diff --git a/libavcodec/alac.c b/libavcodec/alac.c
index 9cd17374a8..5653e9aff5 100644
--- a/libavcodec/alac.c
+++ b/libavcodec/alac.c
@@ -621,6 +621,7 @@ static av_cold int alac_decode_init(AVCodecContext * avctx)
                                    alac->sample_size);
              return AVERROR_PATCHWELCOME;
     }
+    avctx->bits_per_raw_sample = alac->sample_size;
 
     if (alac->channels < 1) {
         av_log(avctx, AV_LOG_WARNING, "Invalid channel count\n");
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 3936d5e1c0..8ee08148b5 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -2249,7 +2249,13 @@ typedef struct AVCodecContext {
 
     /* The following data should not be initialized. */
     /**
-     * Samples per packet, initialized when calling 'init'.
+     * Number of samples per channel in an audio frame.
+     *
+     * - encoding: set by libavcodec in avcodec_open2(). Each submitted frame
+     *   except the last must contain exactly frame_size samples per channel.
+     *   May be 0 when the codec has CODEC_CAP_VARIABLE_FRAME_SIZE set, then the
+     *   frame size is not restricted.
+     * - decoding: may be set by some decoders to indicate constant frame size
      */
     int frame_size;