[Bug 732326] v4l2: slow UVC webcam startup due to excessive S_FMT

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=732326
  GStreamer | gst-plugins-good | git

--- Comment #7 from Daniel Drake <dan at reactivated.net> 2014-06-30 16:17:07 UTC ---
(In reply to comment #6)
> Right, so the current patch will "break" the fallback as originally designed,
> as it won't restore the state on fallback. I wonder why is it needed to restore
> last state in the first place. I'll try and find an answer in old commit
> messages.

Maybe I misunderstood your comment, but didn't I already answer that above?
The save/restore was added to work around a kernel bug in 2011, that bug is
closed upstream as obsolete, and I can't reproduce it.

> Even if restoring the state is needed, this action should only be done once per
> probe, not for each format being tested. This would keep old behaviour, and fix
> the performance issue (well, will still have 0.34 s overhead according to your
> test, which isn't that low).

If it sounds OK, tomorrow I will try to write a patch which does:
- save settings at start of probe
- during probe, do TRY_FMT with fallback on S_FMT
- is S_FMT fallback is used, set restore_needed flag
- at end of probe, if restore_needed flag is set, restore the saved settings
with another S_FMT

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