[PATCH v2 2/3] ipa: libipa: agc_mean_luminance: Error out when effectiveExposureValue is zero

[Stefan Klug]

In a proper system it never happens that the effectiveExposureValue
drops to zero. If that still happens due to a bug outside of
agc_mean_luminance, the calculated gain goes towards infinity but the
newExposureValue is still 0 because it is the result of multiplying the
effectiveExposureTime with the gain, leading to wild oscillations.

Catch that condition, print an error message and set the new effective
exposure value to an arbitrary 10ms.

Note that in any case the underlying problem must be fixed. The
important change is the added error message to be able to detect such a
situation.

Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally at ideasonboard.com>

---

Changes in v2:
- Collected tags
- Improved log message wording
- Added comment on the arbitrary exposure time
---
 src/ipa/libipa/agc_mean_luminance.cpp | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/ipa/libipa/agc_mean_luminance.cpp b/src/ipa/libipa/agc_mean_luminance.cpp
index 02555a44d271..f617fde81101 100644
--- a/src/ipa/libipa/agc_mean_luminance.cpp
+++ b/src/ipa/libipa/agc_mean_luminance.cpp
@@ -541,6 +541,18 @@ AgcMeanLuminance::calculateNewEv(uint32_t constraintModeIndex,
 	std::shared_ptr<ExposureModeHelper> exposureModeHelper =
 		exposureModeHelpers_.at(exposureModeIndex);
 
+	if (effectiveExposureValue == 0s) {
+		LOG(AgcMeanLuminance, Error)
+			<< "Effective exposure value is 0. This is a bug in AGC "
+			   "and must be fixed for proper operation.";
+		/*
+		 * Return an arbitrary exposure time > 0 to ensure regulation
+		 * doesn't get stuck with 0 in case the sensor driver allows a
+		 * min exposure of 0.
+		 */
+		return exposureModeHelper->splitExposure(10ms);
+	}
+
 	double gain = estimateInitialGain();
 	gain = constraintClampGain(constraintModeIndex, yHist, gain);
 
-- 
2.43.0