lavc: add MMAL hardware decoder wrapper

Based on a patch by Rodger Combs.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

4 files changed, 774 insertions, 1 deletions

diff --git a/libavcodec/Makefile b/libavcodec/Makefile
index c633a4e3da..f2e02fd5b4 100644
--- a/libavcodec/Makefile
+++ b/libavcodec/Makefile
@@ -224,6 +224,7 @@ OBJS-$(CONFIG_H264_DECODER)            += h264.o h264_cabac.o h264_cavlc.o \
                                           h264_direct.o h264_loopfilter.o  \
                                           h264_mb.o h264_picture.o h264_ps.o \
                                           h264_refs.o h264_sei.o h264_slice.o
+OBJS-$(CONFIG_H264_MMAL_DECODER)       += mmaldec.o
 OBJS-$(CONFIG_H264_QSV_DECODER)        += qsvdec_h264.o
 OBJS-$(CONFIG_H264_QSV_ENCODER)        += qsvenc_h264.o
 OBJS-$(CONFIG_HEVC_DECODER)            += hevc.o hevc_mvs.o hevc_ps.o hevc_sei.o \
diff --git a/libavcodec/allcodecs.c b/libavcodec/allcodecs.c
index e185e9841d..d17eb16085 100644
--- a/libavcodec/allcodecs.c
+++ b/libavcodec/allcodecs.c
@@ -77,6 +77,7 @@ void avcodec_register_all(void)
     REGISTER_HWACCEL(H263_VAAPI,        h263_vaapi);
     REGISTER_HWACCEL(H263_VDPAU,        h263_vdpau);
     REGISTER_HWACCEL(H264_DXVA2,        h264_dxva2);
+    REGISTER_HWACCEL(H264_MMAL,         h264_mmal);
     REGISTER_HWACCEL(H264_QSV,          h264_qsv);
     REGISTER_HWACCEL(H264_VAAPI,        h264_vaapi);
     REGISTER_HWACCEL(H264_VDA,          h264_vda);
@@ -161,6 +162,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER(H263I,             h263i);
     REGISTER_ENCODER(H263P,             h263p);
     REGISTER_DECODER(H264,              h264);
+    REGISTER_DECODER(H264_MMAL,         h264_mmal);
     REGISTER_DECODER(H264_QSV,          h264_qsv);
     REGISTER_DECODER(HEVC,              hevc);
     REGISTER_DECODER(HNM4_VIDEO,        hnm4_video);
diff --git a/libavcodec/mmaldec.c b/libavcodec/mmaldec.c
new file mode 100644
index 0000000000..c59a496246
--- /dev/null
+++ b/libavcodec/mmaldec.c
@@ -0,0 +1,770 @@
+/*
+ * MMAL Video Decoder
+ * Copyright (c) 2015 Rodger Combs
+ *
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * MMAL Video Decoder
+ */
+
+#include "avcodec.h"
+#include "internal.h"
+#include "libavutil/atomic.h"
+#include "libavutil/avassert.h"
+#include "libavutil/buffer.h"
+#include "libavutil/common.h"
+#include "libavutil/opt.h"
+#include "libavutil/log.h"
+
+#include <bcm_host.h>
+#include <interface/mmal/mmal.h>
+#include <interface/mmal/util/mmal_util.h>
+#include <interface/mmal/util/mmal_util_params.h>
+#include <interface/mmal/util/mmal_default_components.h>
+
+typedef struct FFBufferEntry {
+    AVBufferRef *ref;
+    void *data;
+    size_t length;
+    int64_t pts, dts;
+    int flags;
+    struct FFBufferEntry *next;
+} FFBufferEntry;
+
+// MMAL_POOL_T destroys all of its MMAL_BUFFER_HEADER_Ts. If we want correct
+// refcounting for AVFrames, we can free the MMAL_POOL_T only after all AVFrames
+// have been unreferenced.
+typedef struct FFPoolRef {
+    volatile int refcount;
+    MMAL_POOL_T *pool;
+} FFPoolRef;
+
+typedef struct FFBufferRef {
+    MMAL_BUFFER_HEADER_T *buffer;
+    FFPoolRef *pool;
+} FFBufferRef;
+
+typedef struct MMALDecodeContext {
+    AVClass *av_class;
+    int extra_buffers;
+
+    AVBitStreamFilterContext *bsfc;
+
+    MMAL_COMPONENT_T *decoder;
+    MMAL_QUEUE_T *queue_decoded_frames;
+    MMAL_POOL_T *pool_in;
+    FFPoolRef *pool_out;
+
+    // Waiting input packets. Because the libavcodec API requires decoding and
+    // returning packets in lockstep, it can happen that queue_decoded_frames
+    // contains almost all surfaces - then the decoder input queue can quickly
+    // fill up and won't accept new input either. Without consuming input, the
+    // libavcodec API can't return new frames, and we have a logical deadlock.
+    // This is avoided by queuing such buffers here.
+    FFBufferEntry *waiting_buffers, *waiting_buffers_tail;
+
+    int64_t packets_sent;
+    int64_t frames_output;
+    int eos_received;
+    int eos_sent;
+} MMALDecodeContext;
+
+// Assume decoder is guaranteed to produce output after at least this many
+// packets (where each packet contains 1 frame).
+#define MAX_DELAYED_FRAMES 16
+
+static void ffmmal_poolref_unref(FFPoolRef *ref)
+{
+    if (ref && avpriv_atomic_int_add_and_fetch(&ref->refcount, -1) == 0) {
+        mmal_pool_destroy(ref->pool);
+        av_free(ref);
+    }
+}
+
+static void ffmmal_release_frame(void *opaque, uint8_t *data)
+{
+    FFBufferRef *ref = (void *)data;
+
+    mmal_buffer_header_release(ref->buffer);
+    ffmmal_poolref_unref(ref->pool);
+
+    av_free(ref);
+}
+
+// Setup frame with a new reference to buffer. The buffer must have been
+// allocated from the given pool.
+static int ffmmal_set_ref(AVFrame *frame, FFPoolRef *pool,
+                          MMAL_BUFFER_HEADER_T *buffer)
+{
+    FFBufferRef *ref = av_mallocz(sizeof(*ref));
+    if (!ref)
+        return AVERROR(ENOMEM);
+
+    ref->pool = pool;
+    ref->buffer = buffer;
+
+    frame->buf[0] = av_buffer_create((void *)ref, sizeof(*ref),
+                                     ffmmal_release_frame, NULL,
+                                     AV_BUFFER_FLAG_READONLY);
+    if (!frame->buf[0]) {
+        av_free(ref);
+        return AVERROR(ENOMEM);
+    }
+
+    avpriv_atomic_int_add_and_fetch(&ref->pool->refcount, 1);
+    mmal_buffer_header_acquire(buffer);
+
+    frame->format = AV_PIX_FMT_MMAL;
+    frame->data[3] = (uint8_t *)ref->buffer;
+    return 0;
+}
+
+static void ffmmal_stop_decoder(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    MMAL_COMPONENT_T *decoder = ctx->decoder;
+    MMAL_BUFFER_HEADER_T *buffer;
+
+    mmal_port_disable(decoder->input[0]);
+    mmal_port_disable(decoder->output[0]);
+    mmal_port_disable(decoder->control);
+
+    mmal_port_flush(decoder->input[0]);
+    mmal_port_flush(decoder->output[0]);
+    mmal_port_flush(decoder->control);
+
+    while ((buffer = mmal_queue_get(ctx->queue_decoded_frames)))
+        mmal_buffer_header_release(buffer);
+
+    while (ctx->waiting_buffers) {
+        FFBufferEntry *buffer = ctx->waiting_buffers;
+
+        ctx->waiting_buffers = buffer->next;
+
+        av_buffer_unref(&buffer->ref);
+        av_free(buffer);
+    }
+    ctx->waiting_buffers_tail = NULL;
+
+    ctx->frames_output = ctx->eos_received = ctx->eos_sent = ctx->packets_sent = 0;
+}
+
+static av_cold int ffmmal_close_decoder(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+
+    if (ctx->decoder)
+        ffmmal_stop_decoder(avctx);
+
+    mmal_component_destroy(ctx->decoder);
+    ctx->decoder = NULL;
+    mmal_queue_destroy(ctx->queue_decoded_frames);
+    mmal_pool_destroy(ctx->pool_in);
+    ffmmal_poolref_unref(ctx->pool_out);
+
+    if (ctx->bsfc)
+        av_bitstream_filter_close(ctx->bsfc);
+
+    return 0;
+}
+
+static void input_callback(MMAL_PORT_T *port, MMAL_BUFFER_HEADER_T *buffer)
+{
+    if (!buffer->cmd) {
+        AVBufferRef *buf = buffer->user_data;
+        av_buffer_unref(&buf);
+    }
+    mmal_buffer_header_release(buffer);
+}
+
+static void output_callback(MMAL_PORT_T *port, MMAL_BUFFER_HEADER_T *buffer)
+{
+    AVCodecContext *avctx = (AVCodecContext*)port->userdata;
+    MMALDecodeContext *ctx = avctx->priv_data;
+
+    mmal_queue_put(ctx->queue_decoded_frames, buffer);
+}
+
+static void control_port_cb(MMAL_PORT_T *port, MMAL_BUFFER_HEADER_T *buffer)
+{
+    AVCodecContext *avctx = (AVCodecContext*)port->userdata;
+    MMAL_STATUS_T status;
+
+    if (buffer->cmd == MMAL_EVENT_ERROR) {
+        status = *(uint32_t *)buffer->data;
+        av_log(avctx, AV_LOG_ERROR, "MMAL error %d on control port\n", (int)status);
+    } else {
+        char s[20];
+        av_get_codec_tag_string(s, sizeof(s), buffer->cmd);
+        av_log(avctx, AV_LOG_WARNING, "Unknown MMAL event %s on control port\n", s);
+    }
+
+    mmal_buffer_header_release(buffer);
+}
+
+// Feed free output buffers to the decoder.
+static int ffmmal_fill_output_port(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    MMAL_BUFFER_HEADER_T *buffer;
+    MMAL_STATUS_T status;
+
+    if (!ctx->pool_out)
+        return AVERROR_UNKNOWN; // format change code failed with OOM previously
+
+    while ((buffer = mmal_queue_get(ctx->pool_out->pool->queue))) {
+        if ((status = mmal_port_send_buffer(ctx->decoder->output[0], buffer))) {
+            mmal_buffer_header_release(buffer);
+            av_log(avctx, AV_LOG_ERROR, "MMAL error %d when sending output buffer.\n", (int)status);
+            return AVERROR_UNKNOWN;
+        }
+    }
+
+    return 0;
+}
+
+static enum AVColorSpace ffmmal_csp_to_av_csp(MMAL_FOURCC_T fourcc)
+{
+    switch (fourcc) {
+    case MMAL_COLOR_SPACE_BT470_2_BG:
+    case MMAL_COLOR_SPACE_BT470_2_M:
+    case MMAL_COLOR_SPACE_ITUR_BT601:   return AVCOL_SPC_BT470BG;
+    case MMAL_COLOR_SPACE_ITUR_BT709:   return AVCOL_SPC_BT709;
+    case MMAL_COLOR_SPACE_FCC:          return AVCOL_SPC_FCC;
+    case MMAL_COLOR_SPACE_SMPTE240M:    return AVCOL_SPC_SMPTE240M;
+    default:                            return AVCOL_SPC_UNSPECIFIED;
+    }
+}
+
