[PATCH] Xwayland: fix output changes not getting applied in some, cases

[Ferdi265]

On 22/04/2019 14:01, Roman Gilg wrote:
> On Mon, Apr 22, 2019 at 1:30 PM Ferdi265 <theferdi265 at gmail.com> wrote:
>>
>> Hello,
>>
>> I investigated a bug I encountered while using the Sway Compositor and
>> traced it back to some behaviour in Xwayland:
>>
>> The xdg_output and wl_output events are handled in such a way that
>> output changes (e.g. position) sometimes don't get applied if only
>> xdg_output is sent instead of both xdg_output and wl_output. (Sway does
>> this)
> 
> This issue was something we noted in KWin as well and fixed it
> downstream by making sure the events get always sent together.
> More info here: https://phabricator.kde.org/D19253
> 
> Would a downstream fix work for you as well? Not saying it can't be
> fixed upstream in XWayland, but I haven't yet looked at your patch in
> detail and I just remember the output code in Xwayland being somewhat
> difficult, so maybe a downstream fix would be easier.

The change to remove some information from the wl_output event and
instead rely only on xdg_output in wlroots was deliberate as it
simplifies the compositor code and might not even make much sense for
some types of compositors (e.g. 3D desktop systems).

The reason the events now don't fire together is that no information in
wl_output actually changed, while the information in xdg_output did.

the downstream discussion for this is here
https://github.com/swaywm/wlroots/issues/1610

I can't really speak for the whole wlroots / sway project, so I added
Simon Ser aka "emersion" as CC, who is one of the main developers of
sway and wlroots.

Personally, I think Xwayland shouldn't attach too much meaning to when
these events happen, and if they happen together. They are separate
events after all, and should be treated this way.

Assuming that this information is always updated at the same time or
even re-sending it without a change (e.g. changing the output position
in sway doesn't change any information in the wl_output event) just so
the events happen together just sounds backwards to me.

>> Below I attached a patch that fixes this. I am not very familiar with
>> the Xorg codebase, so I hope this patch is fine.
>>
>> Here (https://github.com/swaywm/sway/issues/4064) is the Sway bug report
>> in question.
>>
>> Kind Regards,
>> Ferdinand "Ferdi265" Bachmann
>>
>> --- PATCH BELOW ---
>>
>> From 431a176c112d373a07b861fdf95766b3741ba95c Mon Sep 17 00:00:00 2001
>> From: Ferdinand Bachmann <theferdi265 at gmail.com>
>> Date: Mon, 22 Apr 2019 00:35:41 +0200
>> Subject: [PATCH] Xwayland: fix output changes not getting applied in
>> some cases
>>
>> On some wayland compositors wl_output and xdg_output events don't
>> neccessarily happen together, so waiting until the wl_output event is
>> done might wait indefinitely.
>>
>> This fixes output changes not getting applied on newer wlroots-based
>> compositors, such as sway.
>>
>> Since these event's don't always happen together, the changes need to
>> be applied regardless of whether the other event is finished or not,
>> making the wl_output_done and xdg_output_done flags redundant.
>> ---
>>  hw/xwayland/xwayland-output.c | 17 ++---------------
>>  hw/xwayland/xwayland.h        |  2 --
>>  2 files changed, 2 insertions(+), 17 deletions(-)
>>
>> diff --git a/hw/xwayland/xwayland-output.c b/hw/xwayland/xwayland-output.c
>> index cc68f0340..497e56db4 100644
>> --- a/hw/xwayland/xwayland-output.c
>> +++ b/hw/xwayland/xwayland-output.c
>> @@ -218,10 +218,6 @@ apply_output_change(struct xwl_output *xwl_output)
>>      RRModePtr randr_mode;
>>      Bool need_rotate;
>>
>> -    /* Clear out the "done" received flags */
>> -    xwl_output->wl_output_done = FALSE;
>> -    xwl_output->xdg_output_done = FALSE;
>> -
>>      /* xdg-output sends output size in compositor space. so already
>> rotated */
>>      need_rotate = (xwl_output->xdg_output == NULL);
>>
>> @@ -268,13 +264,7 @@ static void
>>  output_handle_done(void *data, struct wl_output *wl_output)
>>  {
>>      struct xwl_output *xwl_output = data;
>> -
>> -    xwl_output->wl_output_done = TRUE;
>> -    /* Apply the changes from wl_output only if both "done" events are
>> received,
>> -     * or if xdg-output is not supported.
>> -     */
>> -    if (xwl_output->xdg_output_done || !xwl_output->xdg_output)
>> -        apply_output_change(xwl_output);
>> +    apply_output_change(xwl_output);
>>  }
>>
>>  static void
>> @@ -313,10 +303,7 @@ static void
>>  xdg_output_handle_done(void *data, struct zxdg_output_v1 *xdg_output)
>>  {
>>      struct xwl_output *xwl_output = data;
>> -
>> -    xwl_output->xdg_output_done = TRUE;
>> -    if (xwl_output->wl_output_done)
>> -        apply_output_change(xwl_output);
>> +    apply_output_change(xwl_output);
>>  }
>>
>>  static const struct zxdg_output_v1_listener xdg_output_listener = {
>> diff --git a/hw/xwayland/xwayland.h b/hw/xwayland/xwayland.h
>> index 92664e812..1ab2f04b5 100644
>> --- a/hw/xwayland/xwayland.h
>> +++ b/hw/xwayland/xwayland.h
>> @@ -371,8 +371,6 @@ struct xwl_output {
>>      RRCrtcPtr randr_crtc;
>>      int32_t x, y, width, height, refresh;
>>      Rotation rotation;
>> -    Bool wl_output_done;
>> -    Bool xdg_output_done;
>>  };
>>
>>  void xwl_sync_events (struct xwl_screen *xwl_screen);
>> --
>> 2.21.0
>>
>> _______________________________________________
>> xorg-devel at lists.x.org: X.Org development
>> Archives: http://lists.x.org/archives/xorg-devel
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