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

[Marek Chalupa]

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
>
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> 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>
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