+static int ffmal_update_format(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    MMAL_STATUS_T status;
+    int ret = 0;
+    MMAL_COMPONENT_T *decoder = ctx->decoder;
+    MMAL_ES_FORMAT_T *format_out = decoder->output[0]->format;
+
+    ffmmal_poolref_unref(ctx->pool_out);
+    if (!(ctx->pool_out = av_mallocz(sizeof(*ctx->pool_out)))) {
+        ret = AVERROR(ENOMEM);
+        goto fail;
+    }
+    ctx->pool_out->refcount = 1;
+
+    if (!format_out)
+        goto fail;
+
+    if ((status = mmal_port_parameter_set_uint32(decoder->output[0], MMAL_PARAMETER_EXTRA_BUFFERS, ctx->extra_buffers)))
+        goto fail;
+
+    if (avctx->pix_fmt == AV_PIX_FMT_MMAL) {
+        format_out->encoding = MMAL_ENCODING_OPAQUE;
+    } else {
+        format_out->encoding_variant = format_out->encoding = MMAL_ENCODING_I420;
+    }
+
+    if ((status = mmal_port_format_commit(decoder->output[0])))
+        goto fail;
+
+    if ((ret = ff_set_dimensions(avctx, format_out->es->video.crop.x + format_out->es->video.crop.width,
+                                        format_out->es->video.crop.y + format_out->es->video.crop.height)) < 0)
+        goto fail;
+
+    if (format_out->es->video.par.num && format_out->es->video.par.den) {
+        avctx->sample_aspect_ratio.num = format_out->es->video.par.num;
+        avctx->sample_aspect_ratio.den = format_out->es->video.par.den;
+    }
+
+    avctx->colorspace = ffmmal_csp_to_av_csp(format_out->es->video.color_space);
+
+    decoder->output[0]->buffer_size =
+        FFMAX(decoder->output[0]->buffer_size_min, decoder->output[0]->buffer_size_recommended);
+    decoder->output[0]->buffer_num =
+        FFMAX(decoder->output[0]->buffer_num_min, decoder->output[0]->buffer_num_recommended) + ctx->extra_buffers;
+    ctx->pool_out->pool = mmal_pool_create(decoder->output[0]->buffer_num,
+                                           decoder->output[0]->buffer_size);
+    if (!ctx->pool_out->pool) {
+        ret = AVERROR(ENOMEM);
+        goto fail;
+    }
+
+    return 0;
+
+fail:
+    return ret < 0 ? ret : AVERROR_UNKNOWN;
+}
+
+static av_cold int ffmmal_init_decoder(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    MMAL_STATUS_T status;
+    MMAL_ES_FORMAT_T *format_in;
+    MMAL_COMPONENT_T *decoder;
+    int ret = 0;
+
+    bcm_host_init();
+
+    if ((ret = ff_get_format(avctx, avctx->codec->pix_fmts)) < 0)
+        return ret;
+
+    avctx->pix_fmt = ret;
+
+    if ((status = mmal_component_create(MMAL_COMPONENT_DEFAULT_VIDEO_DECODER, &ctx->decoder)))
+        goto fail;
+
+    decoder = ctx->decoder;
+
+    format_in = decoder->input[0]->format;
+    format_in->type = MMAL_ES_TYPE_VIDEO;
+    format_in->encoding = MMAL_ENCODING_H264;
+    format_in->es->video.width = FFALIGN(avctx->width, 32);
+    format_in->es->video.height = FFALIGN(avctx->height, 16);
+    format_in->es->video.crop.width = avctx->width;
+    format_in->es->video.crop.height = avctx->height;
+    format_in->es->video.frame_rate.num = 24000;
+    format_in->es->video.frame_rate.den = 1001;
+    format_in->es->video.par.num = avctx->sample_aspect_ratio.num;
