Re: [PATCH 2/2] powerpc/papr_scm: Fix build failure when CONFIG_PERF_EVENTS is not set

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Mar 23, 2022 at 3:05 AM Michael Ellerman <mpe@ellerman.id.au> wrote:
>
> Dan Williams <dan.j.williams@intel.com> writes:
> > On Tue, Mar 22, 2022 at 7:30 AM kajoljain <kjain@linux.ibm.com> wrote:
> >> On 3/22/22 03:09, Dan Williams wrote:
> >> > On Fri, Mar 18, 2022 at 4:42 AM Kajol Jain <kjain@linux.ibm.com> wrote:
> >> >>
> >> >> The following build failure occures when CONFIG_PERF_EVENTS is not set
> >> >> as generic pmu functions are not visible in that scenario.
> >> >>
> >> >> arch/powerpc/platforms/pseries/papr_scm.c:372:35: error: ‘struct perf_event’ has no member named ‘attr’
> >> >>          p->nvdimm_events_map[event->attr.config],
> >> >>                                    ^~
> >> >> In file included from ./include/linux/list.h:5,
> >> >>                  from ./include/linux/kobject.h:19,
> >> >>                  from ./include/linux/of.h:17,
> >> >>                  from arch/powerpc/platforms/pseries/papr_scm.c:5:
> >> >> arch/powerpc/platforms/pseries/papr_scm.c: In function ‘papr_scm_pmu_event_init’:
> >> >> arch/powerpc/platforms/pseries/papr_scm.c:389:49: error: ‘struct perf_event’ has no member named ‘pmu’
> >> >>   struct nvdimm_pmu *nd_pmu = to_nvdimm_pmu(event->pmu);
> >> >>                                                  ^~
> >> >> ./include/linux/container_of.h:18:26: note: in definition of macro ‘container_of’
> >> >>   void *__mptr = (void *)(ptr);     \
> >> >>                           ^~~
> >> >> arch/powerpc/platforms/pseries/papr_scm.c:389:30: note: in expansion of macro ‘to_nvdimm_pmu’
> >> >>   struct nvdimm_pmu *nd_pmu = to_nvdimm_pmu(event->pmu);
> >> >>                               ^~~~~~~~~~~~~
> >> >> In file included from ./include/linux/bits.h:22,
> >> >>                  from ./include/linux/bitops.h:6,
> >> >>                  from ./include/linux/of.h:15,
> >> >>                  from arch/powerpc/platforms/pseries/papr_scm.c:5:
> >> >>
> >> >> Fix the build issue by adding check for CONFIG_PERF_EVENTS config option
> >> >> and disabling the papr_scm perf interface support incase this config
> >> >> is not set
> >> >>
> >> >> Fixes: 4c08d4bbc089 ("powerpc/papr_scm: Add perf interface support") (Commit id
> >> >> based on linux-next tree)
> >> >> Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
> >> >> ---
> >> >>  arch/powerpc/platforms/pseries/papr_scm.c | 15 +++++++++++++++
> >> >
> >> > This is a bit messier than I would have liked mainly because it dumps
> >> > a bunch of ifdefery into a C file contrary to coding style, "Wherever
> >> > possible, don't use preprocessor conditionals (#if, #ifdef) in .c
> >> > files". I would expect this all to move to an organization like:
> >>
> >> Hi Dan,
> >>       Thanks for reviewing the patches. Inorder to avoid the multiple
> >> ifdefs checks, we can also add stub function for papr_scm_pmu_register.
> >> With that change we will just have one ifdef check for
> >> CONFIG_PERF_EVENTS config in both papr_scm.c and nd.h file. Hence we can
> >> avoid adding new files specific for papr_scm perf interface.
> >>
> >> Below is the code snippet for that change, let me know if looks fine to
> >> you. I tested it
> >> with set/unset PAPR_SCM config value and set/unset PERF_EVENTS config
> >> value combinations.
> >>
> >> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c
> >> b/arch/powerpc/platforms/pseries/papr_scm.c
> >> index 4dd513d7c029..38fabb44d3c3 100644
> >> --- a/arch/powerpc/platforms/pseries/papr_scm.c
> >> +++ b/arch/powerpc/platforms/pseries/papr_scm.c
> >> @@ -69,8 +69,6 @@
> >>  #define PAPR_SCM_PERF_STATS_EYECATCHER __stringify(SCMSTATS)
> >>  #define PAPR_SCM_PERF_STATS_VERSION 0x1
> >>
> >> -#define to_nvdimm_pmu(_pmu)    container_of(_pmu, struct nvdimm_pmu, pmu)
> >> -
> >>  /* Struct holding a single performance metric */
> >>  struct papr_scm_perf_stat {
> >>         u8 stat_id[8];
> >> @@ -346,6 +344,9 @@ static ssize_t drc_pmem_query_stats(struct
> >> papr_scm_priv *p,
> >>         return 0;
> >>  }
> >>
> >> +#ifdef CONFIG_PERF_EVENTS
> >> +#define to_nvdimm_pmu(_pmu)    container_of(_pmu, struct nvdimm_pmu, pmu)
> >> +
> >>  static int papr_scm_pmu_get_value(struct perf_event *event, struct
> >> device *dev, u64 *count)
> >>  {
> >>         struct papr_scm_perf_stat *stat;
> >> @@ -558,6 +559,10 @@ static void papr_scm_pmu_register(struct
> >> papr_scm_priv *p)
> >>         dev_info(&p->pdev->dev, "nvdimm pmu didn't register rc=%d\n", rc);
> >>  }
> >>
> >> +#else
> >> +static inline void papr_scm_pmu_register(struct papr_scm_priv *p) { }
> >
> > Since this isn't in a header file, it does not need to be marked
> > "inline" the compiler will figure it out.
> >
> >> +#endif
> >
> > It might be time to create:
> >
> > arch/powerpc/platforms/pseries/papr_scm.h
> >
> > ...there is quite a bit of header material accrued in papr_scm.c and
> > once the ifdefs start landing in it then it becomes a nominal coding
> > style issue. That said, this is certainly more palatable than the
> > previous version. So if you don't want to create papr_scm.h yet for
> > this, at least make a note in the changelog that the first portion of
> > arch/powerpc/platforms/pseries/papr_scm.c is effectively papr_scm.h
> > content and may move there in the future, or something like that.
>
> IMHO the only thing that belongs in a header is content that's needed by
> other C files. As long as those types/declarations are only used in
> papr_scm.c then they should stay in the C file, and there's no need for
> a header.
>
> I know the coding style rule is "avoid ifdefs in .c files", but I'd
> argue that rule should be ignored if you're creating a header file
> purely so that you can use an ifdef :)
>
> Coding style also says:
>
>   Prefer to compile out entire functions, rather than portions of functions or
>   portions of expressions.  Rather than putting an ifdef in an expression, factor
>   out part or all of the expression into a separate helper function and apply the
>   conditional to that function.
>
> Which is what we're doing here with eg. papr_scm_pmu_register().
>
> Certainly for this merge window I think introducing a header is likely
> to cause more problems than it solves, so let's not do that for now. We
> can revisit it for the next merge window.

Fair enough. Kajol, please turn that snippet proposal into a formal patch.