coresight: tmc-etf: Properly set AUX buffer head in snapshot mode

Unify amongst sink drivers how the AUX ring buffer head is communicated
to user space.  That way the same algorithm in user space can be used to
determine where the latest data is and how much of it to access.
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-tmc-etf.c

@@ -542,11 +542,15 @@ static unsigned long tmc_update_etf_buffer(struct coresight_device *csdev,
 		}
 	}
 
-	/* In snapshot mode we have to update the head */
-	if (buf->snapshot) {
-		handle->head = (cur * PAGE_SIZE) + offset;
-		to_read = buf->nr_pages << PAGE_SHIFT;
-	}
+	/*
+	 * In snapshot mode we simply increment the head by the number of byte
+	 * that were written.  User space function  cs_etm_find_snapshot() will
+	 * figure out how many bytes to get from the AUX buffer based on the
+	 * position of the head.
+	 */
+	if (buf->snapshot)
+		handle->head += to_read;
+
 	CS_LOCK(drvdata->base);
 out:
 	spin_unlock_irqrestore(&drvdata->spinlock, flags);