[Buildroot] [RFC v1] libv4l: build utils only on MMU (with fork support) platforms

[Peter Seiderer <ps.report@gmx.net>]

Hello Hugues,

On Mon, 17 Jul 2017 10:07:07 +0000, Hugues FRUCHET <hugues.fruchet@st.com> wrote:

> Hi Peter,
> 
> I have tested on STM32 platform running Cortex F family having no MMU, 
> shared libraries are disabled (no dlopen) and no fork supported on our 
> setup.
> 
> libv4lconvert should not have been built at all, because entire lib/ 
> folder build is conditional to shared library support:
> * configure.ac
> AM_CONDITIONAL([WITH_LIBV4L],       [test x$enable_libv4l    != xno -a 
> x$enable_shared != xno])
> * Makefile.am
> [...]
> +if WITH_LIBV4L
> +SUBDIRS += lib
> +endif

A little bit confused, one of the patches you introduced changed it to:

- without WITH_LIBV4L: no libv4l libraries at all
- without WITH_DYN_LIBV4L: only static libraries, no dynamic libraries

And from configure --help:

  --disable-libv4l        disable libv4l compilation
  --disable-dyn-libv4l    disable dynamic libv4l support

> 
> But checking your log I see that --enable-shared is set, so it seems 
> that you are supporting shared libraries on no-MMU platform but without 
> fork() support, is my understanding correct ?
> 

Yes...

> What you can do to fix build is to force disabling of libs inside libv4l 
> package:
> 
> package/libv4l/libv4l.mk
> +ifeq ($(BR2_USE_MMU),)
> +LIBV4L_CONF_OPTS += --disable-libv4l
> +endif

Works, but feels wrong: A package named libv4l enabled and building/installing
nothing (except when LIBV4L_UTILS is enabled)...

Regards,
Peter

> 
> Hoping that helps.
> 
> Best regards,
> Hugues.
> 
> On 07/11/2017 07:26 PM, Peter Seiderer wrote:
> > Fixes [1]:
> > 
> >      CXXLD    v4l2-compliance
> >    .../output/build/libv4l-1.12.5/lib/libv4lconvert/.libs/libv4lconvert.so: undefined reference to `fork'
> >    collect2: error: ld returned 1 exit status
> > 
> > [1] http://autobuild.buildroot.net/results/7e8/7e8fbd99a8c091d7bbeedd16066297682bbe29fe
> > 
> > Signed-off-by: Peter Seiderer <ps.report@gmx.net>
> > ---
> > The libv4l build for noMMU platforms was enabled with commit 'package/libv4l:
> > allow build of v4l2 utilities on noMMU platforms' (see [2]). But libv4lconvert uses
> > fork internally, so all utils linking against fail to build. Fix this by disabling
> > the utils on noMMU platforms (so keep building all libraries).
> > 
> > Alternatives would be:
> > 
> > - disable libv4l on all noMMU platforms (not only the utils), revert a part
> >    of commit [2]
> > 
> > - create a patch to disable only libv4lconvert (and all dependend utils)
> >    on noMMU platforms
> > 
> > [2] https://git.buildroot.net/buildroot/commit/?id=f837251785e9389f53d695ddb8a094707865171b
> > ---
> >   package/libv4l/Config.in | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/package/libv4l/Config.in b/package/libv4l/Config.in
> > index e7f78dc70f..61c290f251 100644
> > --- a/package/libv4l/Config.in
> > +++ b/package/libv4l/Config.in
> > @@ -19,6 +19,7 @@ comment "libv4l JPEG support not enabled"
> >   
> >   config BR2_PACKAGE_LIBV4L_UTILS
> >   	bool "v4l-utils tools"
> > +	depends on BR2_USE_MMU # fork(), used in libv4lconvert linked by various utils eg. v4l2-compliance
> >   	help
> >   	  v4l-utils is a collection of various video4linux and DVB utilities.
> >   	  Enable this if you want to build the following tools:
> > 
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