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From: Bjorn Helgaas <bhelgaas@google.com>
Date: Thu, 21 Aug 2014 23:39:31 -0500
Message-ID: <CAErSpo6D1B+dPLrYoAF4OA1KeWuW8VZW2_RR5jB1GbCRo+6vHg@mail.gmail.com>
Subject: Re: [PATCH 0/2] x86, ia64: Move EFI_FB vga_default_device()
 initialization to pci_vga_fixup()
To: =?UTF-8?Q?Bruno_Pr=C3=A9mont?= <bonbons@linux-vserver.org>
Cc: Andreas Noever <andreas.noever@gmail.com>,
	DRI mailing list <dri-devel@lists.freedesktop.org>,
	Linux PCI <linux-pci@vger.kernel.org>,
	Dave Airlie <airlied@linux.ie>,
	Matthew Garrett <matthew.garrett@nebula.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Thu, Aug 21, 2014 at 4:34 PM, Bruno Prémont
<bonbons@linux-vserver.org> wrote:

> A second step would then be to tune vgaarb's initial selection.
> Bjorn, is it possible to verify which I/O ports are decoded by a PCI
> device at the time of adding it to vgaarb? If so, how? I would like to
> check for legacy VGA I/O range (0x03B0-0x03DF) and only let vgaarb set
> a device as default if that I/O range is decoded by the device.

I don't know of a way.  I'm pretty sure VGA devices are allowed to
respond to those legacy addresses even if there's no BAR for them, but
I haven't found a spec reference for this.  There is the VGA Enable
bit in bridges, of course (PCI Bridge spec, sec 12.1.1.  If the VGA
device is behind a bridge that doesn't have the VGA Enable bit set, it
probably isn't the default device.

Bjorn