[FFmpeg-trac] #8396(undetermined:new): hwdownload always use 0th device (hwaccel_device 0)

[FFmpeg]

#8396: hwdownload always use 0th device (hwaccel_device 0)
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             Reporter:  darn         |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:
             Keywords:  nvenc        |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by darn):

 Replying to [comment:2 oromit]:
 > hwdownload copies frames _from_ the device, and thus uses the context of
 the frames it gets as input. The outcoming frames are in system RAM and
 not tied to any device.

 Yes.
 hwdownload -- copy from GPU memory to system memory.
 hwupload -- copy from system memory to GPU memory.

 Is it correct?

 > The second CUDA context on the default device (0) you are seeing is
 nvenc getting fed non-CUDA frames, which triggers it to create its own
 CUDA context to re-uploads the frames on.
 > nvenc has its own option (-gpu) controlling on which device it creates
 that context.

 I tried to use "-gpu 2" setting, the result is the same.

 > But really, why even download in the first place, just for nvenc to re-
 upload immediately?

 As far as I understand my configuration on filter_complex

 {{{
 -filter_complex
 "[0:v]scale_npp=-1:-1:format=yuv420p:interp_algo=lanczos,hwdownload,format=yuv420p[v0]"
 }}}

 works as follows:

 1. Input stream "[0:v]" copying to GPU memory;
 2. GPU "scale_npp" input stream to "-1:-1" with
 "format=yuv420p:interp_algo=lanczos";
 3. GPU "hwdownload" with "format=yuv420p" from GPU memory to system memory
 with name [v0].

 Is it correct?

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Ticket URL: <https://trac.ffmpeg.org/ticket/8396#comment:3>
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