[vlc-devel] [PATCH] DxVA2: fix compatibility with avcodec frame multithreading

[Steve Lhomme]

There is a race condition when seeking in H264 :(

Sometimes the Release() callback is called with a garbage `opaque`
variable, which is supposed to be our vlc_va_surface_t object...

On Fri, Apr 3, 2015 at 10:53 AM, Steve Lhomme <robux4 at videolabs.io> wrote:
> Fixes #11693
> Disable thread_safe_callbacks when we detect DXVA2 might be used
>
> --
> the p_va of the decoder is not affected, a dummy p_va is used
>
> the decoding issues appeared on Intel HD Graphics 4000 hardware
> ---
>  modules/codec/avcodec/video.c | 75 ++++++++++++++++++++++++++++++++++---------
>  1 file changed, 60 insertions(+), 15 deletions(-)
>
> diff --git a/modules/codec/avcodec/video.c b/modules/codec/avcodec/video.c
> index 381fe62..c4488c0 100644
> --- a/modules/codec/avcodec/video.c
> +++ b/modules/codec/avcodec/video.c
> @@ -105,6 +105,7 @@ static void ffmpeg_ReleaseFrameBuf( struct AVCodecContext *, AVFrame * );
>  #endif
>  static enum PixelFormat ffmpeg_GetFormat( AVCodecContext *,
>                                            const enum PixelFormat * );
> +static bool ffmpeg_CanHwAccel( AVCodecContext *, const enum PixelFormat * );
>  static picture_t *DecodeVideo( decoder_t *, block_t ** );
>
>  static uint32_t ffmpeg_CodecTag( vlc_fourcc_t fcc )
> @@ -374,7 +375,21 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
>      i_thread_count = __MIN( i_thread_count, 16 );
>      msg_Dbg( p_dec, "allowing %d thread(s) for decoding", i_thread_count );
>      p_context->thread_count = i_thread_count;
> +# if defined(_WIN32)
> +    /* Frame multithreading is not thread safe with DXVA2.
> +     * When a frame is being copied to host memory, the frame
> +     * is locked and cannot be used as a reference frame
> +     * simultaneously and thus decoding fails for some frames. This
> +     * causes major image corruption. */
> +    enum PixelFormat test_formats[] = { AV_PIX_FMT_DXVA2_VLD, AV_PIX_FMT_NONE };
> +    if( ffmpeg_CanHwAccel( p_context, &test_formats ) )
> +    {
> +        msg_Dbg( p_dec, "threaded frame decoding with DXVA2 is not thread safe" );
> +        p_context->thread_safe_callbacks = false;
> +    }
> +#else
>      p_context->thread_safe_callbacks = true;
> +#endif
>
>      switch( p_codec->id )
>      {
> @@ -396,21 +411,6 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
>              break;
>      }
>
> -    /* Workaround: frame multithreading is not compatible with
> -     * DXVA2. When a frame is being copied to host memory, the frame
> -     * is locked and cannot be used as a reference frame
> -     * simultaneously and thus decoding fails for some frames. This
> -     * causes major image corruption. */
> -# if defined(_WIN32)
> -    char *avcodec_hw = var_InheritString( p_dec, "avcodec-hw" );
> -    if( avcodec_hw == NULL || strcasecmp( avcodec_hw, "none" ) )
> -    {
> -        msg_Warn( p_dec, "threaded frame decoding is not compatible with DXVA2, disabled" );
> -        p_context->thread_type &= ~FF_THREAD_FRAME;
> -    }
> -    free( avcodec_hw );
> -# endif
> -
>      if( p_context->thread_type & FF_THREAD_FRAME )
>          p_dec->i_extra_picture_buffers = 2 * p_context->thread_count;
>  #endif
> @@ -1305,6 +1305,51 @@ static void ffmpeg_ReleaseFrameBuf( struct AVCodecContext *p_context,
>  }
>  #endif
>
> +static bool ffmpeg_CanHwAccel( AVCodecContext *p_codec,
> +                               const enum PixelFormat *pi_fmt )
> +{
> +    decoder_t *p_dec = p_codec->opaque;
> +    decoder_sys_t *p_sys = p_dec->p_sys;
> +    vlc_va_t *p_va;
> +
> +    /* Enumerate available formats */
> +    bool can_hwaccel = false;
> +    for( size_t i = 0; pi_fmt[i] != PIX_FMT_NONE; i++ )
> +    {
> +        const AVPixFmtDescriptor *dsc = av_pix_fmt_desc_get(pi_fmt[i]);
> +        if (dsc == NULL)
> +            continue;
> +        if ( dsc->flags & AV_PIX_FMT_FLAG_HWACCEL )
> +        {
> +            can_hwaccel = true;
> +            break;
> +        }
> +    }
> +
> +    if (!can_hwaccel)
> +        goto end;
> +
> +    can_hwaccel = false;
> +
> +    /* Profile and level information is needed now.
> +     * TODO: avoid code duplication with avcodec.c */
> +    if( p_codec->profile != FF_PROFILE_UNKNOWN)
> +        p_dec->fmt_in.i_profile = p_codec->profile;
> +    if( p_codec->level != FF_LEVEL_UNKNOWN)
> +        p_dec->fmt_in.i_level = p_codec->level;
> +
> +    p_va = vlc_va_New( VLC_OBJECT(p_dec), p_codec, &p_dec->fmt_in );
> +    if( p_va == NULL )
> +        goto end;
> +
> +    can_hwaccel = true;
> +
> +end:
> +    if ( p_va != NULL )
> +        vlc_va_Delete( p_va, p_codec );
> +    return can_hwaccel;
> +}
> +
>  static enum PixelFormat ffmpeg_GetFormat( AVCodecContext *p_context,
>                                            const enum PixelFormat *pi_fmt )
>  {
> --
> 2.3.0
>
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