Re: [dpdk-dev] [PATCH v3 5/7] build/pkg-config: output driver libs first for static build

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jul 01, 2020 at 04:42:33PM +0200, Thomas Monjalon wrote:
> 30/06/2020 16:14, Bruce Richardson:
> > When calling pkg-config --static --libs, pkg-config will always output the
> > regular libs first, and then the extra libs from libraries.private field,
> 
> s/libraries.private/Libs.private/
> 
> > since the assumption is that those are additional dependencies for building
> > statically that the .a files depend upon.
> > 
> > However, for DPDK, we only link the driver files for static builds, and
> > those need to come *before* the regular libraries. To get this result, we
> > need two pkgconfig files for DPDK, one for the shared libs, and a second
> > for the static libs and drivers, which depends upon the first. Using a
> > dependency means that the shared libs are printed only after the
> > libraries.private field rather than before.
> 
> s/libraries.private/Libs.private/
> 
> > Without this patch, the linking works in DPDK because in all cases we
> > specify the libraries after the drivers in the Libs.private line, ensuring
> > that the references to the libs from the drivers can be resolved. The
> > current output is therefore of the form, "(shared)libs, drivers,
> > (static)libs", while after this patch the output is, "drivers,
> > (static)libs, (shared)libs". The former case will not work if we use the
> > --whole-archive flag on the static libs as it will lead to duplicate
> > definitions due to some references having been previously resolved from the
> > shared libraries. By ensuring the shared libraries come last in the link
> > link, this issue does not occur, as duplicate references when linking the
> > shared libs will be ignored.
> > 
> > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > Acked-by: Luca Boccassi <bluca@debian.org>
> > Acked-by: Sunil Pai G <sunil.pai.g@intel.com>
> > ---
> > +# When calling pkg-config --static --libs, pkg-config will always output the
> > +# regular libs first, and then the extra libs from libraries.private field,
> > +# since the assumption is that those are additional dependencies for building
> > +# statically that the .a files depend upon. However, for DPDK, we only link
> > +# the driver files for static builds, and those need to come *before* the
> > +# regular libraries. To get this result, we need two pkgconfig files for DPDK,
> > +# one for the shared libs, and a second for the static libs and drivers, which
> > +# depends upon the first. Using a dependency means that the shared libs are
> > +# printed only after the libraries.private field rather than before.
> 
> This is not obvious. In order to avoid messing it up in future,
> I suggest this longer reword:
> 
> # When calling pkg-config --static --libs, pkg-config will always output the
> # regular libs first, and then the extra libs from Libs.private field,
> # since the assumption is that those are additional dependencies for building
> # statically that the .a files depend upon. The output order of .pc fields is:
> #   Cflags   Libs   Libs.private   Requires   Requires.private
> # The fields Requires* are for package names.
> # The flags of the DPDK libraries must be defined in Libs* fields.
> # However, the DPDK drivers are linked only in static builds (Libs.private),
> # and those need to come *before* the regular libraries (Libs field).
> # This requirement is satisfied by moving the regular libs in a separate file
> # included in the field Requires (after Libs.private).
> # Another requirement is to allow linking dependencies as shared libraries,
> # while linking static DPDK libraries and drivers. It is satisfied by
> # listing the static files in Libs.private with the explicit syntax -l:libfoo.a.
> # As a consequence, the regular DPDK libraries are already listed as static
> # in the field Libs.private. The second occurences of DPDK libraries,
> # included from Requires and used for shared library linkage case,
> # are skipped in the case of static linkage thanks to the flag --as-needed.
> 
> # Link order summary:
> #   libdpdk.Libs.private: whole-archive(static drivers/libs), drivers deps flags
> #   libdpdk.Requires: libdpdk-libs package
> #   libdpdk-libs.Libs: as-needed(shared libs)
> #   libdpdk-libs.Libs.private: libs deps flags
> #   libdpdk.pc.Requires.private: deps packages
> 
> 
> If you agree, I could change this comment while merging.
> I would add my Signed-off ;)
>

This seems generally ok, but probably should just be added as part of patch
#7 when all parts of the above have been applied.

Couple of comments:
* One small nit is that cflags are not output as part of the --libs call, so
you can remove them from the list on line 5 of the comment. They aren't
really relevant to this comment/essay.

* I find the link-order summary to actually be more confusing than helpful.
I think the text block is explanatory enough. It just confuses things
introducing the extra details of what goes in the requires-private or
libs-private of the libdpdk.pc file. That's just regular stuff, unrelated
to the changes or to DPDK special-case of needing private libs first.

/Bruce