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NAME

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

SYNOPSIS

     To compile this driver into the kernel, place the following lines in your
     kernel configuration file:

           device scbus
           device ahc

           For one or more VL/EISA cards:
           device eisa

           For one or more PCI cards:
           device pci

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

           To configure one or more controllers to assume the target role:
           options AHC_TMODE_ENABLE <bitmask of units>

     Alternatively, to load the driver as a module at boot time, place the
     following lines in loader.conf(5):

           ahc_load="YES"
           ahc_eisa_load="YES"
           ahc_isa_load="YES"
           ahc_pci_load="YES"

DESCRIPTION

     This driver provides access to the SCSI bus(es) connected to the Adaptec
     AIC77xx and AIC78xx host adapter chips.

     Driver features include support for twin and wide busses, fast, ultra or
     ultra2 synchronous transfers depending on controller type, tagged
     queueing, SCB paging, and target mode.

     Memory mapped I/O can be enabled for PCI devices 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
     available.  In some cases, usually when the PCI device is sitting behind
     a PCI->PCI bridge, the BIOS may fail to properly initialize the chip for
     memory mapped I/O.  The typical symptom of this problem is a system hang
     if memory mapped I/O is attempted.  Most modern motherboards perform the
     initialization correctly and work fine with this option enabled.

     Individual controllers may be configured to operate in the target role
     through the ``AHC_TMODE_ENABLE'' configuration option.  The value
     assigned to this option should be a bitmap of all units where target mode
     is desired.  For example, a value of 0x25, would enable target mode on
     units 0, 2, and 5.  A value of 0x8a enables it for units 1, 3, and 7.

     Per target configuration performed in the SCSI-Select menu, accessible at
     boot in non-EISA models, or through an EISA configuration utility for
     EISA models, is honored by this driver.  This includes
     synchronous/asynchronous transfers, maximum synchronous negotiation rate,
     wide transfers, disconnection, the host adapter's SCSI ID, and, in the
     case of EISA Twin Channel controllers, the primary channel selection.
     For systems that store non-volatile settings in a system specific manner
     rather than a serial eeprom directly connected to the aic7xxx controller,
     the BIOS must be enabled for the driver to access this information.  This
     restriction applies to all EISA and many motherboard configurations.

     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.  The jumpers setting the I/O area for the
     284x should match the EISA slot into which the card is inserted to
     prevent conflicts with other EISA cards.

     Performance and feature sets vary throughout the aic7xxx product line.
     The following table provides a comparison of the different chips
     supported by the ahc 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.

           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 8
           aic7891     20    PCI/64     40MHz     16Bit    16        3 4 5 6 7 8
           aic7892     20    PCI/64     80MHz     16Bit    16        3 4 5 6 7 8
           aic7895     15    PCI/32     20MHz     16Bit    16      2 3 4 5
           aic7895C    15    PCI/32     20MHz     16Bit    16      2 3 4 5     8
           aic7896     20    PCI/32     40MHz     16Bit    16      2 3 4 5 6 7 8
           aic7897     20    PCI/64     40MHz     16Bit    16      2 3 4 5 6 7 8
           aic7899     20    PCI/64     80MHz     16Bit    16      2 3 4 5 6 7 8

           1.   Multiplexed Twin Channel Device - One controller servicing two
                busses.
           2.   Multi-function Twin Channel Device - Two controllers on one
                chip.
           3.   Command Channel Secondary DMA Engine - Allows scatter gather
                list and SCB prefetch.
           4.   64 Byte SCB Support - SCSI CDB is embedded in the SCB to
                eliminate an extra DMA.
           5.   Block Move Instruction Support - Doubles the speed of certain
                sequencer operations.
           6.   'Bayonet' style Scatter Gather Engine - Improves S/G prefetch
                performance.
           7.   Queuing Registers - Allows queueing of new transactions
                without pausing the sequencer.
           8.   Multiple Target IDs - Allows the controller to respond to
                selection as a target on multiple SCSI IDs.

