rtla/timerlat: Add --dma-latency option

Add the --dma-latency to set /dev/cpu_dma_latency to the
specified value, this aims to avoid having exit from idle
states latencies that could be influencing the analysis.

Link: https://lkml.kernel.org/r/72ddb0d913459f13217086dadafad88a7c46dd28.1646247211.git.bristot@kernel.org

Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Clark Williams <williams@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/tools/tracing/rtla/src/utils.c b/tools/tracing/rtla/src/utils.c
index ffaf8ec84001..da2b590edaed 100644
--- a/tools/tracing/rtla/src/utils.c
+++ b/tools/tracing/rtla/src/utils.c
@@ -10,6 +10,7 @@
 #include <unistd.h>
 #include <ctype.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <sched.h>
 #include <stdio.h>
 
@@ -431,3 +432,35 @@ int parse_prio(char *arg, struct sched_attr *sched_param)
 	}
 	return 0;
 }
+
+/*
+ * set_cpu_dma_latency - set the /dev/cpu_dma_latecy
+ *
+ * This is used to reduce the exit from idle latency. The value
+ * will be reset once the file descriptor of /dev/cpu_dma_latecy
+ * is closed.
+ *
+ * Return: the /dev/cpu_dma_latecy file descriptor
+ */
+int set_cpu_dma_latency(int32_t latency)
+{
+	int retval;
+	int fd;
+
+	fd = open("/dev/cpu_dma_latency", O_RDWR);
+	if (fd < 0) {
+		err_msg("Error opening /dev/cpu_dma_latency\n");
+		return -1;
+	}
+
+	retval = write(fd, &latency, 4);
+	if (retval < 1) {
+		err_msg("Error setting /dev/cpu_dma_latency\n");
+		close(fd);
+		return -1;
+	}
+
+	debug_msg("Set /dev/cpu_dma_latency to %d\n", latency);
+
+	return fd;
+}