635 lines
29 KiB
Plaintext
635 lines
29 KiB
Plaintext
/* $FreeBSD$ */
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Driver Theory of Operation Manual
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1. Introduction
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This is a short text document that will describe the background, goals
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for, and current theory of operation for the joint Fibre Channel/SCSI
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HBA driver for QLogic hardware.
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Because this driver is an ongoing project, do not expect this manual
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to remain entirely up to date. Like a lot of software engineering, the
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ultimate documentation is the driver source. However, this manual should
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serve as a solid basis for attempting to understand where the driver
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started and what is trying to be accomplished with the current source.
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The reader is expected to understand the basics of SCSI and Fibre Channel
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and to be familiar with the range of platforms that Solaris, Linux and
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the variant "BSD" Open Source systems are available on. A glossary and
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a few references will be placed at the end of the document.
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There will be references to functions and structures within the body of
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this document. These can be easily found within the source using editor
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tags or grep. There will be few code examples here as the code already
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exists where the reader can easily find it.
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2. A Brief History for this Driver
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This driver originally started as part of work funded by NASA Ames
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Research Center's Numerical Aerodynamic Simulation center ("NAS" for
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short) for the QLogic PCI 1020 and 1040 SCSI Host Adapters as part of my
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work at porting the NetBSD Operating System to the Alpha architectures
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(specifically the AlphaServer 8200 and 8400 platforms). In short, it
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started just as simple single SCSI HBA driver for just the purpose of
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running off a SCSI disk. This work took place starting in January, 1997.
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Because the first implementation was for NetBSD, which runs on a very
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large number of platforms, and because NetBSD supported both systems with
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SBus cards (e.g., Sun SPARC systems) as well as systems with PCI cards,
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and because the QLogic SCSI cards came in both SBus and PCI versions, the
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initial implementation followed the very thoughtful NetBSD design tenet
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of splitting drivers into what are called MI (for Machine Independent)
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and MD (Machine Dependent) portions. The original design therefore was
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from the premise that the driver would drive both SBus and PCI card
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variants. These busses are similar but have quite different constraints,
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and while the QLogic SBus and PCI cards are very similar, there are some
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significant differences.
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After this initial goal had been met, there began to be some talk about
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looking into implementing Fibre Channel mass storage at NAS. At this time
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the QLogic 2100 FC/AL HBA was about to become available. After looking at
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the way it was designed I concluded that it was so darned close to being
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just like the SCSI HBAs that it would be insane to *not* leverage off of
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the existing driver. So, we ended up with a driver for NetBSD that drove
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PCI and SBus SCSI cards, and now also drove the QLogic 2100 FC-AL HBA.
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After this, ports to non-NetBSD platforms became interesting as well.
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This took the driver out of the interest with NAS and into interested
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support from a number of other places. Since the original NetBSD
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development, the driver has been ported to FreeBSD, OpenBSD, Linux,
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Solaris, and two proprietary systems. Following from the original MI/MD
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design of NetBSD, a rather successful attempt has been made to keep the
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Operating System Platform differences segregated and to a minimum.
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Along the way, support for the 2200 as well as full fabric and target
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mode support has been added, and 2300 support as well as an FC-IP stack
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are planned.
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3. Driver Design Goals
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The driver has not started out as one normally would do such an effort.
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Normally you design via top-down methodologies and set an initial goal
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and meet it. This driver has had a design goal that changes from almost
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the very first. This has been an extremely peculiar, if not risque,
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experience. As a consequence, this section of this document contains
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a bit of "reconstruction after the fact" in that the design goals are
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as I perceive them to be now- not necessarily what they started as.
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The primary design goal now is to have a driver that can run both the
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SCSI and Fibre Channel SCSI prototocols on multiple OS platforms with
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as little OS platform support code as possible.
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The intended support targets for SCSI HBAs is to support the single and
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dual channel PCI Ultra2 and PCI Ultra3 cards as well as the older PCI
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Ultra single channel cards and SBus cards.
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The intended support targets for Fibre Channel HBAs is the 2100, 2200
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and 2300 PCI cards.
