[PATCH v2 05/28] coresight: etm4x: Ensure context synchronization is not ignored

[Leo Yan]

As recommended in section 4.3.7 "Synchronization of register updates" of
ARM IHI0064H.b, a self-hosted trace analyzer should always executes an
ISB instruction after programming the trace unit registers.

An ISB works as a context synchronization event; a DSB is not required.
Removes the redundant barrier in the enabling flow.

The ISB was placed at the end of the enable and disable functions.
However, this does not guarantee a context synchronization event in the
calling code, which may speculatively execute across function
boundaries.

ISB instructions are moved into callers to ensure that a context
synchronization is imposed immediately after enabling or disabling trace
unit.

Fixes: 40f682ae5086 ("coresight: etm4x: Extract the trace unit controlling")
Signed-off-by: Leo Yan <leo.yan@xxxxxxx>
---
 drivers/hwtracing/coresight/coresight-etm4x-core.c | 38 +++++++++++++++-------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 0f2a8b8459c93ca29d270b6fa05928244e0761c5..af9d3b2319c5f49ccd40dfa0ccf0f694ce9e2f4f 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -459,13 +459,6 @@ static int etm4_enable_trace_unit(struct etmv4_drvdata *drvdata)
 		return -ETIME;
 	}
 
-	/*
-	 * As recommended by section 4.3.7 ("Synchronization when using the
-	 * memory-mapped interface") of ARM IHI 0064D
-	 */
-	dsb(sy);
-	isb();
-
 	return 0;
 }
 
@@ -579,6 +572,13 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata)
 
 	if (!drvdata->paused)
 		rc = etm4_enable_trace_unit(drvdata);
+
+	/*
+	 * As recommended by section 4.3.7 (Synchronization of register updates)
+	 * of ARM IHI 0064H.b, the self-hosted trace analyzer always executes an
+	 * ISB instruction after programming the trace unit registers.
+	 */
+	isb();
 done:
 	etm4_cs_lock(drvdata, csa);
 
@@ -954,11 +954,6 @@ static void etm4_disable_trace_unit(struct etmv4_drvdata *drvdata)
 	if (etm4x_wait_status(csa, TRCSTATR_PMSTABLE_BIT, 1))
 		dev_err(etm_dev,
 			"timeout while waiting for PM stable Trace Status\n");
-	/*
-	 * As recommended by section 4.3.7 (Synchronization of register updates)
-	 * of ARM IHI 0064H.b.
-	 */
-	isb();
 }
 
 static void etm4_disable_hw(struct etmv4_drvdata *drvdata)
@@ -981,6 +976,13 @@ static void etm4_disable_hw(struct etmv4_drvdata *drvdata)
 
 	etm4_disable_trace_unit(drvdata);
 
+	/*
+	 * As recommended by section 4.3.7 (Synchronization of register updates)
+	 * of ARM IHI 0064H.b, the self-hosted trace analyzer always executes an
+	 * ISB instruction after programming the trace unit registers.
+	 */
+	isb();
+
 	/* read the status of the single shot comparators */
 	for (i = 0; i < drvdata->nr_ss_cmp; i++) {
 		config->ss_status[i] =
@@ -1118,6 +1120,12 @@ static int etm4_resume_perf(struct coresight_device *csdev)
 
 	etm4_cs_unlock(drvdata, csa);
 	etm4_enable_trace_unit(drvdata);
+	/*
+	 * As recommended by section 4.3.7 (Synchronization of register updates)
+	 * of ARM IHI 0064H.b, the self-hosted trace analyzer always executes an
+	 * ISB instruction after programming the trace unit registers.
+	 */
+	isb();
 	etm4_cs_lock(drvdata, csa);
 
 	drvdata->paused = false;
@@ -1134,6 +1142,12 @@ static void etm4_pause_perf(struct coresight_device *csdev)
 
 	etm4_cs_unlock(drvdata, csa);
 	etm4_disable_trace_unit(drvdata);
+	/*
+	 * As recommended by section 4.3.7 (Synchronization of register updates)
+	 * of ARM IHI 0064H.b, the self-hosted trace analyzer always executes an
+	 * ISB instruction after programming the trace unit registers.
+	 */
+	isb();
 	etm4_cs_lock(drvdata, csa);
 
 	drvdata->paused = true;

-- 
2.34.1