+    format_in->es->video.par.den = avctx->sample_aspect_ratio.den;
+    format_in->flags = MMAL_ES_FORMAT_FLAG_FRAMED;
+
+    if (avctx->codec->id == AV_CODEC_ID_H264 && avctx->extradata && avctx->extradata[0] == 1) {
+        uint8_t *dummy_p;
+        int dummy_int;
+        ctx->bsfc = av_bitstream_filter_init("h264_mp4toannexb");
+        if (!ctx->bsfc) {
+            av_log(avctx, AV_LOG_ERROR, "Cannot open the h264_mp4toannexb BSF!\n");
+            ret = AVERROR(ENOSYS);
+            goto fail;
+        }
+        av_bitstream_filter_filter(ctx->bsfc, avctx, NULL, &dummy_p, &dummy_int, NULL, 0, 0);
+    }
+
+    if (avctx->extradata_size) {
+        if ((status = mmal_format_extradata_alloc(format_in, avctx->extradata_size)))
+            goto fail;
+        format_in->extradata_size = avctx->extradata_size;
+        memcpy(format_in->extradata, avctx->extradata, format_in->extradata_size);
+    }
+
+    if ((status = mmal_port_format_commit(decoder->input[0])))
+        goto fail;
+
+    decoder->input[0]->buffer_num =
+        FFMAX(decoder->input[0]->buffer_num_min, 20);
+    decoder->input[0]->buffer_size =
+        FFMAX(decoder->input[0]->buffer_size_min, 512 * 1024);
+    ctx->pool_in = mmal_pool_create(decoder->input[0]->buffer_num, 0);
+    if (!ctx->pool_in) {
+        ret = AVERROR(ENOMEM);
+        goto fail;
+    }
+
+    if ((ret = ffmal_update_format(avctx)) < 0)
+        goto fail;
+
+    ctx->queue_decoded_frames = mmal_queue_create();
+    if (!ctx->queue_decoded_frames)
+        goto fail;
+
+    decoder->input[0]->userdata = (void*)avctx;
+    decoder->output[0]->userdata = (void*)avctx;
+    decoder->control->userdata = (void*)avctx;
+
+    if ((status = mmal_port_enable(decoder->control, control_port_cb)))
+        goto fail;
+    if ((status = mmal_port_enable(decoder->input[0], input_callback)))
+        goto fail;
+    if ((status = mmal_port_enable(decoder->output[0], output_callback)))
+        goto fail;
+
+    if ((status = mmal_component_enable(decoder)))
+        goto fail;
+
+    return 0;
+
+fail:
+    ffmmal_close_decoder(avctx);
+    return ret < 0 ? ret : AVERROR_UNKNOWN;
+}
+
+static void ffmmal_flush(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    MMAL_COMPONENT_T *decoder = ctx->decoder;
+    MMAL_STATUS_T status;
+
+    ffmmal_stop_decoder(avctx);
+
+    if ((status = mmal_port_enable(decoder->control, control_port_cb)))
+        goto fail;
+    if ((status = mmal_port_enable(decoder->input[0], input_callback)))
+        goto fail;
+    if ((status = mmal_port_enable(decoder->output[0], output_callback)))
+        goto fail;
+
+    return;
+
+fail:
+    av_log(avctx, AV_LOG_ERROR, "MMAL flush error: %i\n", (int)status);
+}
+
+// Split packets and add them to the waiting_buffers list. We don't queue them
+// immediately, because it can happen that the decoder is temporarily blocked
+// (due to us not reading/returning enough output buffers) and won't accept
+// new input. (This wouldn't be an issue if MMAL input buffers always were
+// complete frames - then the input buffer just would have to be big enough.)
