linux-kernel - Re: [PATCH v3 4/4] Documentation: coresight: Expand branch broadcast documentation

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Message-ID: <CAJ9a7VigLVCa+S=rmpBs9qrmzOtkmo-yzuJJDb_wfZ5coGJnRA@mail.gmail.com>
Date:   Wed, 18 May 2022 10:28:11 +0100
From:   Mike Leach <mike.leach@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     suzuki.poulose@....com, mathieu.poirier@...aro.org,
        coresight@...ts.linaro.org, leo.yan@...aro.com,
        Leo Yan <leo.yan@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] Documentation: coresight: Expand branch broadcast documentation

On Wed, 11 May 2022 at 15:46, James Clark <james.clark@....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@....com>
> ---
>  .../trace/coresight/coresight-etm4x-reference.rst   | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/trace/coresight/coresight-etm4x-reference.rst b/Documentation/trace/coresight/coresight-etm4x-reference.rst
> index 0439b4006227..fb7578fd9372 100644
> --- a/Documentation/trace/coresight/coresight-etm4x-reference.rst
> +++ b/Documentation/trace/coresight/coresight-etm4x-reference.rst
> @@ -656,7 +656,18 @@ 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.
> +
> +    There is currently no support in Perf for supplying modified binaries to the decoder, so this
> +    feature is only inteded to be used for debugging purposes or with a 3rd party tool.
> +
> +    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@...aro.org>
-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK