Re: [Qemu-devel] [PATCH v5 2/2] enable multi-function hot-add

[Michael S. Tsirkin]

On Tue, Oct 27, 2015 at 05:39:01PM +0800, Cao jin wrote:
> >>>static
> >>>bool pcie_has_upstream_port(PCIDevice *dev)
> >>>{
> >>>     PCIDevice *parent_dev = pci_bridge_get_device(pci_dev->bus);
> >>>
> >>>     /*
> >>>      * Device associated with an upstream port.
> >>>      * As there are several types of these, it's easier to check the
> >>>      * parent device: upstream ports are always connected to
> >>>      * root or downstream ports.
> >>>      */
> >>>     return parent_dev &&
> >>>       pci_is_express(parent_dev) &&
> >>>           parent_dev->exp.exp_cap &&
> >>>           (pcie_cap_get_type(parent_dev) == PCI_EXP_TYPE_ROOT_PORT ||
> >>>           pcie_cap_get_type(parent_dev) == PCI_EXP_TYPE_DOWNSTREAM);
> >>>}
> >>>
> >>
> >>Assume my understanding is right, which means both ARI and non-ARI device
> >>have the upstream port(root port or downstream port), could the existence of
> >>upstream port be the judgment condition?
> >
> >This tells us whether we are behind a port that
> >can address devices in slot != 0.
> >
> >
> 
> Seems I find something, according to spec: Endpoints are classified as
> either legacy, PCI Express, or Root Complex Integrated Endpoints. I think
> this is also what you means in comment "As there are several types of these"
> And "we are behind a port" means the device is not a Root Complex
> Integrated, only the other two types can be behind a port.
> 
> Am I right about this?

No, device can be a bridge itself. It's simple: root and downstream
ports only have 1 slot (and it's a physical one) so you know there's
always 1 device there. It could have multiple functions.

-- 
MST