linux-kernel - [PATCH v8 11/13] coresight: Prepare to allow setting the timestamp interval

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Message-Id: <20251128-james-cs-syncfreq-v8-11-4d319764cc58@linaro.org>
Date: Fri, 28 Nov 2025 11:55:23 +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 v8 11/13] coresight: Prepare to allow setting the timestamp
 interval

Timestamps are currently emitted at the maximum rate possible, which is
much too frequent for most use cases. In the next commit, the timestamp
field will be widened to take a value, so set the interval using the
value now. Granular control is not required, so save space in the config
by interpreting it as 2 ^ timestamp. And then 4 bits (0 - 15) will be
enough to set the interval to be larger than the existing SYNC timestamp
interval.

No sysfs mode support is needed for this attribute because counter
generated timestamps are only configured for Perf mode.

Reviewed-by: Leo Yan <leo.yan@....com>
Tested-by: Leo Yan <leo.yan@....com>
Signed-off-by: James Clark <james.clark@...aro.org>
---
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index b457f182efbe..7e9c923acf4b 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -650,7 +650,7 @@ static void etm4_enable_sysfs_smp_call(void *info)
  *  +--------------+
  *         |
  *  +------v-------+
- *  | Counter x    |   (reload to 1 on underflow)
+ *  | Counter x    |   (reload to 2 ^ (ts_level - 1) on underflow)
  *  +--------------+
  *         |
  *  +------v--------------+
@@ -661,7 +661,8 @@ static void etm4_enable_sysfs_smp_call(void *info)
  *  | Timestamp Generator  |  (timestamp on resource y)
  *  +----------------------+
  */
-static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata)
+static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata,
+				       u8 ts_level)
 {
 	int ctridx;
 	int rselector;
@@ -703,12 +704,8 @@ static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata)
 		return -ENOSPC;
 	}
 
-	/*
-	 * Initialise original and reload counter value to the smallest
-	 * possible value in order to get as much precision as we can.
-	 */
-	config->cntr_val[ctridx] = 1;
-	config->cntrldvr[ctridx] = 1;
+	/* Initialise original and reload counter value. */
+	config->cntr_val[ctridx] = config->cntrldvr[ctridx] = 1 << (ts_level - 1);
 
 	/*
 	 * Trace Counter Control Register TRCCNTCTLRn
@@ -756,6 +753,7 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
 	struct perf_event_attr *attr = &event->attr;
 	unsigned long cfg_hash;
 	int preset, cc_threshold;
+	u8 ts_level;
 
 	/* Clear configuration from previous run */
 	memset(config, 0, sizeof(struct etmv4_config));
@@ -791,13 +789,15 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
 			cc_threshold = drvdata->ccitmin;
 		config->ccctlr = cc_threshold;
 	}
-	if (ATTR_CFG_GET_FLD(attr, timestamp)) {
+
+	ts_level = ATTR_CFG_GET_FLD(attr, timestamp);
+	if (ts_level) {
 		/*
 		 * Configure timestamps to be emitted at regular intervals in
 		 * order to correlate instructions executed on different CPUs
 		 * (CPU-wide trace scenarios).
 		 */
-		ret = etm4_config_timestamp_event(drvdata);
+		ret = etm4_config_timestamp_event(drvdata, ts_level);
 
 		/*
 		 * No need to go further if timestamp intervals can't

-- 
2.34.1