[PATCH v2 0/6] coresight: Add format attribute for setting the timestamp interval

[PATCH v2 0/6] coresight: Add format attribute for setting the timestamp interval

[James Clark]

Do some cleanups then add a new format attribute to set the timestamp
interval for ETMv4 in Perf mode. The current interval is too high for
most use cases, and particularly on the FVP the number of timestamps
generated is excessive.

Although it would be good to make only SYNC timestamps the default and
have counter timestamps opt-in, this would be a breaking change. We
can always do that later, or disable counter timestamps from Perf.

This is added as an event format attribute, rather than a Coresight
config because it's something that the driver is already configuring
automatically in Perf mode with any unused counter, so it's not possible
to modify this with a config.

Applies to coresight/next

Signed-off-by: James Clark <james.clark@linaro.org>
---
Changes in v2:
- Only show the attribute for ETMv4 to improve usability and fix the
  arm32 build error. Wrapping everything in
  IS_ENABLED(CONFIG_CORESIGHT_SOURCE_ETM4X) isn't ideal, but the -perf.c
  file is shared between ETMv3 and ETMv4, and there is already precedent
  for doing it this way.
- Link to v1: https://lore.kernel.org/r/20250811-james-cs-syncfreq-v1-0-b001cd6e3404@linaro.org

---
James Clark (6):
      coresight: Change syncfreq to be a u8
      coresight: Fix holes in struct etmv4_config
      coresight: Repack struct etmv4_drvdata
      coresight: Refactor etm4_config_timestamp_event()
      coresight: Add format attribute for setting the timestamp interval
      coresight: docs: Document etm4x ts_interval

 Documentation/trace/coresight/coresight.rst        |  14 +++
 drivers/hwtracing/coresight/coresight-etm-perf.c   |  13 ++-
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 110 +++++++++++++--------
 drivers/hwtracing/coresight/coresight-etm4x.h      |  86 ++++++++++------
 4 files changed, 151 insertions(+), 72 deletions(-)
---
base-commit: a80198ba650f50d266d7fc4a6c5262df9970f9f2
change-id: 20250724-james-cs-syncfreq-7c2257a38ed3

Best regards,
-- 
James Clark <james.clark@linaro.org>

[PATCH v2 1/6] coresight: Change syncfreq to be a u8

[James Clark]

TRCSYNCPR.PERIOD is the only functional part of TRCSYNCPR and it only
has 5 valid bits so it can be stored in a u8.

Signed-off-by: James Clark <james.clark@linaro.org>
---
 drivers/hwtracing/coresight/coresight-etm4x.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
index ac649515054d..746627476bd3 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -824,7 +824,6 @@ struct etmv4_config {
 	u32				eventctrl1;
 	u32				stall_ctrl;
 	u32				ts_ctrl;
-	u32				syncfreq;
 	u32				ccctlr;
 	u32				bb_ctrl;
 	u32				vinst_ctrl;
@@ -832,6 +831,7 @@ struct etmv4_config {
 	u32				vissctlr;
 	u32				vipcssctlr;
 	u8				seq_idx;
+	u8				syncfreq;
 	u32				seq_ctrl[ETM_MAX_SEQ_STATES];
 	u32				seq_rst;
 	u32				seq_state;

-- 
2.34.1

[PATCH v2 2/6] coresight: Fix holes in struct etmv4_config

[James Clark]

Lots of u8s are mixed with u64s and u32s so repack it to save a bit
of space because there's one of these for each ETM.

Signed-off-by: James Clark <james.clark@linaro.org>
---
 drivers/hwtracing/coresight/coresight-etm4x.h | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
index 746627476bd3..a355a1e9606d 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -832,28 +832,33 @@ struct etmv4_config {
 	u32				vipcssctlr;
 	u8				seq_idx;
 	u8				syncfreq;
+	u8				cntr_idx;
+	u8				res_idx;
+	u8				ss_idx;
+	u8				addr_idx;
+	u8				addr_type[ETM_MAX_SINGLE_ADDR_CMP];
+	u8				ctxid_idx;
+	u8				vmid_idx;
 	u32				seq_ctrl[ETM_MAX_SEQ_STATES];
 	u32				seq_rst;
 	u32				seq_state;
-	u8				cntr_idx;
+
 	u32				cntrldvr[ETMv4_MAX_CNTR];
 	u32				cntr_ctrl[ETMv4_MAX_CNTR];
 	u32				cntr_val[ETMv4_MAX_CNTR];
-	u8				res_idx;
+
 	u32				res_ctrl[ETM_MAX_RES_SEL];
-	u8				ss_idx;
+
 	u32				ss_ctrl[ETM_MAX_SS_CMP];
 	u32				ss_status[ETM_MAX_SS_CMP];
 	u32				ss_pe_cmp[ETM_MAX_SS_CMP];
-	u8				addr_idx;
+
 	u64				addr_val[ETM_MAX_SINGLE_ADDR_CMP];
 	u64				addr_acc[ETM_MAX_SINGLE_ADDR_CMP];
-	u8				addr_type[ETM_MAX_SINGLE_ADDR_CMP];
-	u8				ctxid_idx;
+
 	u64				ctxid_pid[ETMv4_MAX_CTXID_CMP];
 	u32				ctxid_mask0;
 	u32				ctxid_mask1;
-	u8				vmid_idx;
 	u64				vmid_val[ETM_MAX_VMID_CMP];
 	u32				vmid_mask0;
 	u32				vmid_mask1;

