linux-kernel - [PATCH v2 6/6] Documentation: coresight: Expand branch broadcast documentation

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Message-Id: <20220113091056.1297982-7-james.clark@arm.com>
Date:   Thu, 13 Jan 2022 09:10:55 +0000
From:   James Clark <james.clark@....com>
To:     suzuki.poulose@....com, mathieu.poirier@...aro.org,
        coresight@...ts.linaro.org, leo.yan@...aro.com,
        mike.leach@...aro.org
Cc:     James Clark <james.clark@....com>, Leo Yan <leo.yan@...aro.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: [PATCH v2 6/6] 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      | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/trace/coresight/coresight-etm4x-reference.rst b/Documentation/trace/coresight/coresight-etm4x-reference.rst
index 0439b4006227..ec336575919c 100644
--- a/Documentation/trace/coresight/coresight-etm4x-reference.rst
+++ b/Documentation/trace/coresight/coresight-etm4x-reference.rst
@@ -656,7 +656,15 @@ 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.
+
+    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