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From:   Mike Leach <mike.leach@linaro.org>
Date:   Fri, 21 Jan 2022 12:50:34 +0000
Message-ID: <CAJ9a7VihnvK6uwYri0kh_pwQu893+FrdcUNg0Y5iW7Euh3YwuA@mail.gmail.com>
Subject: Re: [PATCH v2 6/6] Documentation: coresight: Expand branch broadcast documentation
To:     James Clark <James.Clark@arm.com>
Cc:     suzuki.poulose@arm.com, mathieu.poirier@linaro.org,
        coresight@lists.linaro.org, leo.yan@linaro.com,
        Leo Yan <leo.yan@linaro.org>,
        John Garry <john.garry@huawei.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org
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On Thu, 13 Jan 2022 at 09:11, James Clark <james.clark@arm.com> wrote:
>
> Now that there is a way of enabling branch broadcast via perf, mention
> the possible use cases and known limitations.
>
> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>  .../trace/coresight/coresight-etm4x-reference.rst      | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/trace/coresight/coresight-etm4x-reference.rst b/Documentation/trace/coresight/coresight-etm4x-reference.rst
> index 0439b4006227..ec336575919c 100644
> --- a/Documentation/trace/coresight/coresight-etm4x-reference.rst
> +++ b/Documentation/trace/coresight/coresight-etm4x-reference.rst
> @@ -656,7 +656,15 @@ Bit assignments shown below:-
>      ETM_MODE_BB
>
>  **description:**
> -    Set to enable branch broadcast if supported in hardware [IDR0].
> +    Set to enable branch broadcast if supported in hardware [IDR0]. The primary use for this feature
> +    is when code is patched dynamically at run time and the full program flow may not be able to be
> +    reconstructed using only conditional branches.
> +
> +    Choosing this option will result in a significant increase in the amount of trace generated -
> +    possible danger of overflows, or fewer instructions covered. Note, that this option also
> +    overrides any setting of :ref:`ETM_MODE_RETURNSTACK <coresight-return-stack>`, so where a branch
> +    broadcast range overlaps a return stack range, return stacks will not be available for that
> +    range.
>
>  .. _coresight-cycle-accurate:
>
> --
> 2.28.0
>

Reviewed-by: Mike Leach <mike.leach@linaro.org>
-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK