[OE-core] [PATCH 4/8] ffmpeg: don't rely on configure to correctly detect ARM vfp or neon

[Andre McCurdy]

On Mon, Sep 16, 2019 at 3:34 PM Adrian Bunk <bunk at stusta.de> wrote:
>
> On Mon, Sep 16, 2019 at 03:08:01PM -0700, Andre McCurdy wrote:
> > On Mon, Sep 16, 2019 at 2:33 PM Christopher Larson <kergoth at gmail.com> wrote:
> > >
> > > This seems like something that could do with a ‘why’ in the commit message and/or comments, to me.
> >
> > To be consistent with:
> >
> >   https://git.openembedded.org/openembedded-core/commit/?id=1a563214caf6bd5b3a026ebe953f8c692ebd640a
> >...
>
> libwebp sets -mfpu=neon when autodetecting,
> is there a similar problem with ffmpeg?
>
> > > +# Do not trust configure to determine if ARM vfp or neon are available.
> > > +#
> > > +EXTRA_OECONF_ARM = " \
> > > + ${@bb.utils.contains('TUNE_FEATURES','vfp','--enable-vfp','--disable-vfp', d)} \
> > > + ${@bb.utils.contains("TUNE_FEATURES",'neon','--enable-neon','--disable-neon',d)} \
> >...
>
> If we cannot trust the vfp autodetection, why can we trust that the
> autodetection of the vfp variant (vfpv2, vfpv3 or vfpv3-d16) and
> the exact architecture (e.g. armv5te, armv6, armv6k or armv6t2)
> are correct?

In general we can't. However these are all corner cases which are
rarely (if ever) tested.

For ffmpeg configuring with --enable-runtime-cpudetect (which I think
we effectively do by default) probably hides a lot of the configure
time misdetection too.

> Which brings us back to the problem what the actual problem is (if any).

I built for target without VFP or NEON and saw both as being enabled
according to the configure log. Didn't investigate further than that.

> cu
> Adrian
>
> --
>
>        "Is there not promise of rain?" Ling Tan asked suddenly out
>         of the darkness. There had been need of rain for many days.
>        "Only a promise," Lao Er said.
>                                        Pearl S. Buck - Dragon Seed
>