avcodec/rkmpp : Fix broken build due to missing control operation

This patch is taking care of https://trac.ffmpeg.org/ticket/6834.
It seems that one of the control operations that was available to get
the free decoders input slots was removed.

There is another control operation to retrieve the used slots. Given
that the input slot count is hardcoded to 4 in mpp at this point,
replacing the old control operation by the other one.

This was tested on Rockchip ROCK64.
Signed-off-by: wm4 <nfxjfg@googlemail.com>

configure

@@ -5993,10 +5993,8 @@ enabled openssl           && { check_pkg_config openssl openssl openssl/ssl.h OP
                                check_lib openssl openssl/ssl.h SSL_library_init -lssl32 -leay32 ||
                                check_lib openssl openssl/ssl.h SSL_library_init -lssl -lcrypto -lws2_32 -lgdi32 ||
                                die "ERROR: openssl not found"; }
-enabled rkmpp             && { { require_pkg_config rkmpp rockchip_mpp rockchip/rk_mpi.h mpp_create ||
-                                 die "ERROR : Rockchip MPP was not found."; } &&
-                               { check_func_headers rockchip/rk_mpi_cmd.h "MPP_DEC_GET_FREE_PACKET_SLOT_COUNT" ||
-                                 die "ERROR: Rockchip MPP is outdated, please get a more recent one."; } &&
+enabled rkmpp             && { require_pkg_config rkmpp rockchip_mpp  rockchip/rk_mpi.h mpp_create &&
+                               require_pkg_config rockchip_mpp "rockchip_mpp >= 1.3.7" rockchip/rk_mpi.h mpp_create &&
                                { enabled libdrm ||
                                  die "ERROR: rkmpp requires --enable-libdrm"; }
                              }

libavcodec/rkmppdec.c

@@ -40,6 +40,7 @@
 
 #define RECEIVE_FRAME_TIMEOUT   100
 #define FRAMEGROUP_MAX_FRAMES   16
+#define INPUT_MAX_PACKETS       4
 
 typedef struct {
     MppCtx ctx;
@@ -515,16 +516,17 @@ static int rkmpp_receive_frame(AVCodecContext *avctx, AVFrame *frame)
     RKMPPDecoder *decoder = (RKMPPDecoder *)rk_context->decoder_ref->data;
     int ret = MPP_NOK;
     AVPacket pkt = {0};
-    RK_S32 freeslots;
+    RK_S32 usedslots, freeslots;
 
     if (!decoder->eos_reached) {
         // we get the available slots in decoder
-        ret = decoder->mpi->control(decoder->ctx, MPP_DEC_GET_FREE_PACKET_SLOT_COUNT, &freeslots);
+        ret = decoder->mpi->control(decoder->ctx, MPP_DEC_GET_STREAM_COUNT, &usedslots);
         if (ret != MPP_OK) {
-            av_log(avctx, AV_LOG_ERROR, "Failed to get decoder free slots (code = %d).\n", ret);
+            av_log(avctx, AV_LOG_ERROR, "Failed to get decoder used slots (code = %d).\n", ret);
             return ret;
         }
 
+        freeslots = INPUT_MAX_PACKETS - usedslots;
         if (freeslots > 0) {
             ret = ff_decode_get_packet(avctx, &pkt);
             if (ret < 0 && ret != AVERROR_EOF) {
@@ -541,7 +543,7 @@ static int rkmpp_receive_frame(AVCodecContext *avctx, AVFrame *frame)
         }
 
         // make sure we keep decoder full
-        if (freeslots > 1 && decoder->first_frame)
+        if (freeslots > 1)
             return AVERROR(EAGAIN);
     }