Re: [Qemu-devel] [RFC PATCH] pci: Differentiate PCI Express bus

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 12, 2013 at 09:36:49AM -0600, Alex Williamson wrote:
> On Tue, 2013-03-12 at 16:46 +0200, Michael S. Tsirkin wrote:
> > On Mon, Mar 11, 2013 at 03:18:49PM -0600, Alex Williamson wrote:
> > > When creating capabilities devices need to know what kind of bus
> > > they're on.  If we're on an express bus without a parent_dev, then
> > > we're on the root complex and need to use integrated endpoints
> > > rather than standard endpoints.  When we're on an express bus with
> > > a parent_dev we need to negotiate link parameters so that the
> > > endpoint doesn't claim it's running x16, 8GT/s while the root port
> > > above it claims x1, 2.5GT/s capability.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > ---
> > > 
> > > This feels a bit kludgy so I'm sending this out as an RFC looking for
> > > suggestions.  I played a little with creating a PCIBusClass, putting
> > > is_express on the class,
> > 
> > You actually don't even need is_express if you do this, just
> > add a wrapper that checks the type.
> 
> Yeah, that's true.
> 
> > > and instantiating a TYPE_PCIE_BUS that uses
> > > TYPE_PCI_BUS as it's parent, but that gets overly complicated and
> > > means that any time we instantiate a bus we need to figure out whether
> > > to use legacy or express.
> > 
> > This last is probably a plus, not a minus.
> 
> Ok, it just means either duplicating a lot of interfaces to make legacy
> vs express functions or adding ugly is_express options to the functions
> (ex. pci_bus_new).  Preference?

If we do this I'd prefer a type flag I think. But see below.

> > > Any better ideas?  Thanks,
> > > 
> > > Alex
> > 
> > If I understand correctly, the issue is that the root bus does not
> > have a parent device?
> 
> No, that's actually a feature that lets us determine if the express bus
> is a root complex bus or normal express bus.  I'm worried about things
> like xhci which calls pcie_cap_init() to give itself an endpoint
> capability.  If it's connected to the root complex, that actually needs
> to be an integrated endpoint or else windows won't use it.  Devices
> probably don't want to care whether they're an endpoint or integrated
> endpoint, so pcie_cap_init() should probably transparently change types
> and drop the link capabilities.

Confused. Root complex bus doesn't have a parent, isn't this
enough?

> We also need to add pseudo link negotiation.  Root ports and switches
> should report maximum width and transfer rate capability and downstream
> devices should call a function to negotiate link to the device
> capabilities.  I don't think we can just look for is_express on
> bus->parent_dev because the parent_dev could be a PCIe-to-PCI bridge.

That's easy to detect though, right?

> So there seem to be a number of points where it would be convenient to
> test express vs legacy on the bus.
> 
> > >  hw/ioh3420.c            |    2 ++
> > >  hw/pci/pci_bus.h        |    2 ++
> > >  hw/q35.c                |    1 +
> > >  hw/xio3130_downstream.c |    2 ++
> > >  hw/xio3130_upstream.c   |    2 ++
> > >  5 files changed, 9 insertions(+)
> > 
> > This isn't too bad though I'd prefer accessing is_express
> > through an API. E.g.
> > pci_bus_set_type()
> 
> Yep, this is just an example of the minimum footprint for such a change
> pushing it as far out from the core as we can.  If you're onboard that
> we need a way to differentiate the bus type I can make it more
> integrated into the core.  Thanks,
> 
> Alex
> 
> > > diff --git a/hw/ioh3420.c b/hw/ioh3420.c
> > > index 95bceb5..186a46f 100644
> > > --- a/hw/ioh3420.c
> > > +++ b/hw/ioh3420.c
> > > @@ -102,6 +102,8 @@ static int ioh3420_initfn(PCIDevice *d)
> > >          return rc;
> > >      }
> > >  
> > > +    br->sec_bus->is_express = true;
> > > +
> > >      pcie_port_init_reg(d);
> > >  
> > >      rc = pci_bridge_ssvid_init(d, IOH_EP_SSVID_OFFSET,
> > > diff --git a/hw/pci/pci_bus.h b/hw/pci/pci_bus.h
> > > index aef559a..a325f46 100644
> > > --- a/hw/pci/pci_bus.h
> > > +++ b/hw/pci/pci_bus.h
> > > @@ -34,6 +34,8 @@ struct PCIBus {
> > >         Keep a count of the number of devices with raised IRQs.  */
> > >      int nirq;
> > >      int *irq_count;
> > > +
> > > +    bool is_express; /* PCI Express bus or Legacy bus? */
> > >  };
> > >  
> > >  typedef struct PCIBridgeWindows PCIBridgeWindows;
> > > diff --git a/hw/q35.c b/hw/q35.c
> > > index efebc27..f5fdcb0 100644
> > > --- a/hw/q35.c
> > > +++ b/hw/q35.c
> > > @@ -55,6 +55,7 @@ static int q35_host_init(SysBusDevice *dev)
> > >      }
> > >      b = pci_bus_new(&s->host.pci.busdev.qdev, "pcie.0",
> > >                      s->mch.pci_address_space, s->mch.address_space_io, 0);
> > > +    b->is_express = true;
> > >      s->host.pci.bus = b;
> > >      qdev_set_parent_bus(DEVICE(&s->mch), BUS(b));
> > >      qdev_init_nofail(DEVICE(&s->mch));
> > > diff --git a/hw/xio3130_downstream.c b/hw/xio3130_downstream.c
> > > index 7f00bc8..600ec06 100644
> > > --- a/hw/xio3130_downstream.c
> > > +++ b/hw/xio3130_downstream.c
> > > @@ -66,6 +66,8 @@ static int xio3130_downstream_initfn(PCIDevice *d)
> > >          return rc;
> > >      }
> > >  
> > > +    br->sec_bus->is_express = true;
> > > +
> > >      pcie_port_init_reg(d);
> > >  
> > >      rc = msi_init(d, XIO3130_MSI_OFFSET, XIO3130_MSI_NR_VECTOR,
> > > diff --git a/hw/xio3130_upstream.c b/hw/xio3130_upstream.c
> > > index 70b15d3..b6fea60 100644
> > > --- a/hw/xio3130_upstream.c
> > > +++ b/hw/xio3130_upstream.c
> > > @@ -62,6 +62,8 @@ static int xio3130_upstream_initfn(PCIDevice *d)
> > >          return rc;
> > >      }
> > >  
> > > +    br->sec_bus->is_express = true;
> > > +
> > >      pcie_port_init_reg(d);
> > >  
> > >      rc = msi_init(d, XIO3130_MSI_OFFSET, XIO3130_MSI_NR_VECTOR,
> 
>