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Fibre Channel support should include complete fabric and public loop
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as well as private loop and private loop, direct-attach topologies.
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FC-IP support is also a goal.
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For both SCSI and Fibre Channel, simultaneous target/initiator mode support
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is a goal.
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Pure, raw, performance is not a primary goal of this design. This design,
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because it has a tremendous amount of code common across multiple
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platforms, will undoubtedly never be able to beat the performance of a
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driver that is specifically designed for a single platform and a single
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card. However, it is a good strong secondary goal to make the performance
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penalties in this design as small as possible.
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Another primary aim, which almost need not be stated, is that the
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implementation of platform differences must not clutter up the common
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code with platform specific defines. Instead, some reasonable layering
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semantics are defined such that platform specifics can be kept in the
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platform specific code.
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4. QLogic Hardware Architecture
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In order to make the design of this driver more intelligible, some
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description of the Qlogic hardware architecture is in order. This will
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not be an exhaustive description of how this card works, but will
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note enough of the important features so that the driver design is
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hopefully clearer.
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4.1 Basic QLogic hardware
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The QLogic HBA cards all contain a tiny 16-bit RISC-like processor and
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varying sizes of SRAM. Each card contains a Bus Interface Unit (BIU)
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as appropriate for the host bus (SBus or PCI). The BIUs allow access
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to a set of dual-ranked 16 bit incoming and outgoing mailbox registers
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as well as access to control registers that control the RISC or access
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other portions of the card (e.g., Flash BIOS). The term 'dual-ranked'
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means that at the same host visible address if you write a mailbox
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register, that is a write to an (incoming, to the HBA) mailbox register,
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while a read to the same address reads another (outgoing, to the HBA)
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mailbox register with completely different data. Each HBA also then has
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core and auxiliary logic which either is used to interface to a SCSI bus
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(or to external bus drivers that connect to a SCSI bus), or to connect
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to a Fibre Channel bus.
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4.2 Basic Control Interface
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There are two principle I/O control mechanisms by which the driver
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communicates with and controls the QLogic HBA. The first mechanism is to
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use the incoming mailbox registers to interrupt and issue commands to
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the RISC processor (with results usually, but not always, ending up in
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the ougtoing mailbox registers). The second mechanism is to establish,
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via mailbox commands, circular request and response queues in system
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memory that are then shared between the QLogic and the driver. The
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request queue is used to queue requests (e.g., I/O requests) for the
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QLogic HBA's RISC engine to copy into the HBA memory and process. The
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result queue is used by the QLogic HBA's RISC engine to place results of
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requests read from the request queue, as well as to place notification
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of asynchronous events (e.g., incoming commands in target mode).
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To give a bit more precise scale to the preceding description, the QLogic
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HBA has 8 dual-ranked 16 bit mailbox registers, mostly for out-of-band
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control purposes. The QLogic HBA then utilizes a circular request queue
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of 64 byte fixed size Queue Entries to receive normal initiator mode
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I/O commands (or continue target mode requests). The request queue may
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be up to 256 elements for the QLogic 1020 and 1040 chipsets, but may
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be quite larger for the QLogic 12X0/12160 SCSI and QLogic 2X00 Fibre
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Channel chipsets.
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In addition to synchronously initiated usage of mailbox commands by
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the host system, the QLogic may also deliver asynchronous notifications
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solely in outgoing mailbox registers. These asynchronous notifications in
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mailboxes may be things like notification of SCSI Bus resets, or that the
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Fabric Name server has sent a change notification, or even that a specific
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I/O command completed without error (this is called 'Fast Posting'
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and saves the QLogic HBA from having to write a response queue entry).
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The QLogic HBA is an interrupting card, and when servicing an interrupt
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you really only have to check for either a mailbox interrupt or an
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interrupt notification that the response queue has an entry to
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be dequeued.
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4.3 Fibre Channel SCSI out of SCSI
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QLogic took the approach in introducing the 2X00 cards to just treat
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FC-AL as a 'fat' SCSI bus (a SCSI bus with more than 15 targets). All
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of the things that you really need to do with Fibre Channel with respect
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to providing FC-4 services on top of a Class 3 connection are performed
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by the RISC engine on the QLogic card itself. This means that from
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an HBA driver point of view, very little needs to change that would
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distinguish addressing a Fibre Channel disk from addressing a plain
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old SCSI disk.
