Date:   Wed, 23 Mar 2022 22:10:51 +0530
From:   kajoljain <kjain@...ux.ibm.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Santosh Sivaraj <santosh@...six.org>, maddy@...ux.ibm.com,
        rnsastry@...ux.ibm.com,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        atrajeev@...ux.vnet.ibm.com, Vaibhav Jain <vaibhav@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] powerpc/papr_scm: Fix build failure when
 CONFIG_PERF_EVENTS is not set



On 3/23/22 21:02, Dan Williams wrote:
> On Wed, Mar 23, 2022 at 3:05 AM Michael Ellerman <mpe@...erman.id.au> wrote:
>>
>> Dan Williams <dan.j.williams@...el.com> writes:
>>> On Tue, Mar 22, 2022 at 7:30 AM kajoljain <kjain@...ux.ibm.com> wrote:
>>>> On 3/22/22 03:09, Dan Williams wrote:
>>>>> On Fri, Mar 18, 2022 at 4:42 AM Kajol Jain <kjain@...ux.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@...ux.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.

Sure, I will send patchset for the same.

Thanks,
Kajol Jain

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