Re: [FFmpeg-devel] [PATCH 11/13] ffv1dec: reference the current packet into the main context

[Lynne <dev@lynne.ee>]

On 13/03/2025 18:48, Andreas Rheinhardt wrote:
> Lynne:
>> On 13/03/2025 05:57, Andreas Rheinhardt wrote:
>>> Lynne:
>>>>
>>>>
>>>> On 13/03/2025 02:24, Andreas Rheinhardt wrote:
>>>>> Lynne:
>>>>>> On 10/03/2025 18:42, Lynne wrote:
>>>>>>> On 10/03/2025 04:14, Andreas Rheinhardt wrote:
>>>>>>>> Lynne:
>>>>>>>>> ---
>>>>>>>>>      libavcodec/ffv1.h    |  3 +++
>>>>>>>>>      libavcodec/ffv1dec.c | 19 +++++++++++++++++--
>>>>>>>>>      2 files changed, 20 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/libavcodec/ffv1.h b/libavcodec/ffv1.h
>>>>>>>>> index 8c0e71284d..860a5c14b1 100644
>>>>>>>>> --- a/libavcodec/ffv1.h
>>>>>>>>> +++ b/libavcodec/ffv1.h
>>>>>>>>> @@ -174,6 +174,9 @@ typedef struct FFV1Context {
>>>>>>>>>           * NOT shared between frame threads.
>>>>>>>>>           */
>>>>>>>>>          uint8_t           frame_damaged;
>>>>>>>>> +
>>>>>>>>> +    /* Reference to the current packet */
>>>>>>>>> +    AVPacket *pkt_ref;
>>>>>>>>>      } FFV1Context;
>>>>>>>>>      int ff_ffv1_common_init(AVCodecContext *avctx, FFV1Context *s);
>>>>>>>>> diff --git a/libavcodec/ffv1dec.c b/libavcodec/ffv1dec.c
>>>>>>>>> index eaa21eebdf..6396f22f79 100644
>>>>>>>>> --- a/libavcodec/ffv1dec.c
>>>>>>>>> +++ b/libavcodec/ffv1dec.c
>>>>>>>>> @@ -469,6 +469,10 @@ static av_cold int decode_init(AVCodecContext
>>>>>>>>> *avctx)
>>>>>>>>>          f->pix_fmt = AV_PIX_FMT_NONE;
>>>>>>>>>          f->configured_pix_fmt = AV_PIX_FMT_NONE;
>>>>>>>>> +    f->pkt_ref = av_packet_alloc();
>>>>>>>>> +    if (!f->pkt_ref)
>>>>>>>>> +        return AVERROR(ENOMEM);
>>>>>>>>> +
>>>>>>>>>          if ((ret = ff_ffv1_common_init(avctx, f)) < 0)
>>>>>>>>>              return ret;
>>>>>>>>> @@ -701,6 +705,10 @@ static int decode_frame(AVCodecContext *avctx,
>>>>>>>>> AVFrame *rframe,
>>>>>>>>>          /* Start */
>>>>>>>>>          if (hwaccel) {
>>>>>>>>> +        ret = av_packet_ref(f->pkt_ref, avpkt);
>>>>>>>>> +        if (ret < 0)
>>>>>>>>> +            return ret;
>>>>>>>>> +
>>>>>>>>>              ret = hwaccel->start_frame(avctx, avpkt->data, avpkt-
>>>>>>>>>> size);
>>>>>>>>>              if (ret < 0)
>>>>>>>>>                  return ret;
>>>>>>>>> @@ -720,15 +728,21 @@ static int decode_frame(AVCodecContext
>>>>>>>>> *avctx,
>>>>>>>>> AVFrame *rframe,
>>>>>>>>>                  uint32_t len;
>>>>>>>>>                  ret = find_next_slice(avctx, avpkt->data,
>>>>>>>>> buf_end, i,
>>>>>>>>>                                        &pos, &len);
>>>>>>>>> -            if (ret < 0)
>>>>>>>>> +            if (ret < 0) {
>>>>>>>>> +                av_packet_unref(f->pkt_ref);
>>>>>>>>>                      return ret;
>>>>>>>>> +            }
>>>>>>>>>                  buf_end -= len;
>>>>>>>>>                  ret = hwaccel->decode_slice(avctx, pos, len);
>>>>>>>>> -            if (ret < 0)
>>>>>>>>> +            if (ret < 0) {
>>>>>>>>> +                av_packet_unref(f->pkt_ref);
>>>>>>>>>                      return ret;
>>>>>>>>> +            }
>>>>>>>>>              }
>>>>>>>>> +
>>>>>>>>> +        av_packet_unref(f->pkt_ref);
>>>>>>>>>          } else {
>>>>>>>>>              ret = decode_slices(avctx, c, avpkt);
>>>>>>>>>              if (ret < 0)
>>>>>>>>> @@ -827,6 +841,7 @@ static av_cold int
>>>>>>>>> ffv1_decode_close(AVCodecContext *avctx)
>>>>>>>>>          ff_progress_frame_unref(&s->last_picture);