-- 
2.34.1

[PATCH v2 3/6] coresight: Repack struct etmv4_drvdata

[James Clark]

Fix holes and convert the long list of bools to single bits to save
some space because there's one of these for each ETM.

Signed-off-by: James Clark <james.clark@linaro.org>
---
 drivers/hwtracing/coresight/coresight-etm4x.h | 39 ++++++++++++++-------------
 1 file changed, 20 insertions(+), 19 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
index a355a1e9606d..1c67b263b01b 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -1020,29 +1020,30 @@ struct etmv4_drvdata {
 	u8				ns_ex_level;
 	u8				q_support;
 	u8				os_lock_model;
-	bool				sticky_enable;
-	bool				boot_enable;
-	bool				os_unlock;
-	bool				instrp0;
-	bool				q_filt;
-	bool				trcbb;
-	bool				trccond;
-	bool				retstack;
-	bool				trccci;
-	bool				trc_error;
-	bool				syncpr;
-	bool				stallctl;
-	bool				sysstall;
-	bool				nooverflow;
-	bool				atbtrig;
-	bool				lpoverride;
+	bool				sticky_enable : 1;
+	bool				boot_enable : 1;
+	bool				os_unlock : 1;
+	bool				instrp0 : 1;
+	bool				q_filt : 1;
+	bool				trcbb : 1;
+	bool				trccond : 1;
+	bool				retstack : 1;
+	bool				trccci : 1;
+	bool				trc_error : 1;
+	bool				syncpr : 1;
+	bool				stallctl : 1;
+	bool				sysstall : 1;
+	bool				nooverflow : 1;
+	bool				atbtrig : 1;
+	bool				lpoverride : 1;
+	bool				state_needs_restore : 1;
+	bool				skip_power_up : 1;
+	bool				paused : 1;
 	u64				trfcr;
 	struct etmv4_config		config;
 	u64				save_trfcr;
 	struct etmv4_save_state		*save_state;
-	bool				state_needs_restore;
-	bool				skip_power_up;
-	bool				paused;
+
 	DECLARE_BITMAP(arch_features, ETM4_IMPDEF_FEATURE_MAX);
 };
 

-- 
2.34.1

[PATCH v2 4/6] coresight: Refactor etm4_config_timestamp_event()

[James Clark]

Remove some of the magic numbers and try to clarify some of the
documentation so it's clearer how this sets up the timestamp interval.

Return errors directly instead of jumping to out and returning ret,
nothing needs to be cleaned up at the end and it only obscures the flow
and return value.

Signed-off-by: James Clark <james.clark@linaro.org>
---
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 95 ++++++++++++++--------
 drivers/hwtracing/coresight/coresight-etm4x.h      | 20 +++--
 2 files changed, 77 insertions(+), 38 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index cbea200489c8..1a2d02bdcb88 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -608,18 +608,33 @@ static void etm4_enable_hw_smp_call(void *info)
  * TRCRSCTLR1 (always true) used to get the counter to decrement.  From
  * there a resource selector is configured with the counter and the
  * timestamp control register to use the resource selector to trigger the
- * event that will insert a timestamp packet in the stream.
+ * event that will insert a timestamp packet in the stream:
+ *
+ *  +--------------+
+ *  | Resource 1   |   fixed "always-true" resource
+ *  +--------------+
+ *         |
+ *  +------v-------+
+ *  | Counter x    |   (reload to 1 on underflow)
+ *  +--------------+
+ *         |
+ *  +------v--------------+
+ *  | Resource Selector y |   (trigger on counter x == 0)
+ *  +---------------------+
+ *         |
+ *  +------v---------------+
+ *  | Timestamp Generator  |  (timestamp on resource y)
+ *  +----------------------+
  */
 static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata)
 {
-	int ctridx, ret = -EINVAL;
-	int counter, rselector;
-	u32 val = 0;
+	int ctridx;
+	int rselector;
 	struct etmv4_config *config = &drvdata->config;
 
