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Documentation: coresight: Expand branch broadcast documentation

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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>
Reviewed-by: Mike Leach<mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220511144601.2257870-5-james.clark@arm.com
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James Clark 2022-05-11 15:46:01 +01:00 committed by Suzuki K Poulose
parent 32ee00d86e
commit 774daad3c0
1 changed files with 12 additions and 1 deletions
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13
Documentation/trace/coresight/coresight-etm4x-reference.rst
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@ -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: