ipa: libipa: agc_mean_luminance: Error out when effectiveExposureValue is zero

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@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/ipa/libipa/agc_mean_luminance.cpp b/src/ipa/libipa/agc_mean_luminance.cpp
index 02555a44..f617fde8 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);