[RFC xserver 1/1] xwayland: reduce over-damage

[Pekka Paalanen]

On Wed, 20 Dec 2017 13:18:45 +0200
Pekka Paalanen <ppaalanen at gmail.com> wrote:

> From: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> 
> If an X11 app draws a little here, some there, and a tiny bit in the
> opposite corner, using RegionExtents for the damage to be sent to the
> Wayland compositor will cause massive over-damaging.
> 
> However, we cannot blindly send an arbitrary number of damage
> rectangles, because there is a risk of overflowing the Wayland
> connection. If that happens, it triggers an abort in libwayland-client.
> 
> Try to be more accurate with the damage by sending up to 500 rectangles
> per window, and fall back to extents otherwise. The number is completely
> arbitrary.
> 
> Signed-off-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> ---
>  hw/xwayland/xwayland.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/xwayland/xwayland.c b/hw/xwayland/xwayland.c
> index c5a3ae7ae..383680d7a 100644
> --- a/hw/xwayland/xwayland.c
> +++ b/hw/xwayland/xwayland.c
> @@ -629,6 +629,7 @@ xwl_window_post_damage(struct xwl_window *xwl_window)
>      BoxPtr box;
>      struct wl_buffer *buffer;
>      PixmapPtr pixmap;
> +    int i;
>  
>      assert(!xwl_window->frame_callback);
>  
> @@ -644,9 +645,20 @@ xwl_window_post_damage(struct xwl_window *xwl_window)
>  
>      wl_surface_attach(xwl_window->surface, buffer, 0, 0);
>  
> -    box = RegionExtents(region);
> -    wl_surface_damage(xwl_window->surface, box->x1, box->y1,
> -                        box->x2 - box->x1, box->y2 - box->y1);
> +    /* Arbitrary limit to try to avoid flooding the Wayland
> +     * connection. If we flood it too much anyway, this could
> +     * abort in libwayland-client.
> +     */
> +    if (RegionNumRects(region) > 500) {
> +        box = RegionExtents(region);
> +        wl_surface_damage(xwl_window->surface, box->x1, box->y1,
> +                          box->x2 - box->x1, box->y2 - box->y1);
> +    } else {
> +        box = RegionRects(region);
> +        for (i = 0; i < RegionNumRects(region); i++, box++)
> +            wl_surface_damage(xwl_window->surface, box->x1, box->y1,
> +                              box->x2 - box->x1, box->y2 - box->y1);
> +    }
>  
>      xwl_window->frame_callback = wl_surface_frame(xwl_window->surface);
>      wl_callback_add_listener(xwl_window->frame_callback, &frame_listener, xwl_window);

Hi,

should I invest the effort on a better algorithm than in this patch or
would this be acceptable for landing?


Thanks,
pq