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[Xen-devel] Re: [SeaBIOS] [PATCH] Consider a VGA to be active if it responds to either IO or memory access

On Fri, 2011-05-27 at 16:35 +0100, Marc Jones wrote:
> On Fri, May 27, 2011 at 2:44 AM, Ian Campbell
> <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> > On Fri, 2011-05-27 at 01:59 +0100, Kevin O'Connor wrote:
> >> On Thu, May 26, 2011 at 03:43:28PM +0100, Ian Campbell wrote:
> >> > Under Xen the VGA card ends up configured for memory access only.
> >>
> >> That's odd.  How does the vga bios work if the Xen VGA device doesn't
> >> handle inb/outb accesses?
> >
> > I'm not entirely sure, but it does...
> >
> > The Xen VGA device is the Cirrus GD 5446 provided by qemu. The I/O ports
> > are the VGA control registers at 0x3xx which are not covered by any PCI
> > BARS (FWIW this device has no I/O BARS, and two memory BARS).
> >
> > It's not obvious whether the I/O space enable bit in the PCI cfg command
> > register is supposed to control the availability of non-PCI registers or
> > not. Neither the PCI spec nor the GD-5446 datasheet are particularly
> > clear on the matter.
> >
> > I've just discovered that the ancient pentium box I use as a home
> > firewall has a GD 5446 in it (useful to know!), it doesn't have any I/O
> > BARS but it does have the I/O bit set in the PCI command register.
> >
> > It's not clear who was responsible for setting that bit, in general in
> > the absence of an I/O BAR the BIOS wouldn't know to do so. I expect that
> > either the VGA BIOS is expected to enable it if the hardware it drives
> > requires it or that BIOSen special case class=VGA devices and always
> > enable I/O for one of them.
> >
> > It looks like coreboot always forces this bit on for the VGA device
> > which it determines to be the primary, which is good enough for me --
> > I'll make a patch to the Xen pci setup code to implement that instead of
> > this change to SeaBIOS.
> >
> Hi Ian,
> There are some exceptions around VGA and PCI. The primary display is
> usually the first VGA device found, by checking device class code.
> Enabling the IO bit on a VGA device causes it to claim the legacy
> cycles. It doesn't matter if the device has an IO bar.
> If the VGA device also has the device's palette snoop bit set, it
> claims reads and snarfs writes allowing a downstream device to also
> get the writes. Note that there is also the VGA enable to be set on
> bridges above the device and VGA palette snoop for bridges that a VGA
> device is behind below the primary device bus.

Useful info, thanks Marc!


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