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

[Kasireddy, Vivek]

Hi Daniel,

> 
> 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.
[Kasireddy, Vivek] AFAIU, this property appears to be useful only for virtual
GPU drivers to share the Host output(s) layout with the Guest compositor. The
suggested_x/y properties are specifically used for this purpose but it looks like
the hotplug_mode_update property also needs to be set in order to have Guest
compositors (Mutter cares but Weston does not) look at suggested_x/y.

> 
> I think we need to do a bit of refactoring/documenting here first.
[Kasireddy, Vivek] Just for reference, here is Dave's commit that added this
property for qxl:
commit 4695b03970df378dcb93fe3e7158381f1e980fa2
Author: Dave Airlie <airlied at redhat.com>
Date:   Fri Oct 11 11:05:00 2013 +1000

    qxl: add a connector property to denote hotplug should rescan modes.

    So GNOME userspace has an issue with when it rescans for modes on hotplug
    events, if the monitor has no EDID it assumes that nothing has changed on
    EDID as with real hw we'd never have new modes without a new EDID, and they
    kind off rely on the behaviour now, however with virtual GPUs we would
    like to rescan the modes and get a new preferred mode on hotplug events
    to handle dynamic guest resizing (where you resize the host window and the
    guest resizes with it).

    This is a simple property we can make userspace watch for to trigger new
    behaviour based on it, and can be used to replaced EDID hacks in virtual
    drivers.

Are you suggesting that this property needs to be part of drm_mode_config
just like suggested_x/y properties?

> 
> Also in principle, userspace needs to look at everything in the connector
> again when it gets a hotplug event. We do have hotplug events for specific
> properties nowadays, but those are fairly new.
[Kasireddy, Vivek] From what I understand, Mutter does probe all the
connector properties during hotplug but it still needs this property to be set in
order to consider suggested_x/y values. And, it appears, some customers and
users have relied on this behavior from when these properties were first
introduced for virtual GPU drivers.

Thanks,
Vivek

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch