Re: [FFmpeg-devel] [PATCH v4 15/16] FFHWAccel: add buffer_ref argument to start_frame

[Lynne <dev@lynne.ee>]

On 14/03/2025 00:03, Lynne wrote:
> 
> 
> On 13/03/2025 23:08, Andreas Rheinhardt wrote:
>> Lynne:
>>> This commit adds a reference to the buffer as an argument to
>>> start_frame, and adapts all existing code.
>>>
>>> This allows for asynchronous hardware accelerators to skip
>>> copying packet data by referencing it.
>>> ---
>>> diff --git a/libavcodec/hevc/hevcdec.c b/libavcodec/hevc/hevcdec.c
>>> index da8fdc5935..20ef821819 100644
>>> --- a/libavcodec/hevc/hevcdec.c
>>> +++ b/libavcodec/hevc/hevcdec.c
>>> @@ -3401,7 +3401,10 @@ static int hevc_frame_start(HEVCContext *s, 
>>> HEVCLayerContext *l,
>>>           goto fail;
>>>       if (s->avctx->hwaccel) {
>>> -        ret = FF_HW_CALL(s->avctx, start_frame, NULL, 0);
>>> +        AVCodecInternal *avci = s->avctx->internal;
>>> +        AVPacket *avpkt = avci->in_pkt;
>>> +        ret = FF_HW_CALL(s->avctx, start_frame,
>>> +                         avpkt->buf, NULL, 0);
>>>           if (ret < 0)
>>>               goto fail;
>>>       }
>>> diff --git a/libavcodec/hwaccel_internal.h b/libavcodec/ 
>>> hwaccel_internal.h
>>> index 77df4e0904..4eb74f0b34 100644
>>> --- a/libavcodec/hwaccel_internal.h
>>> +++ b/libavcodec/hwaccel_internal.h
>>> @@ -52,11 +52,13 @@ typedef struct FFHWAccel {
>>>        * Otherwise, this means the whole frame is available at this 
>>> point.
>>>        *
>>>        * @param avctx the codec context
>>> +     * @param buf_ref the frame data buffer reference (optional)
>>>        * @param buf the frame data buffer base
>>>        * @param buf_size the size of the frame in bytes
>>>        * @return zero if successful, a negative value otherwise
>>>        */
>>> -    int (*start_frame)(AVCodecContext *avctx, const uint8_t *buf, 
>>> uint32_t buf_size);
>>> +    int (*start_frame)(AVCodecContext *avctx, AVBufferRef *buf_ref,
>>
>> const AVBufferRef*, we are not passing ownership
> 
> Done locally.
> 
>> Why did you not make this the last parameter (given that only very few
>> hwaccels will access it)?
> 
> It made more sense to have it upfront as it backs up the data given.
> I can change it, but I'd rather not, unless you have a strong opinion.
> 
>>> +                       const uint8_t *buf, uint32_t buf_size);
>>>       /**
>>>        * Callback for parameter data (SPS/PPS/VPS etc).
>>> diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
>>> index bd1b502e50..f3d940671e 100644
>>> --- a/libavcodec/mjpegdec.c
>>> +++ b/libavcodec/mjpegdec.c
>>> @@ -808,7 +808,7 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
>>>           if (!s->hwaccel_picture_private)
>>>               return AVERROR(ENOMEM);
>>> -        ret = hwaccel->start_frame(s->avctx, s->raw_image_buffer,
>>> +        ret = hwaccel->start_frame(s->avctx, NULL, s->raw_image_buffer,
>>>                                      s->raw_image_buffer_size);
>>>           if (ret < 0)
>>>               return ret;
>>> diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
>>> index ffe0710470..c21f220680 100644
>>> --- a/libavcodec/mpeg12dec.c
>>> +++ b/libavcodec/mpeg12dec.c
>>> @@ -1361,7 +1361,7 @@ static int mpeg_field_start(Mpeg1Context *s1, 
>>> const uint8_t *buf, int buf_size)
>>>       }
>>>       if (avctx->hwaccel) {
>>> -        if ((ret = FF_HW_CALL(avctx, start_frame, buf, buf_size)) < 0)
>>> +        if ((ret = FF_HW_CALL(avctx, start_frame, NULL, buf, 
>>> buf_size)) < 0)
>>>               return ret;
>>>       } else if (s->codec_tag == MKTAG('V', 'C', 'R', '2')) {
>>>           // Exchange UV
>>> diff --git a/libavcodec/nvdec_av1.c b/libavcodec/nvdec_av1.c
>>> index 6b408edb87..9b9be702d5 100644
>>> --- a/libavcodec/nvdec_vp9.c
>>> +++ b/libavcodec/nvdec_vp9.c
>>> @@ -29,7 +29,8 @@
>>>   #include "internal.h"
>>>   #include "vp9shared.h"
>>> -static int nvdec_vp9_start_frame(AVCodecContext *avctx, const 
>>> uint8_t *buffer, uint32_t size)
>>> +static int nvdec_vp9_start_frame(AVCodecContext *avctx, AVBufferRef 
>>> *buffer_ref,
>>> +                                 const uint8_t *buffer, uint32_t size)
>>>   {
>>>       VP9SharedContext *h = avctx->priv_data;
>>>       const AVPixFmtDescriptor *pixdesc = av_pix_fmt_desc_get(avctx- 
>>> >sw_pix_fmt);
>>> diff --git a/libavcodec/proresdec.c b/libavcodec/proresdec.c
>>> index 01caa611a0..edc46e1442 100644
>>> --- a/libavcodec/proresdec.c
>>> +++ b/libavcodec/proresdec.c
>>> @@ -793,7 +793,7 @@ static int decode_frame(AVCodecContext *avctx, 
>>> AVFrame *frame,
>>>       if (HWACCEL_MAX && avctx->hwaccel) {
>>>           const FFHWAccel *hwaccel = ffhwaccel(avctx->hwaccel);
>>> -        ret = hwaccel->start_frame(avctx, NULL, 0);
>>> +        ret = hwaccel->start_frame(avctx, avpkt->buf, NULL, 0);
>>
>> Passing an AVBufferRef for NULL data seems fishy.
> 
> I misinterpreted what NULL means.
> Changed this to be NULL.

Looked into proresdec.c and vp9.c, both of them had done the same,
start_frame(NULL, 0), and the code was copy pasted between both. But the 
size of the packet is already known in advance.
Changed to start_frame(avpkt->buf, avpkt->data, avpkt->size) for both.
mjpegdec.c is the only user of the API which has a genuine need to have 
buf as NULL, 0.

This isn't as niche as it sounds. This enables us to add a fast path for 
D3D12, Vulkan and likely VAAPI if anyone is interested.
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