MP3Decoder: take advantage of background callback

Before this, the mp3 file would be read into the in-memory buffer
only when new samples were actually needed.  This meant that the time
to read mp3 content always counted against the ~22ms audio buffer length.

Now, when there's at least 1 full disk block of free space in the input
buffer, we can request that the buffer be filled _after_ returning from
audiomp3_mp3file_get_buffer and actually filling the DMA pointers.  In
this way, the time taken for reading MP3 data from flash/SD is less
likely to cause an underrun of audio DMA.

The existing calls to fill the inbuf remain, but in most cases during
streaming these become no-ops because the buffer will be over half full.

1 files changed, 2 insertions, 0 deletions

diff --git a/shared-module/audiomp3/MP3Decoder.h b/shared-module/audiomp3/MP3Decoder.h
index 9ee1d0949..f91f102a2 100644
--- a/shared-module/audiomp3/MP3Decoder.h
+++ b/shared-module/audiomp3/MP3Decoder.h
@@ -28,6 +28,7 @@
 #ifndef MICROPY_INCLUDED_SHARED_MODULE_AUDIOIO_MP3FILE_H
 #define MICROPY_INCLUDED_SHARED_MODULE_AUDIOIO_MP3FILE_H
 
+#include "supervisor/background_callback.h"
 #include "extmod/vfs_fat.h"
 #include "py/obj.h"
 
@@ -36,6 +37,7 @@
 typedef struct {
     mp_obj_base_t base;
     struct _MP3DecInfo *decoder;
+    background_callback_t inbuf_fill_cb;
     uint8_t* inbuf;
     uint32_t inbuf_length;
     uint32_t inbuf_offset;