linux-kernel - [PATCH v5 13/13] coresight: docs: Document etm4x timestamp interval option

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Message-Id: <20251118-james-cs-syncfreq-v5-13-82efd7b1a751@linaro.org>
Date: Tue, 18 Nov 2025 16:28:03 +0000
From: James Clark <james.clark@...aro.org>
To: Suzuki K Poulose <suzuki.poulose@....com>, 
 Mike Leach <mike.leach@...aro.org>, 
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
 Jonathan Corbet <corbet@....net>, Leo Yan <leo.yan@....com>, 
 Randy Dunlap <rdunlap@...radead.org>
Cc: coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
 James Clark <james.clark@...aro.org>
Subject: [PATCH v5 13/13] coresight: docs: Document etm4x timestamp
 interval option

Document how the new field is used, maximum value and the interaction
with SYNC timestamps.

Signed-off-by: James Clark <james.clark@...aro.org>
---
 Documentation/trace/coresight/coresight.rst | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst
index 806699871b80..d461de4e067e 100644
--- a/Documentation/trace/coresight/coresight.rst
+++ b/Documentation/trace/coresight/coresight.rst
@@ -613,8 +613,20 @@ They are also listed in the folder /sys/bus/event_source/devices/cs_etm/format/
      - Session local version of the system wide setting: :ref:`ETM_MODE_RETURNSTACK
        <coresight-return-stack>`
    * - timestamp
-     - Session local version of the system wide setting: :ref:`ETMv4_MODE_TIMESTAMP
-       <coresight-timestamp>`
+     - Controls generation and interval of timestamps.
+
+       0 = off, 1 = minimum interval .. 15 = maximum interval.
+
+       Values 1 - 14 use a counter that decrements every cycle to generate a
+       timestamp on underflow. The reload value for the counter is 2 ^ (interval
+       - 1). If the value is 1 then the reload value is 1, if the value is 11
+       then the reload value is 1024 etc.
+
+       Setting the maximum interval (15) will disable the counter generated
+       timestamps, freeing the counter resource, leaving only ones emitted when
+       a SYNC packet is generated. The sync interval is controlled with
+       TRCSYNCPR.PERIOD which is every 4096 bytes of trace by default.
+
    * - cc_threshold
      - Cycle count threshold value. If nothing is provided here or the provided value is 0, then the
        default value i.e 0x100 will be used. If provided value is less than minimum cycles threshold

-- 
2.34.1