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?
Thanks, Marcel > I'm referring to this text in the bridge specification: > > The I/O Base and I/O Limit registers are optional and define an address > range that is used > by the bridge to determine when to forward I/O transactions from one > interface to the > other. > If a bridge does not implement an I/O address range, then both the I/O > Base and I/O > Limit registers must be implemented as read-only registers that return > zero when read. If a > bridge supports an I/O address range, then these registers must be > initialized by > configuration software so default states are not specified. > > 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? Thanks, Marcel > - 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 > >