 	/* No point in trying if we don't have at least one counter */
 	if (!drvdata->nr_cntr)
-		goto out;
+		return -EINVAL;
 
 	/* Find a counter that hasn't been initialised */
 	for (ctridx = 0; ctridx < drvdata->nr_cntr; ctridx++)
@@ -629,15 +644,17 @@ static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata)
 	/* All the counters have been configured already, bail out */
 	if (ctridx == drvdata->nr_cntr) {
 		pr_debug("%s: no available counter found\n", __func__);
-		ret = -ENOSPC;
-		goto out;
+		return -ENOSPC;
 	}
 
 	/*
-	 * Searching for an available resource selector to use, starting at
-	 * '2' since every implementation has at least 2 resource selector.
-	 * ETMIDR4 gives the number of resource selector _pairs_,
-	 * hence multiply by 2.
+	 * Searching for an available resource selector to use, starting at '2'
+	 * since resource 0 is the fixed 'always returns false' resource and 1
+	 * is the fixed 'always returns true' resource. See IHI0064H_b '7.3.64
+	 * TRCRSCTLRn, Resource Selection Control Registers, n=2-31'.
+	 *
+	 * ETMIDR4 gives the number of resource selector _pairs_, hence multiply
+	 * by 2.
 	 */
 	for (rselector = 2; rselector < drvdata->nr_resource * 2; rselector++)
 		if (!config->res_ctrl[rselector])
@@ -646,13 +663,9 @@ static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata)
 	if (rselector == drvdata->nr_resource * 2) {
 		pr_debug("%s: no available resource selector found\n",
 			 __func__);
-		ret = -ENOSPC;
-		goto out;
+		return -ENOSPC;
 	}
 
-	/* Remember what counter we used */
-	counter = 1 << ctridx;
-
 	/*
 	 * Initialise original and reload counter value to the smallest
 	 * possible value in order to get as much precision as we can.
@@ -660,26 +673,42 @@ static int etm4_config_timestamp_event(struct etmv4_drvdata *drvdata)
 	config->cntr_val[ctridx] = 1;
 	config->cntrldvr[ctridx] = 1;
 
-	/* Set the trace counter control register */
-	val =  0x1 << 16	|  /* Bit 16, reload counter automatically */
-	       0x0 << 7		|  /* Select single resource selector */
-	       0x1;		   /* Resource selector 1, i.e always true */
-
-	config->cntr_ctrl[ctridx] = val;
-
-	val = 0x2 << 16		| /* Group 0b0010 - Counter and sequencers */
-	      counter << 0;	  /* Counter to use */
-
-	config->res_ctrl[rselector] = val;
+	/*
+	 * Trace Counter Control Register TRCCNTCTLRn
+	 *
+	 * CNTCHAIN = 0, don't reload on the previous counter
+	 * RLDSELF = true, reload counter automatically on underflow
+	 * RLDTYPE = 0, one reload input resource
+	 * RLDSEL = 0, reload on resource 0 (fixed always false resource, never
+	 *	       reload)
+	 * CNTTYPE = 0, one count input resource
+	 * CNTSEL = 1, count on resource 1 (fixed always true resource, always
+	 *	       decrement)
+	 */
+	config->cntr_ctrl[ctridx] = TRCCNTCTLRn_RLDSELF |
+				    FIELD_PREP(TRCCNTCTLRn_CNTSEL_MASK, 1);
 
-	val = 0x0 << 7		| /* Select single resource selector */
-	      rselector;	  /* Resource selector */
+	/*
+	 * Resource Selection Control Register TRCRSCTLRn
+	 *
+	 * PAIRINV = 0, INV = 0, don't invert
+	 * GROUP = 2, SELECT = ctridx, trigger when counter 'ctridx' reaches 0
+	 *
+	 * Multiple counters can be selected, and each bit signifies a counter,
+	 * so set bit 'ctridx' to select our counter.
+	 */
+	config->res_ctrl[rselector] = FIELD_PREP(TRCRSCTLRn_GROUP_MASK, 2) |
+				      FIELD_PREP(TRCRSCTLRn_SELECT_MASK, 1 << ctridx);
 
