multiple separate pci bridges ...

Benjamin Herrenschmidt benh at
Fri Jan 2 15:03:43 EST 2004

On Fri, 2004-01-02 at 05:11, Sven Luther wrote:
> I am currently trying to port linux to the Pegasos 2, which uses the
> Marvell Discovery 2 chip, and has two independent pci controllers,
> of which one is faked as an agp bus. This is with a modified 2.4.23
> kernel from the linuxppc_2_4 branch.

Why "faked" ? It's actually fairly sane to have the AGP bus be a
separate PCI host controller... That's how it's done on pmacs.

> Anyway, these two independent controllers both have one bus 0 on them :
> PCI bus 0 controlled by pci at 80000000
> PCI bus 0 controlled by pci at c0000000
> (This coming from chrp_find_bridges).

Typical junk, Apple does the same. You want PCI domains or bus
renumbering. No PCI domains in 2.4 though, so you want bus renumbering.
Look at what pmac_pci.c does for uninorth which has 3 root busses whose
firmware bus numbers all start at 0

> For the pci bus, at 80000000, a simple indirect access can be used,
> and i setup a specialized io ops for the faked agp bus since it needs
> special care.

What do you mean ? config ops ? You can have different ops per host,
again, look at what pmac_pci.c does

> Later, in pcibios_init, there is a problem in the pci_scan_bus. The
> first bus has no problems :
> Scanning bus 00
> Found 00:00 [11ab/6460] 000600 00
> Found 00:08 [1106/3044] 000c00 00
> Found 00:28 [1000/0001] 000100 00
> Found 00:60 [1106/8231] 000601 00
> Found 00:61 [1106/0571] 000101 00
> Found 00:62 [1106/3038] 000c03 00
> Found 00:63 [1106/3038] 000c03 00
> Found 00:64 [1106/8235] 000000 00
> Found 00:65 [1106/3058] 000401 00
> Found 00:66 [1106/3068] 000780 00
> Found 00:68 [1106/3065] 000200 00
> Fixups for bus 00
> Bus scan for 00 returning with max=00
> But the second fails with :
> PCI: Bus 00 already known
> Which comes because in drivers/pci/pci.c:pci_bus_exists does handle
> only buses, and thus know nothing of separate pci bridges with the
> same bus number, and thus the kernel dies when accessing bus->xxx
> something in pcibios_init.

Yes, in 2.4, you need to do some renumbering.

> Now, i know that the powermacs in particular, and maybe others, also
> have this case of different same numbered pci buses with different
> base addresses, and that this kind of situation has already been
> solved.
> So, what is the workaround here, and where does it get set.

Basically, you do the bus numbering yourself. You never care about the
number of the "root" bus since you basically just send type 0 config
cycles to it, but you do care about bus numbers in PCI<->PCI bridges.
You trigger the proper renumbering code by setting pci_assign_all_busses
to 1.

So basically, what we do is, we keep a "next_busno" variable which is
0 at first. When probing a host, we assign it that bus number (0 for
the first host typically). Then, we call pci_scan_bus(). This one will
do the renumbering for P2P bridges when pci_assign_all_busses is true.
It will also tell us what was the "last" bus number found on the host.
We use that+1 to get the new next_busno and start again for the next

On your side, you don't have much else to do that set
pci_assign_all_busses to 1, and create 2 pci_controller structures with
different config cycle ops.

Note that in 2.4, there is a small risk when doing bus renumbering with
PCI-PCI bridges that you can get temporarily overlapping bus numbers
during the probe, thus screwing it up. this typically happens on
Xserves. The problem is when scanning a given segment with P2P bridges,
when old & new numbers overlap, a not-yet scanned bridge may be set to
the same number as one we are currently scanning after having renumbered
it, thus screwing things up. I think 2.6 is protected against this
problem. For 2.4, what I do is hack to force a 0x10 bus number offset
between hosts.


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