The randomness tests in the NPCM7xx RNG test fail intermittently
but fairly frequently. On my machine running the test in a loop:
 while QTEST_QEMU_BINARY=./qemu-system-aarch64 ./tests/qtest/npcm7xx_rng-test; do true; done

will fail in less than a minute with an error like:
ERROR:../../tests/qtest/npcm7xx_rng-test.c:256:test_first_byte_runs:
assertion failed (calc_runs_p(buf.l, sizeof(buf) * BITS_PER_BYTE) > 0.01): (0.00286205989 > 0.01)

(Failures have been observed on all 4 of the randomness tests,
not just first_byte_runs.)

It's not clear why these tests are failing like this, but intermittent
failures make CI and merge testing awkward, so disable running them
unless a developer specifically sets QEMU_TEST_FLAKY_RNG_TESTS when
running the test suite, until we work out the cause.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201102152454.8287-1-peter.maydell@linaro.org
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
---
 tests/qtest/npcm7xx_rng-test.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tests/qtest/npcm7xx_rng-test.c b/tests/qtest/npcm7xx_rng-test.c
index da6e639bf6f..e7cde85fbbc 100644
--- a/tests/qtest/npcm7xx_rng-test.c
+++ b/tests/qtest/npcm7xx_rng-test.c
@@ -265,10 +265,16 @@ int main(int argc, char **argv)
 
     qtest_add_func("npcm7xx_rng/enable_disable", test_enable_disable);
     qtest_add_func("npcm7xx_rng/rosel", test_rosel);
-    qtest_add_func("npcm7xx_rng/continuous/monobit", test_continuous_monobit);
-    qtest_add_func("npcm7xx_rng/continuous/runs", test_continuous_runs);
-    qtest_add_func("npcm7xx_rng/first_byte/monobit", test_first_byte_monobit);
-    qtest_add_func("npcm7xx_rng/first_byte/runs", test_first_byte_runs);
+    /*
+     * These tests fail intermittently; only run them on explicit
+     * request until we figure out why.
+     */
+    if (getenv("QEMU_TEST_FLAKY_RNG_TESTS")) {
+        qtest_add_func("npcm7xx_rng/continuous/monobit", test_continuous_monobit);
+        qtest_add_func("npcm7xx_rng/continuous/runs", test_continuous_runs);
+        qtest_add_func("npcm7xx_rng/first_byte/monobit", test_first_byte_monobit);
+        qtest_add_func("npcm7xx_rng/first_byte/runs", test_first_byte_runs);
+    }
 
     qtest_start("-machine npcm750-evb");
     ret = g_test_run();
-- 
2.20.1