if memory mapped I/O is attempted. Most modern motherboards perform the initialization correctly and work fine with this option enabled.

Per target configuration performed in the SCSI-Select menu, accesible at boot in non-EISA models, or through an EISA configuration utility for EISA models, is honored by this driver with the stipulation that the BIOS must be enabled for EISA adaptors. This includes synchronous/asynchronous transfers, maximum synchronous negotiation rate, disconnection, and the host adapter's SCSI ID.

Note that I/O addresses are determined automatically by the probe routines, but care should be taken when using a 284x (VESA local bus controller) in an EISA system. Ensure that the jumpers setting the I/O area for the 284x match the EISA slot into which the card is inserted to prevent conflicts with other EISA cards.

BUGS
Some Quantum drives (at least the Empire 2100 and 1080s) will not run on an AIC7870 Rev B in synchronous mode at 10MHz. Controllers with this problem have a 42 MHz clock crystal on them and run slightly above 10MHz. This causes the drive much confusion. Setting a maximum synchronous negotiation rate of 8MHz in the SCSI-Select utility will allow normal function.

SEE ALSO
aha(4), ahb(4), cd(4), scsi(4), sd(4), st(4)

AUTHOR
The ahc driver was written by Justin Gibbs. The AIC7xxx sequencer-code assembler was written by John Aycock.

HISTORY
The ahc driver appeared in FreeBSD 2.1.