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

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Message-ID: <20140702062104.GA3874@redhat.com>
Date:	Wed, 2 Jul 2014 09:21:04 +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>

Making sure all relevant people are Cc'd.

I think we should wait for the hypervisor to settle
on what it wants to implement before making guest
changes. Otherwise we'll just add more guest configurations
that need to be supported.



> ---
>  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)
> -- 
> 1.9.1
> 
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