[FFmpeg-devel] [RFC] DXVA2 decoding and FFmpeg

Stefano Sabatini stefasab at gmail.com
Thu May 14 14:52:29 CEST 2015

On date Thursday 2015-05-14 13:01:51 +0200, Stefano Sabatini encoded:
> On date Tuesday 2015-05-12 15:54:17 +0200, Hendrik Leppkes encoded:
> > One limitation is as the manual said, it needs to be copied from the
> > GPU to system memory. ffmpeg_dxva2.c does not implement a optimized
> > copy function for this, it uses plain old memcpy.
> > Intel introduced a new instruction for this in SSE4, MOVNTDQA, which
> > is optimized for copying from USWC memory (Uncacheable Speculative
> > Write Combining) to system memory. Using this may help speed up the
> > process significantly, and VLC probably uses it.
> Now the question is, how would be possible to optimize GPU to CPU copy
> to get an overall performance gain? At least VLC seems able to get
> better performances when using HW decoding, but I'm not sure it is
> copying decoded data back to the CPU (indeed it may perform direct
> rendering).

commit 62107e563f979c638f9a5f58cdfd5639d9c63ac7
Author: Laurent Aimar <fenrir at videolan.org>
Date:   Tue Nov 17 01:09:43 2009 +0100

    Improved performance when copying video surface in dxva2.

That is, VLC is using optimized GPU->CPU copy when the relevant SSE2
instructions are available.
FFmpeg = Fundamental & Frightening Mean Peaceful EniGma

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