[PATCH net] selftests: drv-net: devmem: Skip when hugetlbfs isn't supported

[Maxime Chevallier]

The devmem selftest requires accessing /proc/sys/vm/nr_hugepages,
which is only available when hugetlbfs is supported. This isn't the
case on all CPU architectures, for example it isn't supported on ARM
32-bits on cores that don't support LPAE.

On these devices, the devmem kselftest doesn't skip gracefully, it hits
an exception while trying to open the nr_hugepages file :

  # Exception|     _reserve_hugepages()
  # Exception|     ~~~~~~~~~~~~~~~~~~^^
  # Exception|   File "/usr/lib/kselftests/drivers/net/hw/devmem_lib.py", line 36, in _reserve_hugepages
  # Exception|     with open(NR_HUGEPAGES_FILE, 'r+', encoding='utf-8') as f:
  # Exception|          ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  # Exception| FileNotFoundError: [Errno 2] No such file or directory: '/proc/sys/vm/nr_hugepages'

Let's check if the file exists before trying to open it, and return
immediately if we can't.

The _reserve_hugepages() runs as part of the "require_devmem()", that
skips if devmem itself isn't supported. Before the blamed commit, this
check was done prior to accessing nr_hugepages, so the problem wasn't
showing.

This has been found by trying to run the selftest on stmmac, on an
Altera CycloneV SoC (Cortex A9, doesn't support hugepages).

Fixes: 8ac4255c1e0c ("selftests/net: devmem.py: add check_rx_large_niov")
Signed-off-by: Maxime Chevallier <maxime.chevallier@xxxxxxxxxxx>
---
 tools/testing/selftests/drivers/net/hw/devmem_lib.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/testing/selftests/drivers/net/hw/devmem_lib.py b/tools/testing/selftests/drivers/net/hw/devmem_lib.py
index 3554954a6691..24637fb180ca 100644
--- a/tools/testing/selftests/drivers/net/hw/devmem_lib.py
+++ b/tools/testing/selftests/drivers/net/hw/devmem_lib.py
@@ -33,6 +33,8 @@ def _restore_nr_hugepages(nr_hugepages):
 
 def _reserve_hugepages(want=64):
     """Raise nr_hugepages to @want and arrange for it to be restored."""
+    if not os.path.exists(NR_HUGEPAGES_FILE):
+        return
     with open(NR_HUGEPAGES_FILE, 'r+', encoding='utf-8') as f:
         nr_hugepages = int(f.read().strip())
         if nr_hugepages >= want:
-- 
2.56.0