--- a/dom/media/platforms/PlatformDecoderModule.h
+++ b/dom/media/platforms/PlatformDecoderModule.h
@@ -170,16 +170,19 @@ public:
   virtual void Output(MediaData* aData) = 0;
 
   // Denotes an error in the decoding process. The reader will stop calling
   // the decoder.
   virtual void Error(MediaDataDecoderError aError) = 0;
 
   // Denotes that the last input sample has been inserted into the decoder,
   // and no more output can be produced unless more input is sent.
+  // A frame decoding session is completed once InputExhausted has been called.
+  // MediaDataDecoder::Input will not be called again until InputExhausted has
+  // been called.
   virtual void InputExhausted() = 0;
 
   virtual void DrainComplete() = 0;
 
   virtual void ReleaseMediaResources() {}
 
   virtual bool OnReaderTaskQueue() = 0;