drm: Introduce documentation for hotspot properties

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>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Michael Banack <banackm@vmware.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023074613.41327-10-aesteve@redhat.com

Documentation/gpu/drm-kms.rst

@@ -579,6 +579,12 @@ 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
 -----------------------
 

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.
+ */
+
 /**
  * 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.
  *
  * RETURNS:
  * Zero for success or -errno