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@xxxxxxx>
---
.../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
