Re: [Qemu-devel] [PATCH V2] pci: fixes to allow booting from extra root pci buses.

[Marcel Apfelbaum]

On 06/11/2015 05:24 PM, Kevin O'Connor wrote:
> On Thu, Jun 11, 2015 at 05:12:33PM +0300, Marcel Apfelbaum wrote:
> > On 06/11/2015 04:58 PM, Kevin O'Connor wrote:
> > > On Thu, Jun 11, 2015 at 04:37:08PM +0300, Marcel Apfelbaum wrote:
> > > > The fixes solves the following issue:
> > > > The PXB device exposes a new  pci root bridge with the
> > > > fw path:  /address@hidden/..., in which 4 is the root bus number.
> > > > Before this patch the fw path was wrongly computed:
> > > >      /address@hidden/address@hidden/...
> > > > Fix the above issues: Correct the bus number and remove the
> > > > extra host bridge description.
> > >
> > > Why is that wrong?  The previous path looks correct to me.
> > The prev path includes both the extra root bridge and *then* the usual host 
> > bridge.
> >   /address@hidden/address@hidden/   ...
> >      ^ new       ^ regular  ^ devices
> >
> > Since the new pci root bridge (and bus) is on "paralel" with the regular one.
> > it is not correct to add it to the path.
> >
> > The architecture is:
> >   /<host bridge>/devices...
> >   /extra root bridge/devices...
> >   /extra root bridge/devices...
> > And not
> > /extra root bridge//<host bridge>/devices
>
> Your patch changed both the "/extra root bridge/devices..." part and
> the "@1" part.  The change of the "@1" in "/address@hidden/" is not
> correct IMO.
Why? @1 should be the unit address which is the text representation
of the physical address, in our case the slot. Since the bus number
in our case is 4, I think /address@hidden/ is the 'correct' address.
> Does open-firmware have any examples for PCI paths and in particular
> PCI paths when there are multiple root-buses?
Maybe Laszlo can say more, but we both agreed that this would be the
berst representation of extra root buses on both OVMF and Seabios.
> It's possible to replace the "address@hidden" with "address@hidden" but that
> seems odd as the extra root bus is accessible via io accesses to
> 0x0cf8.
While this is true, /address@hidden/ may represent also other kind of host
bridges not only PXBs. But we can change this of course, as long as OVMF can 
also
work with it.

> Another option would be to place the address@hidden behind the address@hidden
> as in "/address@hidden/address@hidden/...".  Or, the root bus could be appended
> to the host bridge as in "/address@hidden,1/...".
The latest representation makes sense to me,  but "/address@hidden,4/...", 
after comma
the bus number.

Laszlo, will this work for OVMF?

Thanks,
Marcel

> -Kevin