[Buildroot] [PATCH] ffmpeg: fix static linking build failure when using libavutil

[Giulio Benetti]

Hello Thomas,

Il 16/09/2018 22:22, Thomas Petazzoni ha scritto:
> Hello Giulio,
> 
> On Tue, 11 Sep 2018 22:42:30 +0200, Giulio Benetti wrote:
>> When static linking programs using ffmpeg libraries, if linking against
>> libavutil, -ldrm is listed before -lavutil.
> 
> I think this is not relevant in the explanation, the fact that motion
> links with libdrm directly is independent from the fact that ffmpeg may
> use libdrm internally.
> 
>> This leads to linking failure
>> due to undefined reference of drmGetVersion() and drmFreeVersion().
>> This is why when pkg-config generates libavutil.pc doesn't append -ldrm
> 
> pkg-config does not generate .pc files, it only reads them.
> 
>> after -lavutil.
>> Subsequentely if a package uses pkg-config and ffmpeg it will load
>> library dependencies from libavutil.pc without placing -ldrm at the tail.
> 
> Well, the fact that it's at the tail is a detail, but the main problem
> is that the libavutil.pc file does not list *at all* the dependency on
> libdrm.
> 
>> Without this fix the only way is to workaround the problem directly in
>> the single package, like this:
> 
> "the single package" is not very clear, I would say "in each package
> using ffmpeg".
> 
>> https://github.com/buildroot/buildroot/commit/daf7dd87f4d93923df5e757fd43b3ad214a4a2ae
>>
>> Add patch to assure -ldrm comes after -lavutil when static linking.
> 
> No, that's not what the patch does. The patch ensures that libavutil.pc
> tells pkg-config that linking against libdrm is needed.
> 
> 
>> ++enabled libdrm            && require_pkg_config libdrm libdrm xf86drm.h drmGetVersion && LIBDRM="-ldrm"
>> + enabled libfdk_aac        && { use_pkg_config libfdk_aac fdk-aac "fdk-aac/aacenc_lib.h" aacEncOpen ||
>> +                                { require libfdk_aac fdk-aac/aacenc_lib.h aacEncOpen -lfdk-aac &&
>> +                                  warn "using libfdk without pkg-config"; } }
>> +@@ -7096,7 +7096,7 @@ source_path=${source_path}
>> + LIBPREF=${LIBPREF}
>> + LIBSUF=${LIBSUF}
>> +
>> +-extralibs_avutil="$LIBRT $LIBM"
>> ++extralibs_avutil="$LIBDRM $LIBRT $LIBM"
> 
> This fix is in fact not the most correct one. Indeed, it does the
> following change to libavutil.pc:
> 
> -Libs: -L${libdir}  -lavutil -lm
> +Libs: -L${libdir}  -lavutil -ldrm -lm
> 
> but the proper change would be:
> 
>   Libs: -L${libdir}  -lavutil -lm
> -Libs.private:
> +Libs.private: -ldrm
> 
> Indeed, when we're dynamic linking, there is no need to link consumers
> of libavutil with libdrm. It is only when we're statically linking that
> it should be done. And that's exactly what Libs vs. Libs.private is for.

You've explained me it very well, but by now FFmpeg upstreamed my patch:
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/c50dc77ac708e98d02da7c422a6b9cbf9f565aa5

So, do I rework and resubmit to them or can I re-create a patch for BR
with what you've pointed me above?

Thanks
Kind regards
-- 
Giulio Benetti
CTO

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