Should I always add wl_display_sync() before wl_display_dispatch_queue()?

[Jason Ekstrand]

On Mon, Oct 28, 2013 at 9:39 PM, Zhao, Halley <halley.zhao at intel.com> wrote:

>  Video player use frame callback on an event queue, sometimes there is
> synchronization issue.****
>
> After looked at other usage of event queue, I found:****
>
> In order to avoid blocking by wl_display_dispatch_queue(), a padding
> callback of wl_display_sync is added on purpose.****
>
> Is my understanding correct?****
>
> Here is the changes:****
>
>
> https://gitorious.org/halley-test/gstreamer-vaapi/commit/9846c75e7a2bb223900457beffc3384c77cc5075
> ****
>
> ** **
>
> ** **
>
> however, with or without the above change, I found there is still hangs in
> my player (efl based) : ****
>
>
> https://gitorious.org/gst-plugins-vaapi/simple-player/source/87837fc1b4c11d7a33142c7d4fac1225d8973b80
> :****
>
> it seems to be ECore issue during wl_display_dispatch_pending():
>

The short answer is that you shouldn't have to.  Also, sitting in a
round-trip loop is asking for bad performance if the CPU is under load.

If I had to take a guess, I think it's likely that ecore and gstreamer are
fighting over the file descriptor.  I don't know the codebase of either,
but you have to be careful with handing it back and forth.  In particular,
take a look at wl_display_prepare_read:

http://cgit.freedesktop.org/wayland/wayland/tree/src/wayland-client.c#n1078

I hope that's helpful,
--Jason Ekstrand


> ****
>
> ** **
>
> Rafael:****
>
> Do you think it is an ECore issue?****
>
> ** **
>
> // wl_display_sync callback with wl_display_dispatch_pending()****
>
> (gdb) bt****
>
> #0  0xb7c4ddd0 in _ecore_wl_input_cb_keyboard_enter () from
> /lib/libecore_wayland.so.1****
>
> #1  0xb6f16666 in ffi_call_SYSV () from /lib/libffi.so.6****
>
> #2  0xb6f163eb in ffi_call () from /lib/libffi.so.6****
>
> #3  0xb79a9ae5 in wl_closure_invoke () from /lib/libwayland-client.so.0***
> *
>
> #4  0xb79a74c2 in dispatch_queue () from /lib/libwayland-client.so.0****
>
> #5  0xb79a803e in wl_display_dispatch_queue_pending () from
> /lib/libwayland-client.so.0****
>
> #6  0xb79a8162 in wl_display_dispatch_pending () from
> /lib/libwayland-client.so.0****
>
> #7  0xb3ca72a6 in gst_vaapi_window_wayland_sync (window=0x87d2d50) at
> gstvaapiwindow_wayland.c:162****
>
> #8  gst_vaapi_window_wayland_render (window=0x87d2d50, surface=0xb0a02860,
> src_rect=0xb017dbd0, dst_rect=0x88bdbe0, flags=0) at
> gstvaapiwindow_wayland.c:514****
>
> #9  0xb3c5a2fb in gst_vaapi_window_put_surface (window=0x87d2d50,
> surface=surface at entry=0xb0a02860, src_rect=0xb017dbd0, src_rect at entry=0x0,
> dst_rect=<optimized out>, dst_rect at entry=0x88bdbe0, ****
>
>     flags=flags at entry=0) at gstvaapiwindow.c:435****
>
> #10 0xb3ccc1e1 in gst_vaapisink_put_surface (flags=0,
> surface_rect=<optimized out>, surface=0xb0a02860, sink=0x88bd958) at
> gstvaapisink.c:1150****
>
> ** **
>
> If I remove wl_display_dispatch_pending in frame callback sync:****
>
> There is backtrace, ****
>
> // wl_display_sync callback with wl_display_dispatch_pending()****
>
> (gdb) bt****
>
> #0  0xb7c4ddd0 in _ecore_wl_input_cb_keyboard_enter () from
> /lib/libecore_wayland.so.1****
>
> #1  0xb6f16666 in ffi_call_SYSV () from /lib/libffi.so.6****
>
> #2  0xb6f163eb in ffi_call () from /lib/libffi.so.6****
>
> #3  0xb79a9ae5 in wl_closure_invoke () from /lib/libwayland-client.so.0***
> *
>
> #4  0xb79a74c2 in dispatch_queue () from /lib/libwayland-client.so.0****
>
> #5  0xb79a803e in wl_display_dispatch_queue_pending () from
> /lib/libwayland-client.so.0****
>
> #6  0xb79a8162 in wl_display_dispatch_pending () from
> /lib/libwayland-client.so.0****
>
> #7  0xb3ca72a6 in gst_vaapi_window_wayland_sync (window=0x87d2d50) at
> gstvaapiwindow_wayland.c:162****
>
> #8  gst_vaapi_window_wayland_render (window=0x87d2d50, surface=0xb0a02860,
> src_rect=0xb017dbd0, dst_rect=0x88bdbe0, flags=0) at
> gstvaapiwindow_wayland.c:514****
>
> #9  0xb3c5a2fb in gst_vaapi_window_put_surface (window=0x87d2d50,
> surface=surface at entry=0xb0a02860, src_rect=0xb017dbd0, src_rect at entry=0x0,
> dst_rect=<optimized out>, dst_rect at entry=0x88bdbe0, ****
>
>     flags=flags at entry=0) at gstvaapiwindow.c:435****
>
> #10 0xb3ccc1e1 in gst_vaapisink_put_surface (flags=0,
> surface_rect=<optimized out>, surface=0xb0a02860, sink=0x88bd958) at
> gstvaapisink.c:1150****
>
> ** **
>
> // no wl_display_sync callback witt wl_display_displatch_pending****
>
> #0  0xb7c4ddd0 in _ecore_wl_input_cb_keyboard_enter () from
> /lib/libecore_wayland.so.1****
>
> #1  0xb6f16666 in ffi_call_SYSV () from /lib/libffi.so.6****
>
> #2  0xb6f163eb in ffi_call () from /lib/libffi.so.6****
>
> #3  0xb79a9ae5 in wl_closure_invoke () from /lib/libwayland-client.so.0***
> *
>
> #4  0xb79a74c2 in dispatch_queue () from /lib/libwayland-client.so.0****
>
> #5  0xb79a803e in wl_display_dispatch_queue_pending () from
> /lib/libwayland-client.so.0****
>
> #6  0xb79a8162 in wl_display_dispatch_pending () from
> /lib/libwayland-client.so.0****
>
> #7  0xb3ca72a6 in gst_vaapi_window_wayland_sync (window=0x87d2950) at
> gstvaapiwindow_wayland.c:162****
>
> #8  gst_vaapi_window_wayland_render (window=0x87d2950, surface=0xb0a02860,
> src_rect=0xb017dbd0, dst_rect=0x88bd850, flags=0) at
> gstvaapiwindow_wayland.c:514****
>
> #9  0xb3c5a2fb in gst_vaapi_window_put_surface (window=0x87d2950,
> surface=surface at entry=0xb0a02860, src_rect=0xb017dbd0, src_rect at entry=0x0,
> dst_rect=<optimized out>, dst_rect at entry=0x88bd850, ****
>
>     flags=flags at entry=0) at gstvaapiwindow.c:435****
>
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