Bug#870676: ffmpeg requires NEON on armhf, which is not part of the ARMv7 ABI

[James Cowgill]

Hi Steve,

On 03/08/17 21:03, Steve Langasek wrote:
> Package: ffmpeg
> Version: 7:3.3.3-1
> Severity: important
> Tags: patch
> User: ubuntu-devel at lists.ubuntu.com
> Usertags: origin-ubuntu artful ubuntu-patch autopkgtest
> 
> Dear maintainers,
> 
> The latest release of ffmpeg enables NEON support by default when building
> on armhf; however, NEON support is not a standard part of the ARMv7 ABI, and
> Debian supports running armhf on chips that do not implement NEON.
> 
> Using NEON based on runtime detection of support for it is fine, but the
> existing ffmpeg implementation doesn't appear to do this, instead using NEON
> based on build-time configuration with no fallback.

Are you sure this is true? I tried running the failing test on abel.d.o
(which AFAIK does not have NEON) and harris (which does). The test only
caused ffmpeg to crash on harris, which seems to suggest that the
runtime NEON detection is working properly.

These are the commands to reproduce the autopkgtest fail if you want to
try it:

ffmpeg -f lavfi -i testsrc=s=32x32:d=0.1 -strict -2 -c:v libx264rgb -f avi libx264rgb.avi -y -hide_banner -nostdin
ffmpeg -strict -2 -i libx264rgb.avi -t 1 -c:v rawvideo -c:a pcm_s32le -f nut /dev/null -y -hide_banner -nostdin

> This issue was noticed in Ubuntu only because the autopkgtests for ffmpeg
> and x264 triggered an unaligned access in the NEON code, which is *also* not
> a portable assumption on armhf; however, if the NEON code had not had any
> unaligned access, the fact that NEON was used would have gone unnoticed on
> Ubuntu infrastructure.
> 
>   http://autopkgtest.ubuntu.com/packages/f/ffmpeg/artful/armhf
>   http://autopkgtest.ubuntu.com/packages/x/x264/artful/armhf
> 
> (And if upstream does fix their code to support runtime detection of NEON
> support, then there will be a different bug for us to worry about fixing!)

This is #870622 BTW. If possible, I would much rather fix these bugs
without having to disable all the NEON optimizations.

Thanks,
James

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