CONFIG_LD_DEAD_CODE_DATA_ELIMINATION: Section mismatch in reference from the variable via_pmu_driver to the function .init.text:pmu_init()

[Nicholas Piggin]

On Tue, 3 Jul 2018 13:40:55 +0200
Mathieu Malaterre <malat at debian.org> wrote:

> On Tue, Jul 3, 2018 at 11:40 AM Mathieu Malaterre <malat at debian.org> wrote:
> >
> > Hi Nick,
> >
> > Would you consider this a bug:
> >
> > $ touch drivers/macintosh/via-pmu.c
> > $ CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=n make ARCH=powerpc
> > CROSS_COMPILE=powerpc-linux-gnu-
> > ...
> >   LD      vmlinux.o
> >   MODPOST vmlinux.o
> > WARNING: vmlinux.o(.data+0x216018): Section mismatch in reference from
> > the variable via_pmu_driver to the function .init.text:pmu_init()
> > The variable via_pmu_driver references
> > the function __init pmu_init()
> > If the reference is valid then annotate the
> > variable with __init* or __refdata (see linux/init.h) or name the variable:
> > *_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console
> >
> > While:
> >
> > $ touch drivers/macintosh/via-pmu.c
> > $ CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y make ARCH=powerpc
> > CROSS_COMPILE=powerpc-linux-gnu-
> > ...
> >   AR      init/built-in.a
> >   AR      built-in.a
> >   LD      vmlinux.o
> >   MODPOST vmlinux.o
> >   KSYM    .tmp_kallsyms1.o
> >   KSYM    .tmp_kallsyms2.o
> >   LD      vmlinux
> >   SORTEX  vmlinux
> >   SYSMAP  System.map
> > ...
> >
> > Thanks for comment  
> 
> Just to clarify I reverted 58935176ad17976b7a7f6ea25c0ceb2ca4308a30
> just as to reproduce a warning. So my question (rephrased):
> 
> Is this expected that CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y inhibit
> the behavior of  CONFIG_DEBUG_SECTION_MISMATCH=y ?

Well that's a good question actually. Section mismatch
analysis is done on the throwaway vmlinux.o which is not linked
with --gc-sections (and is not a final link), so the via_pmu_driver
symbol should exist and be picked up.

I wonder if something about the  -ffunction-sections is breaking
the reference detection.

Thanks,
Nick