>>>>>>>>>          av_refstruct_unref(&s->hwaccel_last_picture_private);
>>>>>>>>> +    av_packet_free(&s->pkt_ref);
>>>>>>>>>          ff_ffv1_close(s);
>>>>>>>>>          return 0;
>>>>>>>>
>>>>>>>> Why not simply use a const AVPacket*?
>>>>>>>
>>>>>>> No reason. Fixed locally.
>>>>>>> Thanks.
>>>>>>
>>>>>> *reverted this change.
>>>>>> We need to ref the packet, since we map its memory and let the GPU use
>>>>>> it directly without copying the contents. 6k16bit content at 24fps is
>>>>>> typically around 2Gbps when compressed, so avoiding copies is
>>>>>> important.
>>>>>
>>>>> How long does the hwaccel need this data?
>>>>
>>>> Until the frame has been asynchronously decoded. We give an output frame
>>>> with a semaphore that receivers need to wait on to determine when
>>>> that is.
> 
> Does this mean that the frames the caller of avcodec_receive_frame()
> receives are not completely decoded yet?
> 
>>>>
>>>> On the decoder-side, the hardware has a fixed number of queues where
>>>> submissions can be sent to asynchronously. We treat it as a ring buffer
>>>> and keep a reference to all resources our side for each submission,
>>>> until we need to reuse the slot, at which point we wait on the frame
>>>> decoding to complete (which it usually has), and we release all
>>>> resources used.
>>>>
>>>> Output frames also have a bit of state that has to be freed once the
>>>> frame is marked (unreferenced) by the decoder as no longer being needed
>>>> as a reference, this is done in the FFHWAccel.free_frame_priv callback.
>>>> There, we have to wait for the last internal use of the frame to be
>>>> finished (done via the vp->wait_semaphores() call in vulkan_decode.c).
>>>>
>>>> This is valid for both ASIC hardware decoders and a compute shader based
>>>> implementation, since the two share the same code, except for decode
>>>> submissions.
>>>
>>> 1. If you need a reference to the packet's data, then reference
>>> AVPacket.buf, not the whole AVPacket. This avoids allocating a spare
>>> AVPacket as well as copying side data.
>>> 2. It sounds very wrong and fragile that the decoder has to keep a
>>> reference because the hwaccel might need it. There may be future
>>> hwaccels that don't need such a reference etc. It seems better to extend
>>> e.g. the start_frame callback and pass a reference to the input data (no
>>> need to change this for all other start_frame calls; they can pass NULL
>>> until needed).
>>> 3. If the user closes the decoder (which is allowed at any time, even
>>> without draining the decoder), ff_codec_close() uninitializes the
>>> hwaccel after calling the decoder's close function; the latter
>>> unreferences the reference to the packet. Is this really safe?
>>
>> I wanted to avoid having to change the FFHWAccel API for a single
>> hwaccel, so I opted to simply add a field to the codec private context.
>> I'll add an argument to start_frame and resubmit.
>>
>> I don't see why it wouldn't be safe, upon uninit, all submissions are
>> waited on to complete before unreferencing all resources they held.
>> The output frames themselves have no references to any resources used
>> during decoding, so all resources are guaranteed to be freed at uninit.
> 
> The problem is that codec close might free the packet's data and the
> decoder could still process it (or what guarantees that it doesn't?).

The reference to the packet that the hwaccel keeps.
If a packet is not refcounted, we fallback on copying automatically.
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