Add explanations of the checks and note the way the constants were derived. Tweak the constants to reflect their purpose better. --- tests/kms_setmode.c | 36 +++++++++++++++++++++++++++++++++--- 1 file changed, 33 insertions(+), 3 deletions(-)

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diff --git a/tests/kms_setmode.c b/tests/kms_setmode.c index 4031d81..74eed88 100644 --- a/tests/kms_setmode.c +++ b/tests/kms_setmode.c @@ -474,18 +474,48 @@ static void check_timings(int crtc_idx, const drmModeModeInfo *kmode) mean = igt_stats_get_mean(&stats); stddev = igt_stats_get_std_deviation(&stats); - accuracy = 3.090 * stddev; /* 2-tailed 99% confidence */ + + /* 99.7% samples fall within `accuracy` on both sides of mean in normal + * distribution if `accuracy = 3 * sigma`. + * https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule + * + * The value of 99.7% was chosen to suit requirements of test cases + * which depend on timing, giving the lowest acceptable MTBF of 5.6s + * for 60Hz sampling rate. + */ + accuracy = 3. * stddev; igt_info("Expected frametime: %.0fus; measured %.1fus +- %.3fus accuracy %.2f%% [%.2f scanlines]\n", expected, mean, stddev, 100 * accuracy / mean, accuracy / line_time(kmode)); - /* 99% accuracy within one scanline */ + /* 99.7% samples within one scanline on each side of mean */ igt_assert_f(accuracy < line_time(kmode), "vblank accuracy (%.3fus, %.1f%%) worse than a scanline (%.3fus)\n", accuracy, 100 * accuracy / mean, line_time(kmode)); - igt_assert_f(fabs(mean - expected) < 2*stddev, + /* At least 90% of frame times fall within the one scanline on each + * side of expected mean. + * + * Expected scanline duration: + * (expected - accuracy, expected + accuracy). + * Assuming maximum difference allowed: + * expected = mean + n * sigma + * the scanline duration becomes: + * (mean - accuracy + n * sigma, mean + accuracy + n * sigma) + * The expected scanline captures the following number of samples + * from each side of expected: + * (erf(abs(-(accuracy/sigma) + n) / sqrt(2)) + * + erf((accuracy/sigma) + n) / sqrt(2))) / 2 + * = samples + * + * Solving for samples = 0.9: + * n = 1.718 + * + * See: + * https://en.wikipedia.org/wiki/Standard_deviation#Rules_for_normally_distributed_data + */ + igt_assert_f(fabs(mean - expected) < 1.718 * stddev, "vblank interval differs from modeline! expected %.1fus, measured %1.fus +- %.3fus, difference %.1fus (%.1f sigma)\n", expected, mean, stddev, fabs(mean - expected), fabs(mean - expected) / stddev); -- 2.7.4 _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx