[PATCH i-g-t] tests/kms_setmode: Relax vblank timing tolerance from 1 scanline to 1%

[Jason-JH Lin <jason-jh.lin@mediatek.com>]

The original check_timings() validation required vblank intervals to be
within 1 scanline (~15us for 1080p60) which is overly strict and causes
false failures due to userspace measurement overhead.

Changes:
- Keep original strict checks (1 scanline, 1.718 sigma) as warnings only
- Add relaxed validation with 1% relative tolerance for pass/fail
- Use 3 sigma threshold (99.7% confidence) instead of 1.718 sigma (90%)

Rationale:
Hardware specs (VESA/HDMI/DP) allow ±0.5% pixel clock tolerance, which
translates to ±0.5% frame time due to inverse relationship. However,
userspace measurement via read() syscall adds ~0.3-0.5% overhead from:
- Interrupt latency and scheduling delays (5-50 us)
- System call overhead (1-5 us)
- Variable delays depending on system load

Total tolerance: ±0.5% (hardware) + ~0.5% (software) = ±1.0%

This makes the test ~11x more tolerant while still detecting real timing
issues and aligning with industry hardware specifications.

Signed-off-by: Jason-JH Lin <jason-jh.lin@mediatek.com>
---
 tests/kms_setmode.c | 48 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 5 deletions(-)

diff --git a/tests/kms_setmode.c b/tests/kms_setmode.c
index 1f2849bc26cf..30ab53fef47c 100644
--- a/tests/kms_setmode.c
+++ b/tests/kms_setmode.c
@@ -563,10 +563,9 @@ static bool check_timings(int crtc_idx, const drmModeModeInfo *kmode)
 
 	/* 99.7% samples within one scanline on each side of mean */
 	if (accuracy >= line_time(kmode)) {
-		igt_info("vblank accuracy (%.3fus, %.1f%%) worse than a scanline (%.3fus)\n",
+		igt_warn("vblank accuracy (%.3fus, %.1f%%) worse than a scanline (%.3fus) - "
+			 "strict check failed but continuing with relaxed validation\n",
 			  accuracy, 100 * accuracy / mean, line_time(kmode));
-
-		return false;
 	}
 
 	/* At least 90% of frame times fall within the one scanline on each
@@ -591,16 +590,55 @@ static bool check_timings(int crtc_idx, const drmModeModeInfo *kmode)
 	 * https://en.wikipedia.org/wiki/Standard_deviation#Rules_for_normally_distributed_data
 	 */
 	if (fabs(mean - expected) >= max(line_time(kmode), 1.718 * stddev)) {
-		igt_info("vblank interval differs from modeline! expected %.1fus, "
-			 "measured %1.fus +- %.3fus, difference %.1fus (%.1f sigma, %.1f scanlines)\n",
+		igt_warn("vblank interval differs from modeline! expected %.1fus, "
+			 "measured %1.fus +- %.3fus, difference %.1fus (%.1f sigma, %.1f scanlines) - "
+			 "strict check failed but continuing with relaxed validation\n",
 			  expected, mean, stddev,
 			  fabs(mean - expected),
 			  fabs(mean - expected) / stddev,
 			  fabs(mean - expected) / line_time(kmode));
+	}
+
+	/* Relaxed validation: 1% relative tolerance for accuracy and deviation
+	 *
+	 * Hardware specifications (VESA/HDMI/DP) allow ±0.5% pixel clock tolerance.
+	 * Since frame_time = (htotal × vtotal) / pixel_clock, the relationship is
+	 * inverse: ±0.5% pixel clock → ±0.5% frame time (in opposite direction).
+	 *
+	 * However, userspace measurement introduces additional overhead (~0.3-0.5%):
+	 *   - Interrupt latency and scheduling delays (5-50 μs)
+	 *   - System call overhead for read() (1-5 μs)
+	 *   - Variable delays depending on system load
+	 *
+	 * Total tolerance: ±0.5% (hardware) + ~0.5% (software) ≈ ±1.0%
+	 *
+	 * For stricter validation matching hardware spec only, use 0.005 (0.5%),
+	 * but expect potential false failures due to measurement noise.
+	 */
+	double tolerance_pct = 0.01;  /* 1% tolerance */
+	double tolerance_us = expected * tolerance_pct;
+	double relative_accuracy = accuracy / expected;
+
+	/* Check 1: 99.7% samples within 1% of the mean */
+	if (accuracy >= tolerance_us) {
+		igt_info("FAIL: vblank accuracy (%.3fus, %.2f%%) exceeds 1%% tolerance (%.3fus)\n",
+			 accuracy, 100 * relative_accuracy, tolerance_us);
+		return false;
+	}
 
+	/* Check 2: Mean within 1% of expected, or within 3 sigma (99.7% confidence) */
+	if (fabs(mean - expected) >= max(tolerance_us, 3.0 * stddev)) {
+		igt_info("FAIL: vblank interval differs from expected! expected %.1fus, "
+			 "measured %.1fus +- %.3fus, difference %.1fus (%.1f sigma, %.2f%%), "
+			 "exceeds 1%% tolerance or 3-sigma threshold\n",
+			 expected, mean, stddev,
+			 fabs(mean - expected),
+			 fabs(mean - expected) / stddev,
+			 100 * fabs(mean - expected) / expected);
 		return false;
 	}
 
+	igt_info("Timing validation passed (within 1%% tolerance)\n");
 	return true;
 }
 
-- 
2.43.0