freebsd-skq/share/man/man4/ahc.4

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.Dd October 15, 1998
.Dt AHC 4
.Os FreeBSD
.Sh NAME
.Nm ahc
.Nd Adaptec VL/EISA/PCI SCSI host adapter driver
.Sh SYNOPSIS
For one or more VL/EISA cards:
.Cd device eisa0
.Cd device ahc0
.Pp
For one or more PCI cards:
.Cd device pci0
.Cd device ahc0
.Pp
To allow PCI adapters to use memory mapped I/O if enabled:
.Cd options AHC_ALLOW_MEMIO
.Pp
For one or more SCSI busses:
.Cd device scbus0 at ahc0
.Sh DESCRIPTION
This driver provides access to the
.Tn SCSI
bus(es) connected to Adaptec
.Tn AIC7770,
.Tn AIC7850,
.Tn AIC7860,
.Tn AIC7870,
.Tn AIC7880,
.Tn AIC7890,
.Tn AIC7891,
.Tn AIC7895,
.Tn AIC7896,
or
.Tn AIC7897
host adapter chips.
These chips are found on many motherboards as well as the following
Adaptec SCSI controller cards:
.Tn 274X(W),
.Tn 274X(T),
.Tn 284X,
.Tn 2920C,
.Tn 2930U2,
.Tn 2940,
.Tn 2940U,
.Tn 2940AU,
.Tn 2940UW,
.Tn 2940UW Dual,
.Tn 2940U2W,
.Tn 2940U2B,
.Tn 2950U2W,
.Tn 2950U2B,
.Tn 3940,
.Tn 3940U,
.Tn 3940AU,
.Tn 3940UW,
.Tn 3940AUW,
.Tn 3940U2W,
.Tn 3950U2,
and
.Tn 3985.
.Pp
Driver features include support for twin and wide busses,
fast, ultra and ultra2 synchronous transfers depending on controller type,
tagged queuing,
and SCB paging.
.Pp
Memory mapped I/O can be enabled for PCI devices with the
.Dq Dv 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 available.
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.
.Pp
Per target configuration performed in the 
.Tn SCSI-Select
menu, accessible at boot
in 
.No non- Ns Tn EISA
models,
or through an 
.Tn EISA
configuration utility for 
.Tn EISA
models,
is honored by this driver with the stipulation that the 
.Tn BIOS
must be enabled for 
.Tn EISA
adaptors.  This includes synchronous/asynchronous transfers,
maximum synchronous negotiation rate,
disconnection,
the host adapter's SCSI ID,
and,
in the case of
.Tn EISA
Twin Channel controllers,
the primary channel selection.
.Pp
Note that I/O addresses are determined automatically by the probe routines,
but care should be taken when using a 284x
.Pq Tn VESA No local bus controller
in an
.Tn EISA 
system.  Ensure that the jumpers setting the I/O area for the 284x match the 
.Tn EISA
slot into which the card is inserted to prevent conflicts with other
.Tn EISA
cards.
.Pp
Performance and feature sets vary throughout the aic7xxx product line.
The following table provides a comparison of the different chips supported
by the
.Nm
driver.  Note that wide and twin channel features, although always supported
by a particular chip, may be disabled in a particular motherboard or card
design.
.Pp
.Bd -filled -offset indent
.Bl -column "aic7770 " "10 " "EISA/VL  " "10MHz " "16bit " "SCBs " Features
.Em "Chip       MIPS    Bus      MaxSync   MaxWidth  SCBs  Features"
aic7770     10    EISA/VL    10MHz     16Bit     4    1
aic7850     10    PCI/32     10MHz      8Bit     3
aic7860     10    PCI/32     20MHz      8Bit     3
aic7870     10    PCI/32     10MHz     16Bit    16
aic7880     10    PCI/32     20MHz     16Bit    16 
aic7890     20    PCI/32     40MHz     16Bit    16        3 4 5 6 7
aic7891     20    PCI/64     40MHz     16Bit    16        3 4 5 6 7
aic7895     15    PCI/32     20MHz     16Bit    16      2 3 4 5
aic7896     20    PCI/32     40MHz     16Bit    16      2 3 4 5 6 7
aic7897     20    PCI/64     40MHz     16Bit    16      2 3 4 5 6 7
.El 
.Pp
.Bl -enum -compact
.It
Multiplexed Twin Channel Device - One controller servicing two busses.
.It
Multi-function Twin Channel Device - Two controllers on one chip.
.It
Command Channel Secondary DMA Engine - Allows scatter gather list and
SCB prefetch.
.It
64 Byte SCB Support - SCSI CDB is embedded in the SCB to eliminate an extra DMA.
.It
Block Move Instruction Support - Doubles the speed of certain sequencer
operations.
.It
.Sq Bayonet
style Scatter Gather Engine - Improves S/G prefetch performance.
.It
Queuing Registers - Allows queuing of new transactions without pausing the
sequencer.
.El
.Ed
.Pp

.Sh SCSI CONTROL BLOCKS (SCBs)
Every transaction sent to a device on the SCSI bus is assigned a
.Sq SCSI Control Block
(SCB).  The SCB contains all of the information required by the
controller to process a transaction.  The chip feature table lists
the number of SCBs that can be stored in on chip memory.  All chips
with model numbers greater than or equal to 7870 allow for the on chip
SCB space to be augmented with external SRAM up to a maximum of 255 SCBs.
Very few Adaptec controller have external SRAM.

If external SRAM is not available, SCBs are a limited resource and
using them in a straight forward manner would only allow us to
keep as many transactions as there are SCBs outstanding at a time.
This would not allow enough concurrency to fully utilize the SCSI
bus and it's devices. The solution to this problem is
.Em SCB Paging ,
a concept similar to memory paging.  SCB paging takes advantage of
the fact that devices usually disconnect from the SCSI bus for long
periods of time without talking to the controller.  The SCBs
for disconnected transactions are only of use to the controller
when the transfer is resumed.  When the host queues another transaction
for the controller to execute, the controller firmware will use a
free SCB if one is available.  Otherwise, the state of the most recently
disconnected (and therefor most likely to stay disconnected) SCB is
saved, via dma, to host memory, and the local SCB reused to start
the new transaction.  This allows the controller to queue up to
255 transactions regardless of the amount of SCB space.  Since the
local SCB space serves as a cache for disconnected transactions, the
more SCB space available, the less host bus traffic consumed saving
and restoring SCB data.
.Sh BUGS
Some Quantum drives (at least the Empire 2100 and 1080s) will not run on an
.Tn 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 confuses
the drive and hangs the bus.  Setting a maximum synchronous negotiation rate
of 8MHz in the 
.Tn SCSI-Select
utility
will allow normal operation.
.Sh SEE ALSO
.Xr aha 4 ,
.Xr ahb 4 ,
.Xr cd 4 ,
.Xr da 4 ,
.Xr sa 4 ,
.Xr scsi 4
.Sh AUTHORS
The
.Nm
driver, the
.Tn AIC7xxx
sequencer-code assembler,
and the firmware running on the aic7xxx chips was written by
.An Justin T. Gibbs .
.Sh HISTORY
The
.Nm
driver appeared in
.Fx 2.0 .