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

[Marcel Apfelbaum]

On Mon, 2014-04-07 at 16:51 +0300, Marcel Apfelbaum wrote:
> On Mon, 2014-04-07 at 16:34 +0300, Michael S. Tsirkin wrote:
> > On Mon, Apr 07, 2014 at 02:44:06PM +0200, Gerd Hoffmann wrote:
> > >   Hi,
> > > 
> > > > > +        u8 shpc_cap = pci_find_capability(s->bus_dev, 
> > > > > PCI_CAP_ID_SHPC);
> > > 
> > > > One thing I'd do is maybe check that the relevant memory type is
> > > > enabled in the bridge (probably just by writing fff to base and reading
> > > > it back).
> > > 
> > > > This will give hypervisors an option to avoid wasting resources:
> > > > e.g. it's uncommon for express devices to claim IO.
> > > 
> > > 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?
> 

I signed too soon :), I have another question below,
> Thanks,
> Marcel
> 
[...]
> > 
> > So we should probe bridge for I/O support before wasting I/O resources on 
> > it.
> > The spec does not provide a way to detect this, but we can do it like this:
> > 
> >     - write value ffffffff to I/O base register
> Why write? A simple read would be enough.
> It will never be 0(if I/O or mem is required) because of the "Base Address"
> part of the register which represents the address range, right?
Here ^^^

> 
> Thanks,
> Marcel
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