[ffmpeg-qsv][h264_ffmpeg]decode drop frames on iHD driver.

[a]

Summary of the bug:

ffmpeg-qsv decode reinit-large_420_8-to-small_420_8.h264 show 93 frame, but yamidecode this file show totally 100 frames.

environment

SKL OS: ubuntu 16.04 kernel: 4.15.0 
libva b6c50dad4d7d14c507108d9f468662e2d7ae1c4d ​https://github.com/01org/libva.git 
VPG_Driver ed04556a6676fd77ff845508bc54bfc60915450c ​https://github.com/intel/media-driver
libva-utils 9a10ad663349732decd668a426cfc349b0010d1d ​https://github.com/01org/libva-utils.git
Mediasdk 7ea683585f0f84f1bea035a405855312c92c46c0 ​https://github.com/Intel-Media-SDK/MediaSDK
FFMPEG 56f68a099cc607658118e00cad30569103ae3751 ​https://git.ffmpeg.org/ffmpeg.git

How to reproduce:

1. build env as above lists
2.ffmpeg -hwaccel qsv -hwaccel_device /dev/dri/renderD128 -v verbose -y -c:v h264_qsv -i /media//ffmpeg_h264/reinit-large_420_8-to-small_420_8.h264 -vf hwdownload,format=nv12 -pix_fmt yuv420p -f rawvideo -y dump.yuv -vsync 0
3.  check message:
frame=   93 fps=0.0 q=-0.0 Lsize=   13810kB time=00:00:04.40 bitrate=25712.6kbits/s speed=62.2x
4. yamidecode -i /media//ffmpeg_h264/reinit-large_420_8-to-small_420_8.h264 -m 0
5. check message:
100 frame decoded, fps = 3571.43. fps after 5 frames = 3653.85.

[jkqxz]

libmfx doesn't seem to be producing exact output here, though it does look visually correct? Using VAAPI with iHD does get the expected result, so it's only a problem with libmfx and not the iHD driver.

I get:

$ LIBVA_DRIVER_NAME=iHD ./ffmpeg -y -hwaccel qsv -c:v h264_qsv -i reinit-large_420_8-to-small_420_8.h264 -vf 'hwdownload,format=nv12,format=yuv420p' out_mfx.yuv
...
$ LIBVA_DRIVER_NAME=iHD ./ffmpeg -y -hwaccel vaapi -hwaccel_output_format vaapi -i reinit-large_420_8-to-small_420_8.h264 -vf 'hwdownload,format=nv12,format=yuv420p' out_va.yuv
...
$ ./ffmpeg -y -i reinit-large_420_8-to-small_420_8.h264 out_ref.yuv
...
$ md5sum out_*.yuv
eee1a8af32fc9ae651356f88da040e7a  out_mfx.yuv
151bf4fac480ad53d8f62ed6cd43b55c  out_ref.yuv
151bf4fac480ad53d8f62ed6cd43b55c  out_va.yuv

[Linjie.Fu]

The frame dropping issue was caused by the reinitialization of the input stream.（In this stream，first 50 frames: 352x288,last 50 frames: 240x 196）The reinitialization drops all the buffered pkts in libmfx.

Provided a patch to cache the first frame causing the reinit and decode zero-size pkt to flush the buffered pkt before reinit. After all thebuffered pkts being flushed, resume to reinit and decode.

commit id: 505679cb6d2cd0e7c1714e28e05a6b9cdc917991
​https://patchwork.ffmpeg.org/patch/10515/
​http://ffmpeg.org/pipermail/ffmpeg-devel/2018-September/234703.html

Tested and got the same result with vaapi and ffmpeg ref:

$ ffmpeg -y -loglevel debug -hwaccel qsv -c:v h264_qsv -i reinit-large_420_8-to-small_420_8.h264 -vf 'hwdownload,format=nv12,format=yuv420p' out_mfx.yuv
...
$ md5sum out_mfx.yuv
151bf4fac480ad53d8f62ed6cd43b55c  out_mfx.yuv

[Zhong,Li]

Merged as 87368884a52b09eef96190ff1654d56591ec7038