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

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Date:	Mon, 30 Jun 2014 14:18:20 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Tiejun Chen <tiejun.chen@...el.com>
Cc:	daniel.vetter@...ll.ch, jani.nikula@...ux.intel.com,
	airlied@...ux.ie, intel-gfx@...ts.freedesktop.org,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, qemu-devel@...gnu.org
Subject: Re: [RFC][PATCH] gpu:drm:i915:intel_detect_pch: back to check devfn
 instead of check class type

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@...el.com>
> ---
>  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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