[[!meta title="G-REX"]]

Programming the G-REX PCI bridge

document version 0.1 - THIS IS A WORK IN PROGRESS!

# 0. Introduction

This document describes software/hardware interface of the G-REX PCI bridge for Amiga computers. What you're
reading is a result of reverse engineering, which was long and difficult process. 

Next time when you're going to buy a hardware product for your Amiga, don't forget to ask the vendor to make the
programming documentation publicly available! Remeber that hardware without software is just a piece of junk...
and you can't write software without hardware documentation.

In case you've noticed an error in this document please let me know. 

# 1. Theory of operation

G-REX is an evolution of PCI bridge used previously on CyberVisionPPC and 
BlizzardVisionPPC cards. These products share a lot of similiarities (at 
least when it comes to PCI interface). 

Firmware does the dirty job of assigning PCI resources (BARs, interrupt lines, 
etc.) before the OS is running. Therefore G-REX does not need any special 
initialization.

# 2. Memory map

G-REX is configured as multipie AutoConf boards. Confusingly, they all have the same vendor (8512) and product (101).

0xFFFA0000 - PCI I/O register space, 64KB.

0xFFFC0000 - PCI configuration space, 128KB.

0xFFFE0000 - Bridge configuration registers, 4kB.

0x80000000 - PCI memory space, variable size and number of boards, depending on cards installed. 

# 2a. PCI configuration space

Access to configuration space is a bit tricky. Be warned that access to 
addresses not used by G-REX generates bus error (esp. to configuration 
locations which are unused because there is no card in the slot). Depending on 
how these errors are supported in your OS, it may be important to trap them and
handle correctly. 

Configuration data for first slot seems to be accessible at +0x1000.

[TO BE COMPLETED]

# 2b. PCI I/O registers space

This space offers access to I/O registers of all PCI cards.

BAR addresses in this space are treated as relative to 0xFFFA0000. Card with 
I/O BAR set to 0x100 will actually be available at 0xFFFA0100. 

# 2c. PCI memory space 

This space offers access to memory (and memory-mapped registers) of PCI cards. 
Each PCI memory BAR is assigned a separate AutoConf board during firmware 
initialization. 

Addresses in this space are treated as absolute. Memory BAR register set to 
0x80000000 means it is configured at this address.  

# 2d. Bridge configuration registers

Offset - meaning

0x0000 - Endianness swapper mode, write 0x02 to switch bridge into big endian mode

0x0010 - Interrupt enable, write 0x01 to enable interrupts (INT2 on Amiga side)

No need to fiddle with these registers, as they've been already configured properly by the firmware.

# 3. Reconfiguring the bus.

If needed, it's possible to reconfigure bus just by writing new values into 
configuration space. Keep in mind that any previously initialized chips will 
need to be reset and initialized again (for example 3Dfx Voodoo 3, which is
initialized by the firmware so it can display early startup menu). 

# 4. Interrupts

All interrupts are converted into Amiga INT2 interrupt. There's no such thing 
as interrupt acknowledge register.

# 5. DMA

The bridge is certainly capable of DMA, but it needs further reverse 
engineering.

[TO BE COMPLETED]

There were at least two different revisions of G-REX 1200. Later revision 
probably does support DMA in two slots.

G-REX 4000D probably has busmaster DMA capability in all slots.

# 6. Sample PCI bridge driver implementation

The NetBSD [[p5pb|http://netbsd.gw.com/cgi-bin/man-cgi?p5pb+4.amiga+NetBSD-current]] 
driver serves as an example driver implementation. It was written using the 
same knowledge that went into this document.

The driver consists of several files in [[src/sys/arch/amiga/pci|http://nxr.netbsd.org/xref/src/sys/arch/amiga/pci/]] directory.

p5membar.c - Dummy driver handling AutoConf resources.
p5membarvar.h - Structures used by the p5membar.
p5pb.c - Main driver code.
p5pbreg.h - Inlcude file containing register locations.
p5pbvar.h - Structures used by the p5pb.

The p5pb does attach on top of p5bus, however p5membar drivers attach on top of zbus (since 8512/101 entries are seen as Zorro boards).