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Message-ID: <7676ff3a-4afc-a26a-8e20-521054298c72@arm.com>
Date:   Wed, 16 Aug 2023 10:02:01 +0100
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Subject: Re: [PATCH v5 2/6] perf arm64: Allow version comparisons of CPU IDs
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To:     John Garry <john.g.garry@oracle.com>,
        linux-perf-users@vger.kernel.org, irogers@google.com,
        renyu.zj@linux.alibaba.com
Cc:     Will Deacon <will@kernel.org>, Mike Leach <mike.leach@linaro.org>,
        Leo Yan <leo.yan@linaro.org>,
        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>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Nick Forrington <nick.forrington@arm.com>,
        Kajol Jain <kjain@linux.ibm.com>,
        Eduard Zingerman <eddyz87@gmail.com>,
        Sohom Datta <sohomdatta1@gmail.com>,
        Rob Herring <robh@kernel.org>, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org
References: <20230811144017.491628-1-james.clark@arm.com>
 <20230811144017.491628-3-james.clark@arm.com>
 <47a32410-d9ca-627b-e8a3-f64d4a1ea95f@oracle.com>
 <4c6a4729-8331-5c47-a81e-f92915e2e848@arm.com>
 <eb23ad55-a46c-4ed7-d2b0-0dcf7d536f89@oracle.com>
From:   James Clark <james.clark@arm.com>
In-Reply-To: <eb23ad55-a46c-4ed7-d2b0-0dcf7d536f89@oracle.com>
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On 14/08/2023 15:43, John Garry wrote:
> On 14/08/2023 15:15, James Clark wrote:
>>
>> On 14/08/2023 14:07, John Garry wrote:
>>> On 11/08/2023 15:39, James Clark wrote:
>>>> Currently variant and revision fields are masked out of the MIDR so
>>>> it's not possible to compare different versions of the same CPU.
>>>> In a later commit a workaround will be removed just for N2 r0p3, so
>>>> enable comparisons on version.
>>>>
>>>> This has the side effect of changing the MIDR stored in the header of
>>>> the perf.data file to no longer have masked version fields.
>>> Did you consider adding a raw version of _get_cpuid(), which returns the
>>> full MIDR just for the purpose of caller strcmp_cpuid_str()?
>> I did, but I thought that seeing as it would only be used in one place,
>> and that changing the existing one didn't break anything, that it was
>> better to not fragment the CPU ID interface. I thought it might also
>> have repercussions for the other architectures as well. It would also
>> mean that the MIDR that's stored in the header wouldn't have the version
>> information, which if we're starting to do things with that could be bad.
>>
>> There are already callers of strcmp_cpuid_str() so it's probably best to
>> keep it using the same get_cpuid() string. Unless there is a reason
>> _not_  to do it? There isn't really anything that can't be done with it
>> accepting/returning the full unmasked MIDR. If you want the old
>> behavior, you just set the version fields to 0, which I've also used in
>> a later patch and is already done in mapfile.csv
>>
> 
> ok, fine, so we seems that we would be following x86 on this in terms of
> using strcmp_cpuid_str(). It would be good to mention that there is
> already a weak version of strcmp_cpuid_str() for !x86 in your commit
> message.
> 

Yep I can add this.

> Let me check your code again...
> 
> Thanks,
> John