WaveRT audio driver - Questions regarding allocating audio buffer

I am a novice in writing drivers. I am trying to write WaveRT audio driver for windows 7.

I am looking at the sample code for waveRT provided with win-ddk for ac97 audio device.

I understand that the IMiniportWaveRT stream interface requires us to implement AllocateAudioBuffer method. This method allocates a memory descriptor list using the AllocateContiguousPagesForMdl(since the device in the sample code does not support scatter-gather) method of PORTWAVERTSTREAM interface, an instance of which is passed to the driver while creating a new stream. After this memory allocation succeeds, BDL(buffer descriptor list) is programmed.

I have a few questions regarding the allocation scheme of audio buffer here:

  1. When exactly does the AllocateAudioBuffer callback method gets called?
  2. When does the memory allocated in AllocateAudioBuffer method gets mapped to the user’s address space? Is it done by the miniport driver or the port driver?