Re: [Xen-devel] [RFC][PATCH] gpu:drm:i915:intel_detect_pch: back to check devfn instead of check class type

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

On 2014/6/20 20:32, Daniel Vetter wrote:
> Well I have no clue about forwarding the intel gpu to virtualized
> hosts and also no idea who could review this really. There's been a
> bit a discussion around the iommu mapping forwarding and similar
No, this doesn't affect IOMMU mapping.

> topics though. So I really wonder how well our driver works in this
> use case ...
In native case the truth is intel_detect_pch() needs to probe the 
00:15.0 to determine what type current pch is, then the i915 driver can 
be initialized correctly. Actually the 00:15.0 is just a ISA bridge.
In virtualized case we thought this ISA bridge mayn't be represented 
with 00:15.0 originally by qemu-xen-traditional. So instead of checking 
devfn, the i915 driver check the class type to get this ISA bridge.
But with my investigation,qemu-xen-traditional always set 00:15.0 
explicitly to represent this ISA bridge. And especially, we wouldn't 
provide that ISA bridge with an explicit class type in qemu-upstream, so 
we need to the i915 driver to probe pch by checking devfn.
This should work both on the native case and the virtualized case.

Thanks
Tiejun

> -Daniel
>
> On Fri, Jun 20, 2014 at 11:40 AM, Chen, Tiejun <tiejun.chen@xxxxxxxxx> wrote:
> > Just ping, any comments?
> >
> > Thanks
> > Tiejun
> >
> >
> > On 2014/6/19 17:53, Tiejun Chen wrote:
> > > Originally the reason to probe ISA bridge instead of Dev31:Fun0
> > > is to make graphics device passthrough work easy for VMM, that
> > > only need to expose ISA bridge to let driver know the real
> > > hardware underneath. This is a requirement from virtualization
> > > team. Especially in that virtualized environments, XEN, there
> > > is irrelevant ISA bridge in the system with that legacy qemu
> > > version specific to xen, qemu-xen-traditional. So to work
> > > reliably, we should scan through all the ISA bridge devices
> > > and check for the first match, instead of only checking the
> > > first one.
> > >
> > > But actually, qemu-xen-traditional, is always enumerated with
> > > Dev31:Fun0, 00:1f.0 as follows:
> > >
> > > hw/pt-graphics.c:
> > >
> > > intel_pch_init()
> > >       |
> > >       + pci_isa_bridge_init(bus, PCI_DEVFN(0x1f, 0), ...);
> > >
> > > so this mean that isa bridge is still represented with Dev31:Func0
> > > like the native OS. Furthermore, currently we're pushing VGA
> > > passthrough support into qemu upstream, and with some discussion,
> > > we wouldn't set the bridge class type and just expose this devfn.
> > >
> > > So we just go back to check devfn to make life normal.
> > >
> > > Signed-off-by: Tiejun Chen <tiejun.chen@xxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/i915/i915_drv.c | 19 +++----------------
> > >    1 file changed, 3 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/i915_drv.c
> > > b/drivers/gpu/drm/i915/i915_drv.c
> > > index 651e65e..cb2526e 100644
> > > --- a/drivers/gpu/drm/i915/i915_drv.c
> > > +++ b/drivers/gpu/drm/i915/i915_drv.c
> > > @@ -417,18 +417,8 @@ void intel_detect_pch(struct drm_device *dev)
> > >                  return;
> > >          }
> > >
> > > -       /*
> > > -        * The reason to probe ISA bridge instead of Dev31:Fun0 is to
> > > -        * make graphics device passthrough work easy for VMM, that only
> > > -        * need to expose ISA bridge to let driver know the real hardware
> > > -        * underneath. This is a requirement from virtualization team.
> > > -        *
> > > -        * In some virtualized environments (e.g. XEN), there is
> > > irrelevant
> > > -        * ISA bridge in the system. To work reliably, we should scan
> > > trhough
> > > -        * all the ISA bridge devices and check for the first match,
> > > instead
> > > -        * of only checking the first one.
> > > -        */
> > > -       while ((pch = pci_get_class(PCI_CLASS_BRIDGE_ISA << 8, pch))) {
> > > +       pch = pci_get_bus_and_slot(0, PCI_DEVFN(0x1f, 0));
> > > +       if (pch) {
> > >                  if (pch->vendor == PCI_VENDOR_ID_INTEL) {
> > >                          unsigned short id = pch->device &
> > > INTEL_PCH_DEVICE_ID_MASK;
> > >                          dev_priv->pch_id = id;
> > > @@ -462,10 +452,7 @@ void intel_detect_pch(struct drm_device *dev)
> > >                                  DRM_DEBUG_KMS("Found LynxPoint LP PCH\n");
> > >                                  WARN_ON(!IS_HASWELL(dev));
> > >                                  WARN_ON(!IS_ULT(dev));
> > > -                       } else
> > > -                               continue;
> > > -
> > > -                       break;
> > > +                       }
> > >                  }
> > >          }
> > >          if (!pch)
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