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 | 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: