linux-kernel - Re: [PATCH v1 3/3] perf cs-etm: Remove callback cs_etm_find

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Date:   Thu, 24 Jun 2021 17:46:32 +0100
From:   James Clark <james.clark@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Daniel Kiss <daniel.kiss@....com>,
        Denis Nikitin <denik@...gle.com>, coresight@...ts.linaro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v1 3/3] perf cs-etm: Remove callback
 cs_etm_find_snapshot()



On 28/05/2021 17:15, Leo Yan wrote:
> The callback cs_etm_find_snapshot() is invoked for snapshot mode, its
> main purpose is to find the correct AUX trace data and returns "head"
> and "old" (we can call "old" as "old head") to the caller, the caller
> __auxtrace_mmap__read() uses these two pointers to decide the AUX trace
> data size.
> > cs_etm_find_snapshot() should be removed with below reasons:
> 
> - The first thing in cs_etm_find_snapshot() is to check if the head has
>   wrapped around, if it is not, directly bails out.  The checking is
>   pointless, this is because the "head" and "old" pointers both are
>   monotonical increasing so they never wrap around.
> 
For patch 3:

Reviewed-by: James Clark <james.clark@....com>
Tested-by: James Clark <james.clark@....com>

I think it's a good simplification and it also fixes the duplicate buffers
issue. And I agree with the reasoning about the pointer increasing monotonically.