coresight: Move reference counting inside sink drivers

When operating in CPU-wide mode with an N:1 source/sink HW topology,
multiple CPUs can access a sink concurrently.  As such reference counting
needs to happen when the device's spinlock is held to avoid racing with
other operations (start(), update(), stop()), such as:

session A				Session B
-----					-------

enable_sink
atomic_inc(refcount)  = 1

...

atomic_dec(refcount) = 0		enable_sink
if (refcount == 0) disable_sink
					atomic_inc()

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Robert Walker <robert.walker@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 15 insertions, 6 deletions

diff --git a/drivers/hwtracing/coresight/coresight-etb10.c b/drivers/hwtracing/coresight/coresight-etb10.c
index 23b049cef19a..8e63863cf950 100644
--- a/drivers/hwtracing/coresight/coresight-etb10.c
+++ b/drivers/hwtracing/coresight/coresight-etb10.c
@@ -5,6 +5,7 @@
  * Description: CoreSight Embedded Trace Buffer driver
  */
 
+#include <linux/atomic.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/types.h>
@@ -151,14 +152,15 @@ static int etb_enable_sysfs(struct coresight_device *csdev)
 		goto out;
 	}
 
-	/* Nothing to do, the tracer is already enabled. */
-	if (drvdata->mode == CS_MODE_SYSFS)
-		goto out;
+	if (drvdata->mode == CS_MODE_DISABLED) {
+		ret = etb_enable_hw(drvdata);
+		if (ret)
+			goto out;
 
-	ret = etb_enable_hw(drvdata);
-	if (!ret)
 		drvdata->mode = CS_MODE_SYSFS;
+	}
 
+	atomic_inc(csdev->refcnt);
 out:
 	spin_unlock_irqrestore(&drvdata->spinlock, flags);
 	return ret;
@@ -188,8 +190,10 @@ static int etb_enable_perf(struct coresight_device *csdev, void *data)
 		goto out;
 
 	ret = etb_enable_hw(drvdata);
-	if (!ret)
+	if (!ret) {
 		drvdata->mode = CS_MODE_PERF;
+		atomic_inc(csdev->refcnt);
+	}
 
 out:
 	spin_unlock_irqrestore(&drvdata->spinlock, flags);
@@ -324,6 +328,11 @@ static int etb_disable(struct coresight_device *csdev)
 
 	spin_lock_irqsave(&drvdata->spinlock, flags);
 
+	if (atomic_dec_return(csdev->refcnt)) {
+		spin_unlock_irqrestore(&drvdata->spinlock, flags);
+		return -EBUSY;
+	}
+
 	/* Disable the ETB only if it needs to */
 	if (drvdata->mode != CS_MODE_DISABLED) {
 		etb_disable_hw(drvdata);