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However, in the details it's not *quite* that simple. For example, in
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order to manage Fabric Connections, the HBA driver has to do explicit
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binding of entities it's queried from the name server to specific 'target'
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ids (targets, in this case, being a virtual entity).
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Still- the HBA firmware does really nearly all of the tedious management
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of Fibre Channel login state. The corollary to this sometimes is the
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lack of ability to say why a particular login connection to a Fibre
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Channel disk is not working well.
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There are clear limits with the QLogic card in managing fabric devices.
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The QLogic manages local loop devices (LoopID or Target 0..126) itself,
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but for the management of fabric devices, it has an absolute limit of
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253 simultaneous connections (256 entries less 3 reserved entries).
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5. Driver Architecture
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5.1 Driver Assumptions
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The first basic assumption for this driver is that the requirements for
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a SCSI HBA driver for any system is that of a 2 or 3 layer model where
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there are SCSI target device drivers (drivers which drive SCSI disks,
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SCSI tapes, and so on), possibly a middle services layer, and a bottom
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layer that manages the transport of SCSI CDB's out a SCSI bus (or across
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Fibre Channel) to a SCSI device. It's assumed that each SCSI command is
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a separate structure (or pointer to a structure) that contains the SCSI
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CDB and a place to store SCSI Status and SCSI Sense Data.
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This turns out to be a pretty good assumption. All of the Open Source
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systems (*BSD and Linux) and most of the proprietary systems have this
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kind of structure. This has been the way to manage SCSI subsystems for
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at least ten years.
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There are some additional basic assumptions that this driver makes- primarily
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in the arena of basic simple services like memory zeroing, memory copying,
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delay, sleep, microtime functions. It doesn't assume much more than this.
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5.2 Overall Driver Architecture
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The driver is split into a core (machine independent) module and platform
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and bus specific outer modules (machine dependent).
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The core code (in the files isp.c, isp_inline.h, ispvar.h, ispreg.h and
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ispmbox.h) handles:
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+ Chipset recognition and reset and firmware download (isp_reset)
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+ Board Initialization (isp_init)
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+ First level interrupt handling (response retrieval) (isp_intr)
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+ A SCSI command queueing entry point (isp_start)
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+ A set of control services accessed either via local requirements within
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the core module or via an externally visible control entry point
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(isp_control).
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The platform/bus specific modules (and definitions) depend on each
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platform, and they provide both definitions and functions for the core
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module's use. Generally a platform module set is split into a bus
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dependent module (where configuration is begun from and bus specific
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support functions reside) and relatively thin platform specific layer
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which serves as the interconnect with the rest of this platform's SCSI
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subsystem.
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For ease of bus specific access issues, a centralized soft state
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structure is maintained for each HBA instance (struct ispsoftc). This
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soft state structure contains a machine/bus dependent vector (mdvec)
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for functions that read and write hardware registers, set up DMA for the
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request/response queues and fibre channel scratch area, set up and tear
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down DMA mappings for a SCSI command, provide a pointer to firmware to
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load, and other minor things.
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The machine dependent outer module must provide functional entry points
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for the core module:
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+ A SCSI command completion handoff point (isp_done)
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+ An asynchronous event handler (isp_async)
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+ A logging/printing function (isp_prt)
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The machine dependent outer module code must also provide a set of
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abstracting definitions which is what the core module utilizes heavily
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to do its job. These are discussed in detail in the comments in the
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file ispvar.h, but to give a sense of the range of what is required,
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let's illustrate two basic classes of these defines.
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The first class are "structure definition/access" class. An
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example of these would be:
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XS_T Platform SCSI transaction type (i.e., command for HBA)
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..
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XS_TGT(xs) gets the target from an XS_T
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..
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XS_TAG_TYPE(xs) which type of tag to use
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..
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The second class are 'functional' class definitions. Some examples of
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this class are:
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MEMZERO(dst, src) platform zeroing function
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..