+static int ffmmal_add_packet(AVCodecContext *avctx, AVPacket *avpkt)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    AVBufferRef *buf = NULL;
+    int size = 0;
+    uint8_t *data = (uint8_t *)"";
+    uint8_t *start;
+    int ret = 0;
+
+    ctx->packets_sent++;
+
+    if (avpkt->size) {
+        if (ctx->bsfc) {
+            uint8_t *tmp_data;
+            int tmp_size;
+            if ((ret = av_bitstream_filter_filter(ctx->bsfc, avctx, NULL,
+                                                  &tmp_data, &tmp_size,
+                                                  avpkt->data, avpkt->size,
+                                                  avpkt->flags & AV_PKT_FLAG_KEY)) < 0)
+                goto done;
+            buf = av_buffer_create(tmp_data, tmp_size, NULL, NULL, 0);
+        } else {
+            if (avpkt->buf) {
+                buf = av_buffer_ref(avpkt->buf);
+            } else {
+                buf = av_buffer_alloc(avpkt->size);
+                if (buf)
+                    memcpy(buf->data, avpkt->data, avpkt->size);
+            }
+        }
+        if (!buf) {
+            ret = AVERROR(ENOMEM);
+            goto done;
+        }
+        size = buf->size;
+        data = buf->data;
+    }
+
+    start = data;
+
+    do {
+        FFBufferEntry *buffer = av_mallocz(sizeof(*buffer));
+        if (!buffer) {
+            ret = AVERROR(ENOMEM);
+            goto done;
+        }
+
+        buffer->data = data;
+        buffer->length = FFMIN(size, ctx->decoder->input[0]->buffer_size);
+
+        if (data == start)
+            buffer->flags |= MMAL_BUFFER_HEADER_FLAG_FRAME_START;
+
+        data += buffer->length;
+        size -= buffer->length;
+
+        buffer->pts = avpkt->pts == AV_NOPTS_VALUE ? MMAL_TIME_UNKNOWN : avpkt->pts;
+        buffer->dts = avpkt->dts == AV_NOPTS_VALUE ? MMAL_TIME_UNKNOWN : avpkt->dts;
+
+        if (!size)
+            buffer->flags |= MMAL_BUFFER_HEADER_FLAG_FRAME_END;
+
+        if (!buffer->length) {
+            buffer->flags |= MMAL_BUFFER_HEADER_FLAG_EOS;
+            ctx->eos_sent = 1;
+        }
+
+        if (buf) {
+            buffer->ref = av_buffer_ref(buf);
+            if (!buffer->ref) {
+                av_free(buffer);
+                ret = AVERROR(ENOMEM);
+                goto done;
+            }
+        }
+
+        // Insert at end of the list
+        if (!ctx->waiting_buffers)
+            ctx->waiting_buffers = buffer;
+        if (ctx->waiting_buffers_tail)
+            ctx->waiting_buffers_tail->next = buffer;
+        ctx->waiting_buffers_tail = buffer;
+    } while (size);
+
+done:
+    av_buffer_unref(&buf);
+    return ret;
+}
+
+// Move prepared/split packets from waiting_buffers to the MMAL decoder.
+static int ffmmal_fill_input_port(AVCodecContext *avctx)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+
+    while (ctx->waiting_buffers) {
+        MMAL_BUFFER_HEADER_T *mbuffer;
+        FFBufferEntry *buffer;
+        MMAL_STATUS_T status;
+
+        mbuffer = mmal_queue_get(ctx->pool_in->queue);
+        if (!mbuffer)
+            return 0;
+
+        buffer = ctx->waiting_buffers;
+
+        mmal_buffer_header_reset(mbuffer);
+        mbuffer->cmd = 0;
+        mbuffer->pts = buffer->pts;
+        mbuffer->dts = buffer->dts;
+        mbuffer->flags = buffer->flags;
+        mbuffer->data = buffer->data;
+        mbuffer->length = buffer->length;
+        mbuffer->user_data = buffer->ref;
+        mbuffer->alloc_size = ctx->decoder->input[0]->buffer_size;
+
+        if ((status = mmal_port_send_buffer(ctx->decoder->input[0], mbuffer))) {
+            mmal_buffer_header_release(mbuffer);
+            av_buffer_unref(&buffer->ref);
+        }
+
+        // Remove from start of the list
+        ctx->waiting_buffers = buffer->next;
+        if (ctx->waiting_buffers_tail == buffer)
+            ctx->waiting_buffers_tail = NULL;
+        av_free(buffer);
+
+        if (status) {
+            av_log(avctx, AV_LOG_ERROR, "MMAL error %d when sending input\n", (int)status);
+            return AVERROR_UNKNOWN;
+        }
+    }
+
+    return 0;
+}
+
+static int ffmal_copy_frame(AVCodecContext *avctx,  AVFrame *frame,
+                            MMAL_BUFFER_HEADER_T *buffer)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    int ret = 0;
