Re: [PATCH v5 9/9] drm: Introduce documentation for hotspot properties

[Zack Rusin <zackr@vmware.com>]

On Wed, 2023-07-19 at 11:15 +0300, Pekka Paalanen wrote:
> On Tue, 18 Jul 2023 21:42:18 -0400
> Zack Rusin <zack@kde.org> wrote:
> 
> > From: Michael Banack <banackm@vmware.com>
> > 
> > To clarify the intent and reasoning behind the hotspot properties
> > introduce userspace documentation that goes over cursor handling
> > in para-virtualized environments.
> > 
> > The documentation is generic enough to not special case for any
> > specific hypervisor and should apply equally to all.
> > 
> > Signed-off-by: Zack Rusin <zackr@vmware.com>
> > ---
> >  Documentation/gpu/drm-kms.rst |  6 ++++
> >  drivers/gpu/drm/drm_plane.c   | 58 ++++++++++++++++++++++++++++++++++-
> >  2 files changed, 63 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> > index c92d425cb2dd..7159b3e90a8a 100644
> > --- a/Documentation/gpu/drm-kms.rst
> > +++ b/Documentation/gpu/drm-kms.rst
> > @@ -577,6 +577,12 @@ Variable Refresh Properties
> >  .. kernel-doc:: drivers/gpu/drm/drm_connector.c
> >     :doc: Variable refresh properties
> >  
> > +Cursor Hotspot Properties
> > +---------------------------
> > +
> > +.. kernel-doc:: drivers/gpu/drm/drm_plane.c
> > +   :doc: hotspot properties
> > +
> >  Existing KMS Properties
> >  -----------------------
> >  
> > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > index 1dc00ad4c33c..f3f2eae83cca 100644
> > --- a/drivers/gpu/drm/drm_plane.c
> > +++ b/drivers/gpu/drm/drm_plane.c
> > @@ -230,6 +230,61 @@ static int create_in_format_blob(struct drm_device *dev,
> > struct drm_plane *plane
> >         return 0;
> >  }
> >  
> > +/**
> > + * DOC: hotspot properties
> > + *
> > + * HOTSPOT_X: property to set mouse hotspot x offset.
> > + * HOTSPOT_Y: property to set mouse hotspot y offset.
> > + *
> > + * When the plane is being used as a cursor image to display a mouse pointer,
> > + * the "hotspot" is the offset within the cursor image where mouse events
> > + * are expected to go.
> > + *
> > + * Positive values move the hotspot from the top-left corner of the cursor
> > + * plane towards the right and bottom.
> > + *
> > + * Most display drivers do not need this information because the
> > + * hotspot is not actually connected to anything visible on screen.
> > + * However, this is necessary for display drivers like the para-virtualized
> > + * drivers (eg qxl, vbox, virtio, vmwgfx), that are attached to a user console
> > + * with a mouse pointer.  Since these consoles are often being remoted over a
> > + * network, they would otherwise have to wait to display the pointer movement
> > to
> > + * the user until a full network round-trip has occurred.  New mouse events
> > have
> > + * to be sent from the user's console, over the network to the virtual input
> > + * devices, forwarded to the desktop for processing, and then the cursor
> > plane's
> > + * position can be updated and sent back to the user's console over the
> > network.
> > + * Instead, with the hotspot information, the console can anticipate the new
> > + * location, and draw the mouse cursor there before the confirmation comes in.
> > + * To do that correctly, the user's console must be able predict how the
> > + * desktop will process mouse events, which normally requires the desktop's
> > + * mouse topology information, ie where each CRTC sits in the mouse coordinate
> > + * space.  This is typically sent to the para-virtualized drivers using some
> > + * driver-specific method, and the driver then forwards it to the console by
> > + * way of the virtual display device or hypervisor.
> > + *
> > + * The assumption is generally made that there is only one cursor plane being
> > + * used this way at a time, and that the desktop is feeding all mouse devices
> > + * into the same global pointer.  Para-virtualized drivers that require this
> > + * should only be exposing a single cursor plane, or find some other way
> > + * to coordinate with a userspace desktop that supports multiple pointers.
> > + * If the hotspot properties are set, the cursor plane is therefore assumed to
> > be
> > + * used only for displaying a mouse cursor image, and the position of the
> > combined
> > + * cursor plane + offset can therefore be used for coordinating with input from
> > a
> > + * mouse device.
> > + *
> > + * The cursor will then be drawn either at the location of the plane in the
> > CRTC
> > + * console, or as a free-floating cursor plane on the user's console
> > + * corresponding to their desktop mouse position.
> > + *
> > + * DRM clients which would like to work correctly on drivers which expose
> > + * hotspot properties should advertise DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT.
> > + * Setting this property on drivers which do not special case
> > + * cursor planes will return EOPNOTSUPP, which can be used by userspace to
> > + * gauge requirements of the hardware/drivers they're running on. Advertising
> > + * DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT implies that the userspace client will
> > be
> > + * correctly setting the hotspot properties.
> > + */
> 
> Yes! This is exactly what I was after. Thank you!
> 
> > +
> >  /**
> >   * drm_plane_create_hotspot_properties - creates the mouse hotspot
> >   * properties and attaches them to the given cursor plane
> > @@ -237,7 +292,8 @@ static int create_in_format_blob(struct drm_device *dev,
> > struct drm_plane *plane
> >   * @plane: drm cursor plane
> >   *
> >   * This function enables the mouse hotspot property on a given
> > - * cursor plane.
> > + * cursor plane. Look at the documentation for hotspot properties
> > + * to get a better understanding for what they're used for.
> 
> I haven't seen the rendered HTML, but is there a hyperlink from here to
> the hotspot property doc? I think a link would be neat.

drm_plane_create_hotspot_properties is now a static function so it's not
generated in the docs (but to be fair, I'm also not aware of how to link to drm-
kms.rst section from within function docs because I tried that first only to realize
it's a static function and not in the generated html docs).

I'll give this series a few more hours on the list and if no one objects I'll push
it to drm-misc later today. Thanks!

z