diff --git a/HACKING b/HACKING
index ae420aa0..e8251856 100644
--- a/HACKING
+++ b/HACKING
@@ -81,3 +81,13 @@ automatically includes this option.
 
 For production installations or distribution packaging it is important that
 the "--enable-maintainer-mode" option is NOT used.
+
+Note multiple arguments to -d can be specified, colon, comma or space
+separated. The arguments are relative source code filenames for which
+debugging output should be enabled; output shell-style globs are
+accepted (e.g.: 'plugins/*:src/main.c').
+
+Other debugging settings that can be toggled:
+
+ - Environment variable OFONO_AT_DEBUG (set to 1): enable AT commands
+   debugging
diff --git a/doc/ofonod.8 b/doc/ofonod.8
index 474d7fb3..7bb908cb 100644
--- a/doc/ofonod.8
+++ b/doc/ofonod.8
@@ -18,7 +18,10 @@ is used to manage \fID-Bus\fP permissions for oFono.
 .SH OPTIONS
 .TP
 .B --debug, -d
-Enable debug information output.
+Enable debug information output. Note multiple arguments to -d can be
+specified, colon, comma or space separated. The arguments are relative
+source code filenames for which debugging output should be enabled;
+output shell-style globs are accepted (e.g.: "plugins/*:src/main.c").
 .TP
 .B --nodetach, -n
 Don't run as daemon in background.