+
+    if (avctx->pix_fmt == AV_PIX_FMT_MMAL) {
+        if (!ctx->pool_out)
+            return AVERROR_UNKNOWN; // format change code failed with OOM previously
+
+        if ((ret = ff_decode_frame_props(avctx, frame)) < 0)
+            goto done;
+
+        if ((ret = ffmmal_set_ref(frame, ctx->pool_out, buffer)) < 0)
+            goto done;
+    } else {
+        int w = FFALIGN(avctx->width, 32);
+        int h = FFALIGN(avctx->height, 16);
+        char *ptr;
+        int plane;
+        int i;
+
+        if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
+            goto done;
+
+        ptr = buffer->data + buffer->type->video.offset[0];
+        for (i = 0; i < avctx->height; i++)
+            memcpy(frame->data[0] + frame->linesize[0] * i, ptr + w * i, avctx->width);
+
+        ptr += w * h;
+
+        for (plane = 1; plane < 3; plane++) {
+            for (i = 0; i < avctx->height / 2; i++)
+                memcpy(frame->data[plane] + frame->linesize[plane] * i, ptr + w / 2 * i, (avctx->width + 1) / 2);
+            ptr += w / 2 * h / 2;
+        }
+    }
+
+    if (buffer->pts != MMAL_TIME_UNKNOWN) {
+        frame->pkt_pts = buffer->pts;
+        frame->pts = buffer->pts;
+    }
+
+done:
+    return ret;
+}
+
+// Fetch a decoded buffer and place it into the frame parameter.
+static int ffmmal_read_frame(AVCodecContext *avctx, AVFrame *frame, int *got_frame)
+{
+    MMALDecodeContext *ctx = avctx->priv_data;
+    MMAL_BUFFER_HEADER_T *buffer = NULL;
+    MMAL_STATUS_T status = 0;
+    int ret = 0;
+
+    if (ctx->eos_received)
+        goto done;
+
+    while (1) {
+        // To ensure decoding in lockstep with a constant delay between fed packets
+        // and output frames, we always wait until an output buffer is available.
+        // Except during start we don't know after how many input packets the decoder
+        // is going to return the first buffer, and we can't distinguish decoder
+        // being busy from decoder waiting for input. So just poll at the start and
+        // keep feeding new data to the buffer.
+        // We are pretty sure the decoder will produce output if we sent more input
+        // frames than what a h264 decoder could logically delay. This avoids too
+        // excessive buffering.
+        // We also wait if we sent eos, but didn't receive it yet (think of decoding
+        // stream with a very low number of frames).
+        if (ctx->frames_output || ctx->packets_sent > MAX_DELAYED_FRAMES || ctx->eos_sent) {
+            buffer = mmal_queue_wait(ctx->queue_decoded_frames);
+        } else {
+            buffer = mmal_queue_get(ctx->queue_decoded_frames);
+        }
+        if (!buffer)
+            goto done;
+
+        ctx->eos_received |= !!(buffer->flags & MMAL_BUFFER_HEADER_FLAG_EOS);
+        if (ctx->eos_received)
+            goto done;
+
+        if (buffer->cmd == MMAL_EVENT_FORMAT_CHANGED) {
+            MMAL_COMPONENT_T *decoder = ctx->decoder;
+            MMAL_EVENT_FORMAT_CHANGED_T *ev = mmal_event_format_changed_get(buffer);
+            MMAL_BUFFER_HEADER_T *stale_buffer;
+
+            av_log(avctx, AV_LOG_INFO, "Changing output format.\n");
+
+            if ((status = mmal_port_disable(decoder->output[0])))
+                goto done;
+
+            while ((stale_buffer = mmal_queue_get(ctx->queue_decoded_frames)))
+                mmal_buffer_header_release(stale_buffer);
+
+            mmal_format_copy(decoder->output[0]->format, ev->format);
+
+            if ((ret = ffmal_update_format(avctx)) < 0)
+                goto done;
+
+            if ((status = mmal_port_enable(decoder->output[0], output_callback)))
+                goto done;
+
+            if ((ret = ffmmal_fill_output_port(avctx)) < 0)
+                goto done;
+
+            if ((ret = ffmmal_fill_input_port(avctx)) < 0)
+                goto done;
+
+            mmal_buffer_header_release(buffer);
+            continue;
+        } else if (buffer->cmd) {
+            char s[20];
+            av_get_codec_tag_string(s, sizeof(s), buffer->cmd);
+            av_log(avctx, AV_LOG_WARNING, "Unknown MMAL event %s on output port\n", s);
+            goto done;
+        } else if (buffer->length == 0) {
+            // Unused output buffer that got drained after format change.
