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

[Simon Ser <contact@emersion.fr>]

On Friday, June 3rd, 2022 at 16:38, Zack Rusin <zackr@vmware.com> wrote:

> > On Jun 3, 2022, at 10:32 AM, Simon Ser <contact@emersion.fr> wrote:
> >
> > ⚠ External Email
> >
> > On Friday, June 3rd, 2022 at 16:27, Zack Rusin <zackr@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.

> >>> On Thursday, June 2nd, 2022 at 17:42, Zack Rusin zack@kde.org wrote:
> >>>
> >>>> - all userspace code needs to hardcore a list of drivers which require
> >>>> hotspots because there's no way to query from drm "does this driver
> >>>> require hotspot"
> >>>
> >>> Can you elaborate? I'm not sure I understand what you mean here.
> >>
> >> Only some drivers require informations about cursor hotspot, user space
> >> currently has no way of figuring out which ones are those, i.e. amdgpu
> >> doesn’t care about hotspots, qxl does so when running on qxl without
> >> properly set cursor hotspot atomic kms will result in a desktop where
> >> mouse clicks have incorrect coordinates.
> >>
> >> There’s no way to differentiate between drivers that do not care about
> >> cursor hotspots and ones that absolutely require it.
> >
> > Only VM drivers should expose the hotspot properties. Real drivers like
> > amdgpu must not expose it.
>
> Yes, that’s what I said. There’s no way to differentiate between amdgpu that
> doesn’t have those properties and virtio_gpu driver from a kernel before
> hotspot properties were added.

See cap proposal above.