Created attachment 343733 [details] [review]
0001-decklinkaudiosrc-Option-to-use-max-channels-supporte.patch

This works, but it feels a bit clumsy in some parts. Please review.

Review of attachment 343733 [details] [review]:

::: sys/decklink/gstdecklinkaudiosrc.cpp
@@ +742,3 @@
+    if (self->input->attributes) {
+      HRESULT ret = self->input->attributes->GetInt
+          (BMDDeckLinkMaximumAudioChannels, &self->channels_found);

I would move that to gstdecklink.cpp in the loop where we detect all the cards, and then just reuse that value here

@@ +758,3 @@
+        self->channels_found = DEFAULT_CHANNELS;
+      }
+    }

You might also want to negotiate here the maximum of the channels supported by the card and downstream, i.e. always return the whole range here and in fixate_caps() fixate to the maximum

Created attachment 344321 [details] [review]
0001-decklinkaudiosrc-Option-to-use-max-channels-supporte.patch

Thanks, here is the new patch. I didn't move the channel detection code BTW (as discussed IRL) because a "git grep" showed me that other parts of the code do it the same way as here, so I kept it consistent. I only changed the negotiation.

commit f23277e55df363ec8e3686f19edf21d1810ab62d
Author: Vivia Nikolaidou <vivia@toolsonair.com>
Date:   Wed Jan 18 17:53:00 2017 +0200

    decklinkaudiosrc: Option to use max channels supported by device
    
    Query the device for the maximum number of channels supported and have
    an option to use that. Default is still 2.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=777458