+            mmal_buffer_header_release(buffer);
+            continue;
+        }
+
+        ctx->frames_output++;
+
+        if ((ret = ffmal_copy_frame(avctx, frame, buffer)) < 0)
+            goto done;
+
+        *got_frame = 1;
+        break;
+    }
+
+done:
+    if (buffer)
+        mmal_buffer_header_release(buffer);
+    if (status && ret >= 0)
+        ret = AVERROR_UNKNOWN;
+    return ret;
+}
+
+static int ffmmal_decode(AVCodecContext *avctx, void *data, int *got_frame,
+                         AVPacket *avpkt)
+{
+    AVFrame *frame = data;
+    int ret = 0;
+
+    if ((ret = ffmmal_add_packet(avctx, avpkt)) < 0)
+        return ret;
+
+    if ((ret = ffmmal_fill_input_port(avctx)) < 0)
+        return ret;
+
+    if ((ret = ffmmal_fill_output_port(avctx)) < 0)
+        return ret;
+
+    if ((ret = ffmmal_read_frame(avctx, frame, got_frame)) < 0)
+        return ret;
+
+    // ffmmal_read_frame() can block for a while. Since the decoder is
+    // asynchronous, it's a good idea to fill the ports again.
+
+    if ((ret = ffmmal_fill_output_port(avctx)) < 0)
+        return ret;
+
+    if ((ret = ffmmal_fill_input_port(avctx)) < 0)
+        return ret;
+
+    return ret;
+}
+
+AVHWAccel ff_h264_mmal_hwaccel = {
+    .name       = "h264_mmal",
+    .type       = AVMEDIA_TYPE_VIDEO,
+    .id         = AV_CODEC_ID_H264,
+    .pix_fmt    = AV_PIX_FMT_MMAL,
+};
+
+static const AVOption options[]={
+    {"extra_buffers", "extra buffers", offsetof(MMALDecodeContext, extra_buffers), AV_OPT_TYPE_INT, {.i64 = 10}, 0, 256, 0},
+    {NULL}
+};
+
+static const AVClass ffmmaldec_class = {
+    .class_name = "mmaldec",
+    .option     = options,
+    .version    = LIBAVUTIL_VERSION_INT,
+};
+
+AVCodec ff_h264_mmal_decoder = {
+    .name           = "h264_mmal",
+    .long_name      = NULL_IF_CONFIG_SMALL("h264 (mmal)"),
+    .type           = AVMEDIA_TYPE_VIDEO,
+    .id             = AV_CODEC_ID_H264,
+    .priv_data_size = sizeof(MMALDecodeContext),
+    .init           = ffmmal_init_decoder,
+    .close          = ffmmal_close_decoder,
+    .decode         = ffmmal_decode,
+    .flush          = ffmmal_flush,
+    .priv_class     = &ffmmaldec_class,
+    .capabilities   = CODEC_CAP_DELAY,
+    .pix_fmts       = (const enum AVPixelFormat[]) { AV_PIX_FMT_MMAL,
+                                                     AV_PIX_FMT_YUV420P,
+                                                     AV_PIX_FMT_NONE},
+};
diff --git a/libavcodec/version.h b/libavcodec/version.h
index ecfd6d8a7f..e99e8f98ec 100644
--- a/libavcodec/version.h
+++ b/libavcodec/version.h
@@ -29,7 +29,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR 56
-#define LIBAVCODEC_VERSION_MINOR 20
+#define LIBAVCODEC_VERSION_MINOR 21
 #define LIBAVCODEC_VERSION_MICRO  0
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \