Avago Technologies LSI53C1020 - Manual (Page 33)

Fusion-MPT Architecture Overview 2-7
Version 2.4 Copyright © 2001–2004 by LSI Logic Corporation. All rights reserved.
2.2 Fusion-MPT Architecture Overview
The Fusion-MPT architecture provides two I/O methods for the host
system to communicate with the IOP: the system interface doorbell and
the message queues.
The system interface doorbell is a simple, message-passing mechanism
that allows the PCI host system and IOP to exchange single, 32-bit
dword messages. When the host system writes to the doorbell, the
LSI53C1020 hardware generates a maskable interrupt to the IOP, which
can then read the doorbell value and take the appropriate action. When
the IOP writes a value to the doorbell, the LSI53C1020 hardware
generates a maskable interrupt to the host system. The host system can
then read the doorbell value and take the appropriate action.
There are two 32-bit message queues: the request message queue and
the reply message queue. The host uses the request queue to request
an action by the LSI53C1020, and the LSI53C1020 uses the reply queue
to return status information to the host. The request message queue
consists of only the request post FIFO. The reply message queue
consists of both the reply post FIFO and the reply free FIFO. The shared
RAM contains the message queues.
Communication using the message queues occurs through request
messages and reply messages. Request message frame descriptors are
pointers to the request message frames and are passed through the
request post FIFO. The request message frame data structure is up to
128 bytes in length and includes a message header and a payload. The
header uniquely identifies the message. The payload contains
information that is specific to the request. Reply message frame
descriptors have one of two formats and are passed through the reply
post FIFO. When indicating the successful completion of a SCSI I/O, the
IOP writes the reply message frame descriptor using the Context Reply
format, which is a message context. If a SCSI I/O does not complete
successfully, the IOP uses the Address Reply format. In this case, the
IOP pops a reply message frame from the reply free FIFO, generates a
reply message describing the error, writes the reply message to system
memory, and writes the address of the reply message frame to the reply
post FIFO. The host can then read the reply message and take the
appropriate action.