[PATCH weston v2] simple-shm: explain two initial roundtrips

[Pekka Paalanen]

On Wed, 26 Nov 2014 16:12:27 +0100
Marek Chalupa <mchqwerty at gmail.com> wrote:

> On 26 November 2014 at 09:04, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> 
> > From: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> >
> > Explain carefully why we need two roundtrips, not just one, not just
> > dispatch and roundtrip, but two roundtrips after creating the
> > wl_registry object.
> >
> > v2: Explain what initial events are, and that this is a general
> > technique.
> >
> > Signed-off-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>
> > ---
> >  clients/simple-shm.c | 40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> >
> > diff --git a/clients/simple-shm.c b/clients/simple-shm.c
> > index c1cb386..0844565 100644
> > --- a/clients/simple-shm.c
> > +++ b/clients/simple-shm.c
> > @@ -388,6 +388,46 @@ create_display(void)
> >
> >         wl_display_roundtrip(display->display);
> >
> > +       /*
> > +        * Why do we need two roundtrips here?
> > +        *
> > +        * wl_display_get_registry() sends a request to the server, to
> > which
> > +        * the server replies by emitting the wl_registry.global events.
> > +        * The first wl_display_roundtrip() sends wl_display.sync. The
> > server
> > +        * first processes the wl_display.get_registry which includes
> > sending
> > +        * the global events, and then processes the sync. Therefore when
> > the
> > +        * sync (roundtrip) returns, we are guaranteed to have received and
> > +        * processed all the global events.
> > +        *
> > +        * While we are inside the first wl_display_roundtrip(), incoming
> > +        * events are dispatched, which causes registry_handle_global() to
> > +        * be called for each global. One of these globals is wl_shm.
> > +        * registry_handle_global() sends wl_registry.bind request for the
> > +        * wl_shm global. However, wl_registry.bind request is sent only
> > after
> >
> 
> Probably it's just due to my bad English, but the 'only' seems redundant to
> me here.
> 
> 
> > +        * the first wl_display.sync, so the reply to the sync comes before
> > +        * the initial events of the wl_shm object.
> > +        *
> > +        * The initial events that get sent as a reply to binding to wl_shm
> > +        * include wl_shm.format. These tell us which pixel formats are
> > +        * supported, and we need them before we can create buffers. They
> > +        * don't change at runtime, so we receive them as part of init.
> > +        *
> > +        * When the reply to the first sync comes, the server may or may
> > not
> > +        * have sent the initial wl_shm events. Therefore we need the
> > second
> > +        * wl_display_roundtrip() call here.
> > +        *
> > +        * The server processes the wl_registry.bind for wl_shm first, and
> > +        * the second wl_display.sync next. During our second call to
> > +        * wl_display_roundtrip() the initial wl_shm events are received
> > and
> > +        * processed. Finally, when the reply to the second wl_display.sync
> > +        * arrives, it guarantees we have processed all wl_shm initial
> > events.
> > +        *
> > +        * This sequence contains two examples on how
> > wl_display_roundtrip()
> > +        * can be used to guarantee, that all reply events to a request
> > +        * have been received and processed. This is a general Wayland
> > +        * technique.
> > +        */
> > +
> >         if (!(display->formats & (1 << WL_SHM_FORMAT_XRGB8888))) {
> >                 fprintf(stderr, "WL_SHM_FORMAT_XRGB32 not available\n");
> >                 exit(1);
> > --
> > 2.0.4
> >
> > _______________________________________________
> > wayland-devel mailing list
> > wayland-devel at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/wayland-devel
> >
> 
> Looks fine to me.
> 
> Maybe it'd be worth to mention this 'general technique' on appropriate
> places in the source code, i. e. in wl_display_get_registry() documentation
> or so.
> Everybody who first runs into Wayland will probably read documentation to
> wl_display_get_registry() :)
> 
> Reviewed-by: Marek Chalupa <mchqwerty at gmail.com>

Thanks, removed the one word, pushed. A good idea about get_registry.


Thanks,
pq