HELP: Power Mac G3 PCI Strangeness in 2.2 and 2.3

[Geert Uytterhoeven]

On Sun, 5 Mar 2000, Grant Erickson wrote:
> After modifying the Linux kernel to accept PCI devices with the invalid
> vendor/device ID combination of 0xffff/0x0000, I noticed something odd
> about the way the system PROM (and to a lesser extent Linux) configured
> the chip's base addresses. Looking at a number of other devices in the
> system:
>
>         grant at marathon% cat /proc/iomem
>         81800000-8180001f : PCI device 1022:2000 AMD Lance Ethernet
>         81900000-819fffff : PCI device 1022:2000 AMD Lance Ethernet
>         82000000-82ffffff : PCI device 1002:4754 ATI Mach 128
>         f3000000-f307ffff : PCI device 106b:0010 Apple Heathrow I/O
>         fffc0000-ffffffff : PCI device ffff:0000 ???
>
> Clue 1: The address range programmed for the chip looks suspiciously like
> the state it might be left in after determining how much address space it
> needs.

Yes.

> Before Linux does this address space sizing, it saves the base address.
> In the case of my chip, it would have read "0x0" since nothing was
> programmed there previously. If Linux does find 0x0, it just leaves the
> result of the address sizing there and moves on.
>
> So, that led me to wonder where the addresses in the other devices were
> coming from...
>
> I found that even before Linux executes a single line of code, all the
> other devices above have valid addresses programmed into their base
> address registers. Who's doing this?
>
> I believe that the Open Firmware PROM in the PowerPC system probes all
> PCI devices in the system at power up. It then assigns valid base
> addresses to those devices with VALID vendor and device IDs.

OF (should) assign addresses. On my box (CHRP), it does so for _recognized_
devices only.

> Because my chip uses an invalid vendor ID of 0xFFFF, Open Firmware must
> just ignore it and never programs in a valid base address! Therefore,
> when we go and try to access the chip at 0xFFFC0000 (which is outside the
> valid PCI address space on PReP/CHRP/PowerMac systems?) we're off probing
> never-never land. The PCI controller never assumes the transaction!

How difficult is it to make the PCI chip use a valid vendor/device ID? I don't
think you can assume any boot ROM will try to assign a region to it. There's no
way to distinguish a device with an invalid vendor/device ID from a device
that's not even present.

Go read the PCI spec and use valid IDs.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven ------------- Sony Software Development Center Europe (SDCE)
Geert.Uytterhoeven at sonycom.com ------------------- Sint-Stevens-Woluwestraat 55
Voice +32-2-7248638 Fax +32-2-7262686 ---------------- B-1130 Brussels, Belgium


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