[PATCH 0/6] drm: Add mouse cursor hotspot support to atomic KMS

Zack Rusin zackr at vmware.com
Fri Jun 10 14:24:01 UTC 2022

On Fri, 2022-06-10 at 10:59 +0200, Daniel Vetter wrote:
> ⚠ External Email
> On Fri, Jun 10, 2022 at 10:41:05AM +0200, Daniel Vetter wrote:
> > Hi all,
> > 
> > Kinda top post because the thread is sprawling and I think we need a
> > summary/restart. I think there's at least 3 issues here:
> > 
> > - lack of hotspot property support, which means compositors can't really
> >   support hotspot with atomic. Which isn't entirely true, because you
> >   totally can use atomic for the primary planes/crtcs and the legacy
> >   cursor ioctls, but I understand that people might find that a bit silly :-)
> > 
> >   Anyway this problme is solved by the patch set here, and I think results
> >   in some nice cleanups to boot.
> > 
> > - the fact that cursors for virtual drivers are not planes, but really
> >   special things. Which just breaks the universal plane kms uapi. That
> >   part isn't solved, and I do agree with Simon and Pekka that we really
> >   should solve this before we unleash even more compositors onto the
> >   atomic paths of virtual drivers.
> > 
> >   I think the simplest solution for this is:
> >   1. add a new DRM_PLANE_TYPE_VIRTUAL_CURSOR, and set that for these
> >   special cursor planes on all virtual drivers
> >   2. add the new "I understand virtual cursors planes" setparam, filter
> >   virtual cursor planes for userspace which doesn't set this (like we do
> >   right now if userspace doesn't set the universal plane mode)
> >   3. backport the above patches to all stable kernels
> >   4. make sure the hotspot property is only set on VIRTUAL_CURSOR planes
> >   and nothing else in the rebased patch series
> Simon also mentioned on irc that these special planes must not expose the
> CRTC_X/Y property, since that doesn't really do much at all. Or is our
> understanding of how this all works for commandeered cursors wrong?

Yes, that's the part I don't understand. I don't think I see how the CRTC_X|Y
properties aren't used.

I think the confusion might stem from the fact that it appears that the
hypervisors/hosts are moving that plane, which is not the case. We never move the
plane itself, we redirect the mouse focus/movement, that's what's reducing the
latency but don't touch drm internals. I can't speak for every virtualized stack,
but what is happening on ours is that we set the image that's on the cursor plane as
the cursor on the machine that's running the guest. So when you move the mouse
across the screen as you enter the VM window the cursor plane isn't touched, but the
local machines cursor changes to what's inside the cursor plane. When the mouse is
clicked the mouse device in the guest generates the event with the proper
coordinates (hence we need the hotspot to route that event correctly). That's when
the guest reacts just like it would normally on native if a mouse event was

The actual cursor plane might not be visible while this is happening but even when
it's not visible it's still at whatever crtc_x|y the guest thinks it is. crtc_x|y
are still only driven by the guests mouse device.

We control the mouse device and visibility of the cursor plane itself based on
what's happening in the system but I don't think that's that different from a native

This is easy to see by running the "weston-simple-damage --width=64 --height=64" if
you click on that window and move the cursor to the very edge of the virtualized
window you'll notice that it's coming out outside of the window, that's because it's
the local machines cursor, but if you stop the press the window will jump to
wherever it was because the real guest plane is still at crtc_x|y (and because in
weston that window doesn't react to mouse move events like a cursor would it never
sets crtc_x|y to the mouse directed coordinates). 

The problem stems from the fact that the cursor plane has something that's not a
cursor and doesn't react to mouse events like a cursor would. So the flag (or new
plane) that we'd be adding is simply making user-space give the following promise:

"I understand that what's inside the cursor plane needs to react and deal with mouse
events like a mouse cursor would, i.e. it should be a mouse cursor"


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