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

[Simon Ser]

On Friday, June 3rd, 2022 at 17:32, Zack Rusin <zackr at vmware.com> wrote:

> > On Jun 3, 2022, at 11:22 AM, Simon Ser <contact at emersion.fr> wrote:
> > ⚠ External Email
> >
> > On Friday, June 3rd, 2022 at 17:17, Zack Rusin <zackr at vmware.com> wrote:
> >
> >
> > >
> > >
> > > > On Jun 3, 2022, at 10:56 AM, Simon Ser <contact at emersion.fr> wrote:
> > > > ⚠ External Email
> > > >
> > > > On Friday, June 3rd, 2022 at 16:38, Zack Rusin <zackr at vmware.com> wrote:
> > > >
> > > >
> > > > > > On Jun 3, 2022, at 10:32 AM, Simon Ser <contact at emersion.fr> wrote:
> > > > > >
> > > > > > ⚠ External Email
> > > > > >
> > > > > > On Friday, June 3rd, 2022 at 16:27, Zack Rusin <zackr at vmware.com> wrote:
> > > > > >
> > > > > >
> > > > > > > > In particular: since the driver will ignore the KMS cursor plane
> > > > > > > > position set by user-space, I don't think it's okay to just expose
> > > > > > > > without opt-in from user-space (e.g. with a DRM_CLIENT_CAP).
> > > > > > > >
> > > > > > > > cc wayland-devel and Pekka for user-space feedback.
> > > > > > >
> > > > > > >
> > > > > > > I think Thomas expressed our concerns and reasons why those wouldn’t
> > > > > > > work for us back then. Is there something else you’d like to add?
> > > > > >
> > > > > >
> > > > > > I disagreed, and I don't understand Thomas' reasoning.
> > > > > >
> > > > > > KMS clients will need an update to work correctly. Adding a new prop
> > > > > > without a cap leaves existing KMS clients broken. Adding a cap allows
> > > > > > both existing KMS clients and updated KMS clients to work correctly.
> > > > >
> > > > >
> > > > > I’m not sure what you mean here. They are broken right now. That’s what we’re
> > > > > fixing. That’s the reason why the virtualized drivers are on deny-lists for
> > > > > all atomic kms. So nothing needs to be updated. If you have a kms client that
> > > > > was using virtualized drivers with atomic kms then mouse clicks never worked
> > > > > correctly.
> > > > >
> > > > > So, yes, clients need to set cursor hotspot if they want mouse to work
> > > > > correctly with virtualized drivers.
> > > >
> > > >
> > > > My proposal was:
> > > >
> > > > - Introduce DRM_CLIENT_CAP_CURSOR_PLANE_NO_POSITION (or a better name). Only
> > > > user-space which supports the hotspot props will enable it.
> > > > - By default, don't expose a cursor plane, because current user-space doesn't
> > > > support it (Weston might put a window in the cursor plane, and nobody can
> > > > report hotspot).
> > > > - If the KMS client enables the cap, advertise the cursor
> > > > plane, and make it so the plane doesn't have the CRTC_X/CRTC_Y properties
> > > > since the driver will pick the position.
> > > >
> > > > That way both old and new user-space are fixed.
> > >
> > >
> > > I don’t think I see how that fixes anything. In particular I don’t see a way
> > > of fixing the old user space at all. We require hotspot info, old user space
> > > doesn’t set it because there’s no way of setting it on atomic kms.
> >
> >
> > Old atomic user-space is fixed by removing the cursor plane. Then old
> > atomic user-space will fallback to drawing the cursor itself, e.g. via
> > OpenGL.
>
> But it’s not fixed, because the driver is still on a deny-list and
> nothing implements this. You’re saying you could potentially “fix” by
> implementing client side cursor handling in all kms clients? That’s a
> hard sell if the user space can just put the virtualized driver on
> deny-lists and fallback to use old kms which supports hotspots.

What deny-list are you referring to?

All compositors I know of implement a fallback when no cursor plane is
usable.

> > New user-space supports setting the hotspot prop, and is aware that it can't
> > set the cursor plane position, so the cursor plane can be exposed again when
> > the cap is enabled.
>
> But we still use cursor plane position. Hotspots are offsets from
> cursor plane positions. Both are required.

No, VM drivers don't need and disregard the cursor position AFAIK. They
replace it with the host cursor's position.

This is what breaks Weston, breaks uAPI expectations, and IMHO is
unacceptable without KMS client opt-in.