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MBOX_WAIT_COMPLETE(struct ispsoftc *) wait for mailbox cmd to be done
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Note that the former is likely to be simple replacement with bzero or
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memset on most systems, while the latter could be quite complex.
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This soft state structure also contains different parameter information
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based upon whether this is a SCSI HBA or a Fibre Channel HBA (which is
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filled in by the code module).
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In order to clear up what is undoubtedly a seeming confusion of
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interconnects, a description of the typical flow of code that performs
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boards initialization and command transactions may help.
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5.3 Initialization Code Flow
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Typically a bus specific module for a platform (e.g., one that wants
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to configure a PCI card) is entered via that platform's configuration
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methods. If this module recognizes a card and can utilize or construct the
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space for the HBA instance softc, it does so, and initializes the machine
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dependent vector as well as any other platform specific information that
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can be hidden in or associated with this structure.
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Configuration at this point usually involves mapping in board registers
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and registering an interrupt. It's quite possible that the core module's
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isp_intr function is adequate to be the interrupt entry point, but often
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it's more useful have a bus specific wrapper module that calls isp_intr.
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After mapping and interrupt registry is done, isp_reset is called.
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Part of the isp_reset call may cause callbacks out to the bus dependent
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module to perform allocation and/or mapping of Request and Response
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queues (as well as a Fibre Channel scratch area if this is a Fibre
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Channel HBA). The reason this is considered 'bus dependent' is that
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only the bus dependent module may have the information that says how
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one could perform I/O mapping and dependent (e.g., on a Solaris system)
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on the Request and Response queues. Another callback can enable the *use*
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of interrupts should this platform be able to finish configuration in
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interrupt driven mode.
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If isp_reset is successful at resetting the QLogic chipset and downloading
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new firmware (if available) and setting it running, isp_init is called. If
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isp_init is successful in doing initial board setups (including reading
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NVRAM from the QLogic card), then this bus specicic module will call the
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platform dependent module that takes the appropriate steps to 'register'
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this HBA with this platform's SCSI subsystem. Examining either the
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OpenBSD or the NetBSD isp_pci.c or isp_sbus.c files may assist the reader
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here in clarifying some of this.
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5.4 Initiator Mode Command Code Flow
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A successful execution of isp_init will lead to the driver 'registering'
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itself with this platform's SCSI subsystem. One assumed action for this
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is the registry of a function that the SCSI subsystem for this platform
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will call when it has a SCSI command to run.
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The platform specific module function that receives this will do whatever
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it needs to to prepare this command for execution in the core module. This
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sounds vague, but it's also very flexible. In principle, this could be
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a complete marshalling/demarshalling of this platform's SCSI command
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structure (should it be impossible to represent in an XS_T). In addition,
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this function can also block commands from running (if, e.g., Fibre
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Channel loop state would preclude successful starting of the command).
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When it's ready to do so, the function isp_start is called with this
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command. This core module tries to allocate request queue space for
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this command. It also calls through the machine dependent vector
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function to make sure any DMA mapping for this command is done.
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Now, DMA mapping here is possibly a misnomer, as more than just
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DMA mapping can be done in this bus dependent function. This is
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also the place where any endian byte-swizzling will be done. At any
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rate, this function is called last because the process of establishing
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DMA addresses for any command may in fact consume more Request Queue
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entries than there are currently available. If the mapping and other
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functions are successful, the QLogic mailbox inbox pointer register
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is updated to indicate to the QLogic that it has a new request to
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read.
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If this function is unsuccessful, policy as to what to do at this point is
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left to the machine dependent platform function which called isp_start. In
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some platforms, temporary resource shortages can be handled by the main
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SCSI subsystem. In other platforms, the machine dependent code has to
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handle this.
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In order to keep track of commands that are in progress, the soft state
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structure contains an array of 'handles' that are associated with each
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active command. When you send a command to the QLogic firmware, a portion
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of the Request Queue entry can contain a non-zero handle identifier so
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that at a later point in time in reading either a Response Queue entry
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or from a Fast Posting mailbox completion interrupt, you can take this
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handle to find the command you were waiting on. It should be noted that
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this is probably one of the most dangerous areas of this driver. Corrupted
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handles will lead to system panics.
