[PATCH v1 2/2] drm/virtio: Add the hotplug_mode_update property for rescanning of modes

[Gerd Hoffmann]

On Fri, Jan 06, 2023 at 10:35:15AM +0100, Daniel Vetter wrote:
> On Fri, Jan 06, 2023 at 09:56:40AM +0100, Gerd Hoffmann wrote:
> > On Thu, Nov 17, 2022 at 05:30:54PM -0800, Vivek Kasireddy wrote:
> > > Setting this property will allow the userspace to look for new modes or
> > > position info when a hotplug event occurs.
> > 
> > This works just fine for modes today.
> > 
> > I assume this is this need to have userspace also check for position
> > info updates added by patch #1)?
> 
> What does this thing even do? Quick grep says qxl and vmwgfx also use
> this, but it's not documented anywhere, and it's also not done with any
> piece of common code. Which all looks really fishy.

It's again a virtualization-specific thing.  On physical hardware you
typically have no idea which of your two monitors stands left and which
stands right.  On virtual hardware the host knows how the two windows
for the two heads are arranged and can pass on that information to the
guest.  suggested_x/y properties added by patch #1 do pass that
information to userspace so the display server can arrange things
correctly without manual invention.

I have no clue though why this hotplug_mode_update property exists in
the first place and why mutter checks it.  IMHO mutter could just check
for suggested_x/y directly.

take care,
  Gerd