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

[Adrian Bunk <bunk@stusta.de>]

On Mon, Sep 16, 2019 at 03:43:42PM -0700, Andre McCurdy wrote:
> On Mon, Sep 16, 2019 at 3:34 PM Adrian Bunk <bunk@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@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.
>...

Building with --disable-runtime-cpudetect for qemuarmv5 gives:

ARCH                      arm (armv5te)
big-endian                no
runtime cpu detection     no
ARMv5TE enabled           yes
ARMv6 enabled             yes
ARMv6T2 enabled           yes
VFP enabled               yes
NEON enabled              yes


These 4 misdetections are all affecting a certain range of CPUs,
ARMv6T2 misdetection should affect more CPUs than VFP misdetection.

There likely is a common root cause that should be fixed,
instead of manually specifying every autodetected feature.


cu
Adrian

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