Re: [Qemu-ppc] [Qemu-devel] [PATCH 2/3] mac_newworld: enable access to EDID data for the VGA device

[Gerd Hoffmann]

On Wed, Dec 12, 2018 at 08:54:37AM +0000, Mark Cave-Ayland wrote:
> On 12/12/2018 08:32, Gerd Hoffmann wrote:
> 
> > On Fri, Dec 07, 2018 at 04:08:05PM +0000, Mark Cave-Ayland wrote:
> >> This is in preparation for some upcoming QEMU NDRV driver changes that pass
> >> display information from the host to the guest.
> > 
> >> -    pci_vga_init(pci_bus);
> >> +    dev = qdev_create(BUS(pci_bus), "VGA");
> >> +    qdev_prop_set_int32(dev, "addr", -1);
> >> +    qdev_prop_set_bit(dev, "edid", true);
> >> +    qdev_init_nofail(dev);
> > 
> > Hmm.  IMO you should not overwrite the device defaults here.
> > 
> > edid is off by default only because it is new and I'm conservative.
> > I want a release (or two) with it being available for user testing.
> > If no issues pop up flip it to default on.
> 
> Oh, okay. I already have some unreleased guest code that makes use of this, 
> so my
> questions would be: how can EDID be enabled from the command line for 
> in-built VGA
> devices,

"-global VGA.edid=on" should do.

> and how do I detect whether EDID support is present from the guest?

Check whenever the header is present.  The edid blob has a fixed 8 byte
header (00 FF FF FF FF FF FF 00).  The linux kernel driver is lazy and
checks the first two bytes only.

> Otherwise a guest driver that assumes it is always present and tries
> to read from that area of memory will crash.

Oh, reading should work no matter what.  You just don't find valid edid
data there if it is turned off.

cheers,
  Gerd