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At some later point in time an interrupt will occur. Eventually,
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isp_intr will be called. This core module will determine what the cause
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of the interrupt is, and if it is for a completing command. That is,
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it'll determine the handle and fetch the pointer to the command out of
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storage within the soft state structure. Skipping over a lot of details,
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the machine dependent code supplied function isp_done is called with the
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pointer to the completing command. This would then be the glue layer that
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informs the SCSI subsystem for this platform that a command is complete.
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5.5 Asynchronous Events
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Interrupts occur for events other than commands (mailbox or request queue
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started commands) completing. These are called Asynchronous Mailbox
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interrupts. When some external event causes the SCSI bus to be reset,
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or when a Fibre Channel loop changes state (e.g., a LIP is observed),
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this generates such an asynchronous event.
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Each platform module has to provide an isp_async entry point that will
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handle a set of these. This isp_async entry point also handles things
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which aren't properly async events but are simply natural outgrowths
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of code flow for another core function (see discussion on fabric device
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management below).
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5.6 Target Mode Code Flow
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This section could use a lot of expansion, but this covers the basics.
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The QLogic cards, when operating in target mode, follow a code flow that is
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essentially the inverse of that for intiator mode describe above. In this
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scenario, an interrupt occurs, and present on the Response Queue is a
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queue entry element defining a new command arriving from an initiator.
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This is passed to possibly external target mode handler. This driver
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provides some handling for this in a core module, but also leaves
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things open enough that a completely different target mode handler
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may accept this incoming queue entry.
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The external target mode handler then turns around forms up a response
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to this 'response' that just arrived which is then placed on the Request
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Queue and handled very much like an initiator mode command (i.e., calling
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the bus dependent DMA mapping function). If this entry completes the
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command, no more need occur. But often this handles only part of the
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requested command, so the QLogic firmware will rewrite the response
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to the initial 'response' again onto the Response Queue, whereupon the
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target mode handler will respond to that, and so on until the command
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is completely handled.
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Because almost no platform provides basic SCSI Subsystem target mode
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support, this design has been left extremely open ended, and as such
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it's a bit hard to describe in more detail than this.
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5.7 Locking Assumptions
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The observant reader by now is likely to have asked the question, "but what
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about locking? Or interrupt masking" by now.
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The basic assumption about this is that the core module does not know
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anything directly about locking or interrupt masking. It may assume that
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upon entry (e.g., via isp_start, isp_control, isp_intr) that appropriate
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locking and interrupt masking has been done.
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The platform dependent code may also therefore assume that if it is
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called (e.g., isp_done or isp_async) that any locking or masking that
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was in place upon the entry to the core module is still there. It is up
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to the platform dependent code to worry about avoiding any lock nesting
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issues. As an example of this, the Linux implementation simply queues
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up commands completed via the callout to isp_done, which it then pushes
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out to the SCSI subsystem after a return from it's calling isp_intr is
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|
executed (and locks dropped appropriately, as well as avoidance of deep
|
|
interrupt stacks).
|
|
|
|
Recent changes in the design have now eased what had been an original
|
|
requirement that the while in the core module no locks or interrupt
|
|
masking could be dropped. It's now up to each platform to figure out how
|
|
to implement this. This is principally used in the execution of mailbox
|
|
commands (which are principally used for Loop and Fabric management via
|
|
the isp_control function).
|
|
|
|
5.8 SCSI Specifics
|
|
|
|
The driver core or platform dependent architecture issues that are specific
|
|
to SCSI are few. There is a basic assumption that the QLogic firmware
|
|
supported Automatic Request sense will work- there is no particular provision
|
|
for disabling it's usage on a per-command basis.
|
|
|
|
5.9 Fibre Channel Specifics
|
|
|
|
Fibre Channel presents an interesting challenge here. The QLogic firmware
|
|
architecture for dealing with Fibre Channel as just a 'fat' SCSI bus
|
|
is fine on the face of it, but there are some subtle and not so subtle
|
|
problems here.