HARDWARE

     The ahc driver supports the following SCSI host adapter chips and SCSI
     controller cards:

     +o   Adaptec AIC7770 host adapter chip
     +o   Adaptec AIC7850 host adapter chip
     +o   Adaptec AIC7860 host adapter chip
     +o   Adaptec AIC7870 host adapter chip
     +o   Adaptec AIC7880 host adapter chip
     +o   Adaptec AIC7890 host adapter chip
     +o   Adaptec AIC7891 host adapter chip
     +o   Adaptec AIC7892 host adapter chip
     +o   Adaptec AIC7895 host adapter chip
     +o   Adaptec AIC7896 host adapter chip
     +o   Adaptec AIC7897 host adapter chip
     +o   Adaptec AIC7899 host adapter chip
     +o   Adaptec 274X(W)
     +o   Adaptec 274X(T)
     +o   Adaptec 284X
     +o   Adaptec 2910
     +o   Adaptec 2915
     +o   Adaptec 2920C
     +o   Adaptec 2930C
     +o   Adaptec 2930U2
     +o   Adaptec 2940
     +o   Adaptec 2940J
     +o   Adaptec 2940N
     +o   Adaptec 2940U
     +o   Adaptec 2940AU
     +o   Adaptec 2940UW
     +o   Adaptec 2940UW Dual
     +o   Adaptec 2940UW Pro
     +o   Adaptec 2940U2W
     +o   Adaptec 2940U2B
     +o   Adaptec 2950U2W
     +o   Adaptec 2950U2B
     +o   Adaptec 19160B
     +o   Adaptec 29160B
     +o   Adaptec 29160N
     +o   Adaptec 3940
     +o   Adaptec 3940U
     +o   Adaptec 3940AU
     +o   Adaptec 3940UW
     +o   Adaptec 3940AUW
     +o   Adaptec 3940U2W
     +o   Adaptec 3950U2
     +o   Adaptec 3960
     +o   Adaptec 39160
     +o   Adaptec 3985
     +o   Adaptec 4944UW
     +o   NEC PC-9821Xt13 (PC-98)
     +o   NEC RvII26 (PC-98)
     +o   NEC PC-9821X-B02L/B09 (PC-98)
     +o   NEC SV-98/2-B03 (PC-98)
     +o   Many motherboards with on-board SCSI support

SCSI CONTROL BLOCKS (SCBs)

     Every transaction sent to a device on the SCSI bus is assigned a '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 configurations have external SRAM.

     If external SRAM is not available, SCBs are a limited resource.  Using
     the SCBs in a straight forward manner would only allow the driver to
     handle as many concurrent transactions as there are physical SCBs.  To
     fully utilize the SCSI bus and the devices on it, requires much more
     concurrency.  The solution to this problem is 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 therefore
     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.

SEE ALSO

     aha(4), ahb(4), cd(4), da(4), sa(4), scsi(4)

HISTORY

     The ahc driver appeared in FreeBSD 2.0.

AUTHORS

     The ahc driver, the AIC7xxx sequencer-code assembler, and the firmware
     running on the aic7xxx chips was written by Justin T. Gibbs.

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 confuses the drive and hangs the bus.  Setting a maximum synchronous
     negotiation rate of 8MHz in the SCSI-Select utility will allow normal
     operation.

     Although the Ultra2 and Ultra160 products have sufficient instruction ram
     space to support both the initiator and target roles concurrently, this
     configuration is disabled in favor of allowing the target role to respond
     on multiple target ids.  A method for configuring dual role mode should
     be provided.

     Tagged Queuing is not supported in target mode.

     Reselection in target mode fails to function correctly on all high
     voltage differential boards as shipped by Adaptec.  Information on how to
     modify HVD board to work correctly in target mode is available from
     Adaptec.