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

[Simon Ser]

On Wednesday, July 19th, 2023 at 03:42, Zack Rusin <zack at kde.org> wrote:

> +/**
> + * 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.

Nit: an ampersand ("&") can be used to linkify that cap.

> + * 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.

Thanks a lot for writing these docs! It's super helpful!

Reviewed-by: Simon Ser <contact at emersion.fr>