[FFmpeg-user] Correct configuration for Mac Pro 6, 1 AMD FirePro D500s?

[Scott Lutz]

I have been trying for a couple of days to leverage the h264_videotoolbox
hardware acceleration capabilities of ffmpeg on dual AMD FirePro D500s of
my Mac Pro 6,1, but without success. Here is an example of the output I've
been getting most recently:

ffmpeg -i infile.mkv -s hd720 -c:v h264_videotoolbox  -c:a aac outfile.mkv

[h264_videotoolbox @ 0x7f8494b06b40] Error: cannot create compression
session: -12903
[h264_videotoolbox @ 0x7f8494b06b40] Try -allow_sw 1. The hardware encoder
may be busy, or not supported.
[vost#0:0/h264_videotoolbox @ 0x7f8494b074c0] Error while opening encoder -
maybe incorrect parameters such as bit_rate, rate, width or height.
[vf#0:0 @ 0x7f8493f065c0] Error sending frames to consumers: Generic error
in an external library
[vf#0:0 @ 0x7f8493f065c0] Task finished with error code: -542398533
(Generic error in an external library)
[vf#0:0 @ 0x7f8493f065c0] Terminating thread with return code -542398533
(Generic error in an external library)
[vost#0:0/h264_videotoolbox @ 0x7f8494b074c0] Could not open encoder before
EOF
[vost#0:0/h264_videotoolbox @ 0x7f8494b074c0] Task finished with error
code: -22 (Invalid argument)
[vost#0:0/h264_videotoolbox @ 0x7f8494b074c0] Terminating thread with
return code -22 (Invalid argument)
[out#0/matroska @ 0x7f8483f04080] Nothing was written into output file,
because at least one of its streams received no packets.

Possibly due to the age of these machines, there isn't a load of info on
the wider interwebs, so I was wondering if anyone has any suggestions on
how to make this work, ideally for both x264 and x265?

Thanks!
Scott