document buffer requirement, add sidechain constraint.

diff --git a/libs/ardour/plugin_insert.cc b/libs/ardour/plugin_insert.cc
index f7664d995130e5895d67de123a43c67c667f3a9e..74c169196d25854c60892f573a1c5ef4c0fdbcec 100644 (file)
--- a/libs/ardour/plugin_insert.cc
+++ b/libs/ardour/plugin_insert.cc
@@ -1464,8 +1464,21 @@ PluginInsert::configure_io (ChanCount in, ChanCount out)
        /* only the "noinplace_buffers" thread buffers need to be this large,
         * this can be optimized. other buffers are fine with
         * ChanCount::max (natural_input_streams (), natural_output_streams())
+        * and route.cc's max (configured_in, configured_out)
+        *
+        * no-inplace copies "thru" outputs (to emulate in-place) for
+        * all outputs (to prevent overwrite) into a temporary space
+        * which also holds input buffers (in case the plugin does process
+        * in-place and overwrites those).
+        *
+        * this buffers need to be at least as
+        *   natural_input_streams () + possible outputs.
+        *
+        * sidechain inputs add a constraint on the input:
+        * configured input + sidechain (=_configured_internal)
         */
-       _required_buffers = natural_input_streams () + natural_output_streams() * get_count();
+       _required_buffers =ChanCount::max (_configured_internal,
+                       natural_input_streams () + ChanCount::max (_configured_out, natural_output_streams () * get_count ()));
 
        if (old_in != in || old_out != out || old_internal != _configured_internal
                        || (old_match.method != _match.method && (old_match.method == Split || _match.method == Split))