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

[Daniel Vetter]

On Tue, Jan 10, 2023 at 10:28:39AM +0100, Gerd Hoffmann wrote:
> 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.

Yeah suggested_x/y I know about. Would still be good to fix the
documentation situation for that.

> 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.

This one is the complete wtf here. I have no idea why this exists.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch