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Date:   Thu, 13 Jul 2023 10:15:33 +0100
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Subject: Re: [PATCH 4/4] perf: Remove unused PERF_PMU_CAP_HETEROGENEOUS_CPUS
 capability
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Will Deacon <will@kernel.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-perf-users@vger.kernel.org, irogers@google.com
References: <20230710122138.1450930-1-james.clark@arm.com>
 <20230710122138.1450930-5-james.clark@arm.com>
 <96a3d12a-71d8-1779-28a3-316e1838292e@arm.com>
 <6143e1cd-4db7-d980-a189-b9b06f99d7c4@arm.com>
 <ca559765-0f07-b187-bdcd-826dcd99a312@arm.com>
Content-Language: en-US
From:   James Clark <james.clark@arm.com>
In-Reply-To: <ca559765-0f07-b187-bdcd-826dcd99a312@arm.com>
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On 13/07/2023 08:30, Anshuman Khandual wrote:
> 
> 
> On 7/11/23 19:45, James Clark wrote:
>>
>>
>> On 11/07/2023 13:10, Anshuman Khandual wrote:
>>>
>>>
>>> On 7/10/23 17:51, James Clark wrote:
>>>> Since commit bd2756811766 ("perf: Rewrite core context handling") the
>>>> relationship between perf_event_context and PMUs has changed so that
>>>> the error scenario that PERF_PMU_CAP_HETEROGENEOUS_CPUS originally
>>>> silenced no longer exists.
>>>>
>>>> Remove the capability to avoid confusion that it actually influences
>>>> any perf core behavior. This change should be a no-op.
>>>>
>>>> Signed-off-by: James Clark <james.clark@arm.com>
>>>> ---
>>>>  include/linux/perf_event.h | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>>>> index d5628a7b5eaa..3f4d941fd6c5 100644
>>>> --- a/include/linux/perf_event.h
>>>> +++ b/include/linux/perf_event.h
>>>> @@ -288,7 +288,7 @@ struct perf_event_pmu_context;
>>>>  #define PERF_PMU_CAP_EXTENDED_REGS		0x0008
>>>>  #define PERF_PMU_CAP_EXCLUSIVE			0x0010
>>>>  #define PERF_PMU_CAP_ITRACE			0x0020
>>>> -#define PERF_PMU_CAP_HETEROGENEOUS_CPUS		0x0040
>>>> +/* Unused					0x0040 */
>>>
>>> Small nit, "Unused" marking might not be required here.
>>>
>>
>> But then it would be very easy to miss that there is a free bit if I
>> don't leave the comment. Is it really better without it?
>>
>> I could shift all the following ones down by one bit, but it would be a
> 
> Sounds as a better option IMHO.
> 
>> lot of work to make sure that nobody has hard coded some check for one
>> of the bits instead of using the define somewhere.
> 
> These are not user visible ABI and hence defined in include/linux/perf_event.h
> to be used by drivers registering a PMU for capability enumeration. I am just
> wondering why they might have been hard coded any where ?
> 

You never know, I've seen worse! I suppose we'll find out if I change it
and anything breaks. It will probably be fine though, I'll make the
change on the next version.

>>
>>>>  #define PERF_PMU_CAP_NO_EXCLUDE			0x0080
>>>>  #define PERF_PMU_CAP_AUX_OUTPUT			0x0100
>>>>  #define PERF_PMU_CAP_EXTENDED_HW_TYPE		0x0200