|
|
|
|
5.9.1 Firmware State
|
|
|
|
Part of the initialization (isp_init) for Fibre Channel HBAs involves
|
|
sending a command (Initialize Control Block) that establishes Node
|
|
and Port WWNs as well as topology preferences. After this occurs,
|
|
the QLogic firmware tries to traverese through serveral states:
|
|
|
|
FW_CONFIG_WAIT
|
|
FW_WAIT_AL_PA
|
|
FW_WAIT_LOGIN
|
|
FW_READY
|
|
FW_LOSS_OF_SYNC
|
|
FW_ERROR
|
|
FW_REINIT
|
|
FW_NON_PART
|
|
|
|
It starts with FW_CONFIG_WAIT, attempts to get an AL_PA (if on an FC-AL
|
|
loop instead of being connected as an N-port), waits to log into all
|
|
FC-AL loop entities and then hopefully transitions to FW_READY state.
|
|
|
|
Clearly, no command should be attempted prior to FW_READY state is
|
|
achieved. The core internal function isp_fclink_test (reachable via
|
|
isp_control with the ISPCTL_FCLINK_TEST function code). This function
|
|
also determines connection topology (i.e., whether we're attached to a
|
|
fabric or not).
|
|
|
|
5.9.2. Loop State Transitions- From Nil to Ready
|
|
|
|
Once the firmware has transitioned to a ready state, then the state of the
|
|
connection to either arbitrated loop or to a fabric has to be ascertained,
|
|
and the identity of all loop members (and fabric members validated).
|
|
|
|
This can be very complicated, and it isn't made easy in that the QLogic
|
|
firmware manages PLOGI and PRLI to devices that are on a local loop, but
|
|
it is the driver that must manage PLOGI/PRLI with devices on the fabric.
|
|
|
|
In order to manage this state an eight level staging of current "Loop"
|
|
(where "Loop" is taken to mean FC-AL or N- or F-port connections) states
|
|
in the following ascending order:
|
|
|
|
LOOP_NIL
|
|
LOOP_LIP_RCVD
|
|
LOOP_PDB_RCVD
|
|
LOOP_SCANNING_FABRIC
|
|
LOOP_FSCAN_DONE
|
|
LOOP_SCANNING_LOOP
|
|
LOOP_LSCAN_DONE
|
|
LOOP_SYNCING_PDB
|
|
LOOP_READY
|
|
|
|
When the core code initializes the QLogic firmware, it sets the loop
|
|
state to LOOP_NIL. The first 'LIP Received' asynchronous event sets state
|
|
to LOOP_LIP_RCVD. This should be followed by a "Port Database Changed"
|
|
asynchronous event which will set the state to LOOP_PDB_RCVD. Each of
|
|
these states, when entered, causes an isp_async event call to the
|
|
machine dependent layers with the ISPASYNC_CHANGE_NOTIFY code.
|
|
|
|
After the state of LOOP_PDB_RCVD is reached, the internal core function
|
|
isp_scan_fabric (reachable via isp_control(..ISPCTL_SCAN_FABRIC)) will,
|
|
if the connection is to a fabric, use Simple Name Server mailbox mediated
|
|
commands to dump the entire fabric contents. For each new entity, an
|
|
isp_async event will be generated that says a Fabric device has arrived
|
|
(ISPASYNC_FABRIC_DEV). The function that isp_async must perform in this
|
|
step is to insert possibly remove devices that it wants to have the
|
|
QLogic firmware log into (at LOOP_SYNCING_PDB state level)).
|
|
|
|
After this has occurred, the state LOOP_FSCAN_DONE is set, and then the
|
|
internal function isp_scan_loop (isp_control(...ISPCTL_SCAN_LOOP)) can
|
|
be called which will then scan for any local (FC-AL) entries by asking
|
|
for each possible local loop id the QLogic firmware for a Port Database
|
|
entry. It's at this level some entries cached locally are purged
|
|
or shifting loopids are managed (see section 5.9.4).
