[PATCH v2] rt-tests: hwlatdetect: Gracefully handle lack of /dev/cpu_dma_latency

[Punit Agrawal <punitagrawal@gmail.com>]

From: Punit Agrawal <punit1.agrawal@toshiba.co.jp>

On systems where cpu idle is disabled to reduce latencies,
"/dev/cpu_dma_latency" does not exist and leads to the following
exception report from python when using hwlatdetect.py -

    FileNotFoundError: [Errno 2] No such file or directory: '/dev/cpu_dma_latency

Update hwlatdetect to check whether the file exists before attemping
to write values to it.

Reported-by: Suresh Hegde <suresh.c11@toshiba-tsip.com>
Signed-off-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
---
v1 -> v2:
* Correct Suresh's email

 src/hwlatdetect/hwlatdetect.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/hwlatdetect/hwlatdetect.py b/src/hwlatdetect/hwlatdetect.py
index 12228f45f852..89537a6e371f 100755
--- a/src/hwlatdetect/hwlatdetect.py
+++ b/src/hwlatdetect/hwlatdetect.py
@@ -216,8 +216,11 @@ class Detector(object):
     # use c_states_on() to close the file descriptor and re-enable c-states
     #
     def c_states_off(self):
-        self.dma_latency_handle = os.open("/dev/cpu_dma_latency", os.O_WRONLY)
-        os.write(self.dma_latency_handle, b'\x00\x00\x00\x00')
+        if os.path.exists("/dev/cpu_dma_latency"):
+            self.dma_latency_handle = os.open("/dev/cpu_dma_latency", os.O_WRONLY)
+            os.write(self.dma_latency_handle, b'\x00\x00\x00\x00')
+        else:
+            self.dma_latency_handle = None
         debug("c-states disabled")
 
     def c_states_on(self):
-- 
2.32.0