Re: [Qemu-devel] [SeaBIOS] [PATCH] hw/pci: reserve IO and mem for pci-2-pci bridges with no devices attached

[Michael S. Tsirkin]

On Mon, Apr 07, 2014 at 05:16:13PM +0300, Marcel Apfelbaum wrote:
> On Mon, 2014-04-07 at 17:09 +0300, Michael S. Tsirkin wrote:
> > On Mon, Apr 07, 2014 at 04:51:54PM +0300, Marcel Apfelbaum wrote:
> [...]
> > > > > I don't think we'll need that for the SHPC bridge.
> > > > 
> > > > Why not?
> > > Because "has shpc" => not an PCIe port. (as far as I know)
> > > Anyway, why have shpc capability but no I/O or mem to support it?
> ^^^
> [...]
> > 
> > 
> > AFAIK the spec does not list reset value for this register.
> > IIRC QEMU resets both to 0.
> Thanks, what do you think about the above ? ^^^
> While I am not against it, it seems redundant.
> It has shpc => it needs I/O or mem space.
> 
> Marcel

Fair enough but it can have shpc and memory without io,
or shpc and io without memory.

> > 
> > 
> > > >         - read back value
> > > > 
> > > > value 0 means bridge does not support I/O.
> > > > 
> > > > 
> > > > A similar trick should work for other optional resources.
> > > > 
> > > > 
> > > > > For express it indeed makes sense to avoid claiming IO address space.
> > > > > I'd try to find something more automatic though, where you don't need
> > > > > some kind of "disable io for this express port" config option.
> > > > 
> > > > Won't same trick as above work?
> > > > 
> > > > > For express ports which can only have a single device underneath we 
> > > > > can
> > > > > check whenever we have a device and if one is present already don't
> > > > > bother claiming extra resources for hotplug.
> > > > > 
> > > > > > > +    for (cap = pci_config_readb(pci->bdf, PCI_CAPABILITY_LIST); 
> > > > > > > cap;
> > > > > > > +                cap = pci_config_readb(pci->bdf, cap + 
> > > > > > > PCI_CAP_LIST_NEXT))
> > > > > > > +        if (pci_config_readb(pci->bdf, cap + PCI_CAP_LIST_ID) == 
> > > > > > > cap_id)
> > > > > > > +            return cap;
> > > > > > 
> > > > > > I would also limit this to 256 iterations, to make sure
> > > > > > we dont' get into an infinite loop with a broken device.
> > > > > 
> > > > > Good point.
> > > > > 
> > > > > cheers,
> > > > >   Gerd
> > > > > 
> > > 
> > > 
> 
>