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AHC(4)                   BSD Kernel Interfaces Manual                   AHC(4)

NAME
     ahc -- Adaptec VL/EISA/PCI SCSI interface

SYNOPSIS
     ahc0 at isa?    (VL)
     ahc* at eisa?   (EISA)
     ahc* at pci?    (PCI)
     option AHC_ALLOW_MEMIO
     option AHC_TMODE_ENABLE

DESCRIPTION
     This driver provides access to the SCSI bus(es) connected to Adaptec
     AIC7770, AIC7850, AIC7860, AIC7870, AIC7880, AIC7890, AIC7891, AIC7892,
     AIC7895, AIC7896, AIC7897 and AIC7899 host adapter chips.  These chips
     are found on many motherboards as well as the following Adaptec SCSI con-
     troller cards: 274X(W), 274X(T), 284X, 2910, 2915, 2920, 2930C, 2930U2,
     2940, 2940J, 2940N, 2940U, 2940AU, 2940UW, 2940UW Dual, 2940UW Pro,
     2940U2W, 2940U2B, 2950U2W, 2950U2B, 19160B, 29160B, 29160N, 3940, 3940U,
     3940AU, 3940UW, 3940AUW, 3940U2W, 3950U2, 3960, 39160, 3985, and 4944UW.

     Driver features include support for twin and wide buses, fast, ultra,
     ultra2 and ultra160 synchronous transfers depending on controller type,
     tagged queuing, and 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.  This is
     the default mode of operation on every architecture except i386.

     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/asyn-
     chronous 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 rou-
     tines, 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 pre-
     vent 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 sup-
     ported by the ahc driver.  Note that wide and twin channel features,
     although always supported by a particular chip, may be disabled in a par-
     ticular 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
          buses.
     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 per-
          formance.
     7.   Queuing Registers - Allows queuing of new transactions without paus-
          ing the sequencer.
     8.   Ultra160 support.
     9.   Multiple Target IDs - Allows the controller to respond to selection
          as a target on multiple SCSI IDs.

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 han-
     dle as many concurrent transactions as there are physical SCBs.  To fully
     utilize the SCSI bus and the devices on it, requires much more concur-
     rency.  The solution to this problem is SCB Paging, a concept similar to
     memory paging.  SCB paging takes advantage of the fact that devices usu-
     ally disconnect from the SCSI bus for long periods of time without talk-
     ing 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 dis-
     connected) 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
     ahd(4), cd(4), ch(4), eisa(4), intro(4), isa(4), pci(4), scsi(4), sd(4),
     st(4), uk(4)

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

     The OpenBSD platform dependent code was written by Steve P. Murphree, Jr
     and Kenneth R. Westerback.

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 volt-
     age differential boards as shipped by Adaptec.  Information on how to
     modify HVD board to work correctly in target mode is available from
     Adaptec.

BSD                             April 25, 2017                             BSD