|
|
|
|
The final step after this is to call the internal function isp_pdb_sync
|
|
(isp_control(..ISPCTL_PDB_SYNC)). The purpose of this function is to
|
|
then perform the PLOGI/PRLI functions for fabric devices. The next state
|
|
entered after this is LOOP_READY, which means that the driver is ready
|
|
to process commands to send to Fibre Channel devices.
|
|
|
|
5.9.3 Fibre Channel variants of Initiator Mode Code Flow
|
|
|
|
The code flow in isp_start for Fibre Channel devices is the same as it is
|
|
for SCSI devices, but with a notable exception.
|
|
|
|
Maintained within the fibre channel specific portion of the driver soft
|
|
state structure is a distillation of the existing population of both
|
|
local loop and fabric devices. Because Loop IDs can shift on a local
|
|
loop but we wish to retain a 'constant' Target ID (see 5.9.4), this
|
|
is indexed directly via the Target ID for the command (XS_TGT(xs)).
|
|
|
|
If there is a valid entry for this Target ID, the command is started
|
|
(with the stored 'Loop ID'). If not the command is completed with
|
|
the error that is just like a SCSI Selection Timeout error.
|
|
|
|
This code is currently somewhat in transition. Some platforms to
|
|
do firmware and loop state management (as described above) at this
|
|
point. Other platforms manage this from the machine dependent layers. The
|
|
important function to watch in this respect is isp_fc_runstate (in
|
|
isp_inline.h).
|
|
|
|
5.9.4 "Target" in Fibre Channel is a fixed virtual construct
|
|
|
|
Very few systems can cope with the notion that "Target" for a disk
|
|
device can change while you're using it. But one of the properties of
|
|
for arbitrated loop is that the physical bus address for a loop member
|
|
(the AL_PA) can change depending on when and how things are inserted in
|
|
the loop.
|
|
|
|
To illustrate this, let's take an example. Let's say you start with a
|
|
loop that has 5 disks in it. At boot time, the system will likely find
|
|
them and see them in this order:
|
|
|
|
disk# Loop ID Target ID
|
|
disk0 0 0
|
|
disk1 1 1
|
|
disk2 2 2
|
|
disk3 3 3
|
|
disk4 4 4
|
|
|
|
The driver uses 'Loop ID' when it forms requests to send a comamnd to
|
|
each disk. However, it reports to NetBSD that things exist as 'Target
|
|
ID'. As you can see here, there is perfect correspondence between disk,
|
|
Loop ID and Target ID.
|
|
|
|
Let's say you add a new disk between disk2 and disk3 while things are
|
|
running. You don't really often see this, but you *could* see this where
|
|
the loop has to renegotiate, and you end up with:
|
|
|
|
disk# Loop ID Target ID
|
|
disk0 0 0
|
|
disk1 1 1
|
|
disk2 2 2
|
|
diskN 3 ?
|
|
disk3 4 ?
|
|
disk4 5 ?
|
|
|
|
Clearly, you don't want disk3 and disk4's "Target ID" to change while you're
|
|
running since currently mounted filesystems will get trashed.
|
|
|
|
What the driver is supposed to do (this is the function of isp_scan_loop),
|
|
is regenerate things such that the following then occurs:
|
|
|
|
disk# Loop ID Target ID
|
|
disk0 0 0
|
|
disk1 1 1
|
|
disk2 2 2
|
|
diskN 3 5
|
|
disk3 4 3
|
|
disk4 5 4
|
|
|
|
So, "Target" is a virtual entity that is maintained while you're running.
|
|
|
|
6. Glossary
|
|
|
|
HBA - Host Bus Adapter
|
|
|
|
SCSI - Small Computer
|
|
|
|
7. References
|
|
|
|
Various URLs of interest:
|
|
|
|
http://www.netbsd.org - NetBSD's Web Page
|
|
http://www.openbsd.org - OpenBSD's Web Page
|
|
http://www.freebsd.org - FreeBSD's Web Page
|
|
|
|
http://www.t10.org - ANSI SCSI Commitee's Web Page
|
|
(SCSI Specs)
|
|
http://www.t11.org - NCITS Device Interface Web Page
|
|
(Fibre Channel Specs)
|
|
|