PCM Memory Management

snd_pcm_lib_preallocate_free_for_all - release all pre-allocated buffers on the pcm
snd_pcm_lib_preallocate_pages - pre-allocation for the given DMA type
snd_pcm_lib_preallocate_pages_for_all - pre-allocation for continuous memory type (all substreams)
snd_pcm_sgbuf_ops_page - get the page struct at the given offset
snd_pcm_lib_malloc_pages - allocate the DMA buffer
snd_pcm_lib_free_pages - release the allocated DMA buffer.
snd_pcm_lib_free_vmalloc_buffer - free vmalloc buffer
snd_pcm_lib_get_vmalloc_page - map vmalloc buffer offset to page struct