linux-kernel - [PATCH v4 12/13] coresight: Allow setting the timestamp interval

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Message-Id: <20251112-james-cs-syncfreq-v4-12-165ba21401dc@linaro.org>
Date: Wed, 12 Nov 2025 15:22:18 +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>
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 v4 12/13] coresight: Allow setting the timestamp interval

Timestamps are currently emitted at the maximum rate possible, which is
much too frequent for most use cases. Set the interval using the value
from the timestamp field. Granular control is not required, so save
space in the config by interpreting it as 2 ^ timestamp. And then 4
bits (0 - 15) is 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-etm-perf.h   |  1 +
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 28 +++++++++++++++-------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.h b/drivers/hwtracing/coresight/coresight-etm-perf.h
index 24d929428633..128f80bb1443 100644
--- a/drivers/hwtracing/coresight/coresight-etm-perf.h
+++ b/drivers/hwtracing/coresight/coresight-etm-perf.h
@@ -7,6 +7,7 @@
 #ifndef _CORESIGHT_ETM_PERF_H
 #define _CORESIGHT_ETM_PERF_H
 
+#include <linux/bits.h>
 #include <linux/percpu-defs.h>
 #include "coresight-priv.h"
 
diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index d4e294cd48ae..cb6f08510dc0 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -660,7 +660,7 @@ static void etm4_enable_sysfs_smp_call(void *info)
  *  +--------------+
  *         |
  *  +------v-------+
- *  | Counter x    |   (reload to 1 on underflow)
+ *  | Counter x    |   (reload to 2 ^ timestamp on underflow)
  *  +--------------+
  *         |
  *  +------v--------------+
@@ -671,11 +671,25 @@ 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,
+				       struct perf_event_attr *attr)
 {
 	int ctridx;
 	int rselector;
 	struct etmv4_config *config = &drvdata->config;
+	struct perf_event_attr max_timestamp = {
+		.ATTR_CFG_FLD_timestamp_CFG = U64_MAX,
+	};
+
+	/* timestamp may be 0 if deprecated_timestamp is used, so make min 1 */
+	u8 ts_level = max(1, ATTR_CFG_GET_FLD(attr, timestamp));
+
+	/*
+	 * Disable counter generated timestamps when timestamp == MAX. Leave
+	 * only SYNC timestamps.
+	 */
+	if (ts_level == ATTR_CFG_GET_FLD(&max_timestamp, timestamp))
+		return 0;
 
 	/* No point in trying if we don't have at least one counter */
 	if (!drvdata->nr_cntr)
@@ -713,12 +727,8 @@ 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
@@ -808,7 +818,7 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
 		 * order to correlate instructions executed on different CPUs
 		 * (CPU-wide trace scenarios).
 		 */
-		ret = etm4_config_timestamp_event(drvdata);
+		ret = etm4_config_timestamp_event(drvdata, attr);
 
 		/*
 		 * No need to go further if timestamp intervals can't

-- 
2.34.1