mmaldec: limit internal buffering

This uses a new MMAL feature, which limits the number of extra frames
that can be buffered within the decoder. VIDEO_MAX_NUM_CALLBACKS can
be defined as positive or negative number. Positive numbers are
absolute, and can lead to deadlocks if the user underestimates the
number of required buffers. Negative numbers specify the number of extra
buffers, e.g. -1 means no extra buffer, (-1-N) means N extra buffers.

Set a gratuitous default of -11 (N=10). This is much lower than the
firmware default, which appears to be 96.

This is backwards compatible, but needs a symbol only present in newer
firmware headers. (It's an enum item, so it requires a check in
configure.)

configure

@@ -5579,6 +5579,10 @@ enabled mmal              && { check_lib interface/mmal/mmal.h mmal_port_connect
                                     check_lib interface/mmal/mmal.h mmal_port_connect ; }
                                 check_lib interface/mmal/mmal.h mmal_port_connect ; } ||
                                die "ERROR: mmal not found"; }
+enabled mmal &&
+    (check_code cc interface/mmal/mmal.h "MMAL_PARAMETER_VIDEO_MAX_NUM_CALLBACKS" ||
+     die "ERROR: mmal firmware headers too old")
+
 enabled netcdf            && require_pkg_config netcdf netcdf.h nc_inq_libvers
 enabled nvenc             && { check_header nvEncodeAPI.h || die "ERROR: nvEncodeAPI.h not found."; } &&
                              { check_cpp_condition nvEncodeAPI.h "NVENCAPI_MAJOR_VERSION >= 5" ||

libavcodec/mmaldec.c

@@ -66,6 +66,7 @@ typedef struct FFBufferRef {
 typedef struct MMALDecodeContext {
     AVClass *av_class;
     int extra_buffers;
+    int extra_decoder_buffers;
 
     MMAL_COMPONENT_T *decoder;
     MMAL_QUEUE_T *queue_decoded_frames;
@@ -382,6 +383,11 @@ static av_cold int ffmmal_init_decoder(AVCodecContext *avctx)
     av_get_codec_tag_string(tmp, sizeof(tmp), format_in->encoding);
     av_log(avctx, AV_LOG_DEBUG, "Using MMAL %s encoding.\n", tmp);
 
+    if (mmal_port_parameter_set_uint32(decoder->input[0], MMAL_PARAMETER_VIDEO_MAX_NUM_CALLBACKS,
+                                       -1 - ctx->extra_decoder_buffers)) {
+        av_log(avctx, AV_LOG_WARNING, "Could not set input buffering limit.\n");
+    }
+
     if ((status = mmal_port_format_commit(decoder->input[0])))
         goto fail;
 
@@ -812,6 +818,7 @@ AVHWAccel ff_vc1_mmal_hwaccel = {
 
 static const AVOption options[]={
     {"extra_buffers", "extra buffers", offsetof(MMALDecodeContext, extra_buffers), AV_OPT_TYPE_INT, {.i64 = 10}, 0, 256, 0},
+    {"extra_decoder_buffers", "extra MMAL internal buffered frames", offsetof(MMALDecodeContext, extra_decoder_buffers), AV_OPT_TYPE_INT, {.i64 = 10}, 0, 256, 0},
     {NULL}
 };