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

[Pekka Paalanen <ppaalanen@gmail.com>]

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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.

>   *
>   * RETURNS:
>   * Zero for success or -errno

Anyway:

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>


Thanks,
pq

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