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

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Date:	Tue, 24 Jun 2014 10:59:44 +0800
From:	Zhenyu Wang <zhenyuw@...ux.intel.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: [Intel-gfx] [RFC][PATCH] gpu:drm:i915:intel_detect_pch: back to
 check devfn instead of check class type

On 2014.06.19 17:53:51 +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>

This was added historically when supporting graphics device passthrough.
Looks qemu upstream can't accept multiple ISA bridge and our PCH is always
on device 31: func0 as far as I know. Looks good to me.

Reviewed-by: Zhenyu Wang <zhenyuw@...ux.intel.com> 

-- 
Open Source Technology Center, Intel ltd.

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