-	config->ts_ctrl = val;
+	/*
+	 * Global Timestamp Control Register TRCTSCTLR
+	 *
+	 * TYPE = 0, one input resource
+	 * SEL = rselector, generate timestamp on resource 'rselector'
+	 */
+	config->ts_ctrl = FIELD_PREP(TRCTSCTLR_SEL_MASK, rselector);
 
-	ret = 0;
-out:
-	return ret;
+	return 0;
 }
 
 static int etm4_parse_event_config(struct coresight_device *csdev,
diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
index 1c67b263b01b..aaa6633b2d67 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -224,6 +224,16 @@
 #define TRCRSCTLRn_GROUP_MASK			GENMASK(19, 16)
 #define TRCRSCTLRn_SELECT_MASK			GENMASK(15, 0)
 
+#define TRCCNTCTLRn_CNTCHAIN			BIT(17)
+#define TRCCNTCTLRn_RLDSELF			BIT(16)
+#define TRCCNTCTLRn_RLDTYPE			BIT(15)
+#define TRCCNTCTLRn_RLDSEL_MASK			GENMASK(12, 8)
+#define TRCCNTCTLRn_CNTTYPE_MASK		BIT(7)
+#define TRCCNTCTLRn_CNTSEL_MASK			GENMASK(4, 0)
+
+#define TRCTSCTLR_TYPE				BIT(7)
+#define TRCTSCTLR_SEL_MASK			GENMASK(4, 0)
+
 /*
  * System instructions to access ETM registers.
  * See ETMv4.4 spec ARM IHI0064F section 4.3.6 System instructions
@@ -823,7 +833,7 @@ struct etmv4_config {
 	u32				eventctrl0;
 	u32				eventctrl1;
 	u32				stall_ctrl;
-	u32				ts_ctrl;
+	u32				ts_ctrl; /* TRCTSCTLR */
 	u32				ccctlr;
 	u32				bb_ctrl;
 	u32				vinst_ctrl;
@@ -843,11 +853,11 @@ struct etmv4_config {
 	u32				seq_rst;
 	u32				seq_state;
 
-	u32				cntrldvr[ETMv4_MAX_CNTR];
-	u32				cntr_ctrl[ETMv4_MAX_CNTR];
-	u32				cntr_val[ETMv4_MAX_CNTR];
+	u32				cntrldvr[ETMv4_MAX_CNTR]; /* TRCCNTRLDVRn */
+	u32				cntr_ctrl[ETMv4_MAX_CNTR]; /* TRCCNTCTLRn */
+	u32				cntr_val[ETMv4_MAX_CNTR]; /* TRCCNTVRn */
 
-	u32				res_ctrl[ETM_MAX_RES_SEL];
+	u32				res_ctrl[ETM_MAX_RES_SEL]; /* TRCRSCTLRn */
 
 	u32				ss_ctrl[ETM_MAX_SS_CMP];
 	u32				ss_status[ETM_MAX_SS_CMP];

-- 
2.34.1

[PATCH v2 5/6] coresight: Add format attribute for setting the timestamp interval

[James Clark]

Timestamps are currently emitted at the maximum rate possible, which is
much too frequent for most use cases. Add an attribute to be able to set
the interval. Granular control is not required, so save space in the
config by interpreting it as 2 ^ ts_interval. And then 4 bits (0 - 15) is
enough to set the interval to be larger than the existing SYNC timestamp
interval.

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

Only show this attribute for ETM4x because timestamps aren't configured
in the same way for ETM3x. The attribute is only ever read in
coresight-etm4x-core.c.

Signed-off-by: James Clark <james.clark@linaro.org>
---
 drivers/hwtracing/coresight/coresight-etm-perf.c   | 13 ++++++++++++-
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 21 ++++++++++++---------
 drivers/hwtracing/coresight/coresight-etm4x.h      |  6 ++++++
 3 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
index f677c08233ba..af937bbb933c 100644
--- a/drivers/hwtracing/coresight/coresight-etm-perf.c
+++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
@@ -25,6 +25,11 @@
 #include "coresight-syscfg.h"
 #include "coresight-trace-id.h"
 
+#if IS_ENABLED(CONFIG_CORESIGHT_SOURCE_ETM4X)
+#include <linux/perf/arm_pmu.h>
+#include "coresight-etm4x.h"
+#endif
+
 static struct pmu etm_pmu;
 static bool etm_perf_up;
 
