The adaptive-rx and adaptive-tx checks use 'ethtool -c | grep -q' under
'set -o pipefail'. grep -q exits as soon as it finds a match, which can
happen before ethtool finishes writing its output. When that occurs,
ethtool receives SIGPIPE causing (uninformative):

 # selftests: drivers/net/netdevsim: ethtool-coalesce.sh
 # FAILED 1/22 checks
 not ok 1 selftests: drivers/net/netdevsim: ethtool-coalesce.sh # exit=1

This happens on debug kernels in NIPA, ~4% of the time.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: andrew@lunn.ch
CC: shuah@kernel.org
CC: linux-kselftest@vger.kernel.org
---
 .../drivers/net/netdevsim/ethtool-coalesce.sh          | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/drivers/net/netdevsim/ethtool-coalesce.sh b/tools/testing/selftests/drivers/net/netdevsim/ethtool-coalesce.sh
index 9adfba8f87e6..b9fcafad4258 100755
--- a/tools/testing/selftests/drivers/net/netdevsim/ethtool-coalesce.sh
+++ b/tools/testing/selftests/drivers/net/netdevsim/ethtool-coalesce.sh
@@ -116,12 +116,14 @@ done
 
 # bool settings which ethtool displays on the same line
 ethtool -C $NSIM_NETDEV adaptive-rx on
-s=$(ethtool -c $NSIM_NETDEV | grep -q "Adaptive RX: on  TX: off")
-check $? "$s" ""
+s=$(ethtool -c $NSIM_NETDEV)
+echo "$s" | grep -q "Adaptive RX: on  TX: off"
+check $? "" ""
 
 ethtool -C $NSIM_NETDEV adaptive-tx on
-s=$(ethtool -c $NSIM_NETDEV | grep -q "Adaptive RX: on  TX: on")
-check $? "$s" ""
+s=$(ethtool -c $NSIM_NETDEV)
+echo "$s" | grep -q "Adaptive RX: on  TX: on"
+check $? "" ""
 
 if [ $num_errors -eq 0 ]; then
     echo "PASSED all $((num_passes)) checks"
-- 
2.55.0