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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Thu, Jun 19, 2014 at 05:53:51PM +0800, 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) {

Then if you want to use this slot for something else, what happens?
If you want to relax the PCI_CLASS_BRIDGE_ISA requirement when
running on top of a hypervisor, just scan all devices.

>               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)
> -- 
> 1.9.1
> 

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