[PATCH] documentation: clarify the need for wl_surface.damage

[Jonas Ådahl]

On Tue, Oct 03, 2017 at 10:02:03AM +0300, Pekka Paalanen wrote:
> On Mon,  2 Oct 2017 17:39:56 +0200
> Mahdi Khanalizadeh <mahdi at khanalizadeh.com> wrote:
> 
> > Add an explanation for wl_surface.attach why a wl_surface.damage request
> > is necessary. Explicitly declare it implementation defined behaviour if the
> > wl_surface.damage request is omitted to give the compositor some leeway
> > on how it handles attaches.
> > 
> > Signed-off-by: Mahdi Khanalizadeh <mahdi at khanalizadeh.com>
> > ---
> >  protocol/wayland.xml | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/protocol/wayland.xml b/protocol/wayland.xml
> > index aabc7ae..6f6cc11 100644
> > --- a/protocol/wayland.xml
> > +++ b/protocol/wayland.xml
> > @@ -1365,6 +1365,11 @@
> >  	wl_buffer before receiving the wl_buffer.release event, the surface
> >  	contents become undefined immediately.
> >  
> > +	Attaching a buffer should always be accompanied by a
> > +	wl_surface.damage request to signal the compositor that the
> > +	contents of the buffer have changed. Otherwise it is implementation
> > +	defined whether the wl_surface.attach request has any visible effect.
> 
> Hi,
> 
> thank you for the clarification, I assume this is as a response to our
> IRC discussion.
> 
> How about phrasing it: "Attaching a buffer should always be accompanied
> by at least one wl_surface.damage request to signal the compositor
> which parts of the surface contents are changed."?
> 
> - There can be multiple damage requests to build up a complex region.
> - It's not about buffer contents, it is about surface contents.
> - We don't want to imply that you must always give full surface-sized
>   damage when the buffer is swapped.
> 
> However, most of the new paragraph are redundant with the first
> paragraph for the damage request. Could you instead add one more
> sentence to the first paragraph explaining that the compositor has no
> obligation to use the new contents outside of the given damage?

In a way, the compositor MAY be obliged to use the new content if it
didn't make a copy of the previous one. As in, it must switch to the new
buffer and send the corrpesponding wl_buffer::release events on the old
one, even if there was no damage, if the buffers are used in a zero-copy
manner. If it already made a copy, there is of course no obligation to use
the new content, as the wl_buffer::release event would probably already
be sent after having made the copy.

I don't think we should imply that it is illegal to not attach damage
anyhow, as there may be valid use cases. We should rather make it clear
that there the regular semantics are still applied (regarding use count
etc) and that it also means the the client are saying the buffers are
equal pixel by pixel.


Jonas

> 
> There is also the request damage_buffer which is otherwise the same but
> uses a different coordinate system.
> 
> > +
> >  	If wl_surface.attach is sent with a NULL wl_buffer, the
> >  	following wl_surface.commit will remove the surface content.
> >        </description>
> 
> Thanks,
> pq



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