@@ -69,7 +74,10 @@ PMU_FORMAT_ATTR(sinkid,		"config2:0-31");
 /* config ID - set if a system configuration is selected */
 PMU_FORMAT_ATTR(configid,	"config2:32-63");
 PMU_FORMAT_ATTR(cc_threshold,	"config3:0-11");
-
+#if IS_ENABLED(CONFIG_CORESIGHT_SOURCE_ETM4X)
+/* Interval = (2 ^ ts_level) */
+GEN_PMU_FORMAT_ATTR(ts_level);
+#endif
 
 /*
  * contextid always traces the "PID".  The PID is in CONTEXTIDR_EL1
@@ -103,6 +111,9 @@ static struct attribute *etm_config_formats_attr[] = {
 	&format_attr_configid.attr,
 	&format_attr_branch_broadcast.attr,
 	&format_attr_cc_threshold.attr,
+#if IS_ENABLED(CONFIG_CORESIGHT_SOURCE_ETM4X)
+	&format_attr_ts_level.attr,
+#endif
 	NULL,
 };
 
diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 1a2d02bdcb88..42277c201d4f 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -28,6 +28,7 @@
 #include <linux/amba/bus.h>
 #include <linux/seq_file.h>
 #include <linux/uaccess.h>
+#include <linux/perf/arm_pmu.h>
 #include <linux/perf_event.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
@@ -615,7 +616,7 @@ static void etm4_enable_hw_smp_call(void *info)
  *  +--------------+
  *         |
  *  +------v-------+
- *  | Counter x    |   (reload to 1 on underflow)
+ *  | Counter x    |   (reload to 2 ^ ts_level on underflow)
  *  +--------------+
  *         |
  *  +------v--------------+
@@ -626,11 +627,17 @@ static void etm4_enable_hw_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;
+	u8 ts_level = ATTR_CFG_GET_FLD(attr, ts_level);
+
+	/* Disable when ts_level == MAX */
+	if (ts_level == FIELD_GET(ATTR_CFG_FLD_ts_level_MASK, UINT_MAX))
+		return 0;
 
 	/* No point in trying if we don't have at least one counter */
 	if (!drvdata->nr_cntr)
@@ -666,12 +673,8 @@ 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;
 
 	/*
 	 * Trace Counter Control Register TRCCNTCTLRn
@@ -761,7 +764,7 @@ 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
diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
index aaa6633b2d67..54558de158fa 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -598,6 +598,12 @@
 #define ETM_CNTR_MAX_VAL		0xFFFF
 #define ETM_TRACEID_MASK		0x3f
 
+#define ATTR_CFG_FLD_ts_level_CFG	config3
+#define ATTR_CFG_FLD_ts_level_LO	12
+#define ATTR_CFG_FLD_ts_level_HI	15
+#define ATTR_CFG_FLD_ts_level_MASK	GENMASK(ATTR_CFG_FLD_ts_level_HI, \
+						ATTR_CFG_FLD_ts_level_LO)
+
 /* ETMv4 programming modes */
 #define ETM_MODE_EXCLUDE		BIT(0)
 #define ETM_MODE_LOAD			BIT(1)

-- 
2.34.1

[PATCH v2 6/6] coresight: docs: Document etm4x ts_interval

[James Clark]

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

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

diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst
index 806699871b80..0cd83119b83f 100644
--- a/Documentation/trace/coresight/coresight.rst
+++ b/Documentation/trace/coresight/coresight.rst
@@ -619,6 +619,20 @@ They are also listed in the folder /sys/bus/event_source/devices/cs_etm/format/
      - 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
        value, as indicated via TRCIDR3.CCITMIN, then the minimum value will be used instead.
+   * - ts_level
+     - Controls frequency of timestamps. The reload value of the
+       timestamp counter is 2 raised to the power of this value. If the value is
+       0 then the reload value is 1, if the value is 10 then the reload value is
+       1024. Maximum allowed value is 15, and setting the maximum disables
+       generation of timestamps via the counter, freeing the counter resources.
+       Timestamps will be generated after 2 ^ ts_level cycles.
+
+       Separately to this value, timestamps will also be emitted when a SYNC
+       packet is generated, although this is only for every 4096 bytes of trace.
+       Therefore it's not possible to generate timestamps less frequently than
+       that and ts_level timestamps are always in addition to SYNC timestamps.
+       Timestamps must be enabled for this to have effect.
+
 
 How to use the STM module
 -------------------------

-- 
2.34.1