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

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Message-Id: <20220511144601.2257870-5-james.clark@arm.com>
Date:   Wed, 11 May 2022 15:46:01 +0100
From:   James Clark <james.clark@....com>
To:     suzuki.poulose@....com, mathieu.poirier@...aro.org,
        coresight@...ts.linaro.org, mike.leach@...aro.org
Cc:     leo.yan@...aro.com, James Clark <james.clark@....com>,
        Leo Yan <leo.yan@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 4/4] Documentation: coresight: Expand branch broadcast documentation

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