ipa: libipa: Fix bug in ExposureModeHelper that leads to oscillations in AEGC

The ExposureModeHelper::splitExposures() runs through the configured
stages to find the best gain/exposure time pair. It first raises the
exposure time until it reaches the limit of the current stage. Then it
raises the gain until that also reaches the limit of the current stage.
After that it continues with the next stage until a match is found.

Due to a slight mistake in the initial code, the second step doesn't
work as expected because the exposure time gets divided by the gain of
the current stage, effectively leading to a jump of the gain value from
the maximum gain of the last stage to the maximum gain of the current
stage instead of gradually increasing the gain value.

Depending on the tuning file this leads to very visible oscillations and
jumps in the brightness.

Fix by clamping the exposure time in the second step to the maximum
exposure time of the current stage.

While at it, add two comments for easier understanding.

Fixes: 34c9ab62827b ("ipa: libipa: Add ExposureModeHelper")
Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/ipa/libipa/exposure_mode_helper.cpp b/src/ipa/libipa/exposure_mode_helper.cpp
index f235316d..0c1e99e3 100644
--- a/src/ipa/libipa/exposure_mode_helper.cpp
+++ b/src/ipa/libipa/exposure_mode_helper.cpp
@@ -182,6 +182,7 @@ ExposureModeHelper::splitExposure(utils::Duration exposure) const
 		 * the stage limits are initialised.
 		 */
 
+		/* Clamp the gain to lastStageGain and regulate exposureTime. */
 		if (stageExposureTime * lastStageGain >= exposure) {
 			exposureTime = clampExposureTime(exposure / clampGain(lastStageGain));
 			gain = clampGain(exposure / exposureTime);
@@ -189,8 +190,9 @@ ExposureModeHelper::splitExposure(utils::Duration exposure) const
 			return { exposureTime, gain, exposure / (exposureTime * gain) };
 		}
 
+		/* Clamp the exposureTime to stageExposureTime and regulate gain. */
 		if (stageExposureTime * stageGain >= exposure) {
-			exposureTime = clampExposureTime(exposure / clampGain(stageGain));
+			exposureTime = clampExposureTime(stageExposureTime);
 			gain = clampGain(exposure / exposureTime);
 
 			return { exposureTime, gain, exposure / (exposureTime * gain) };