ipa: workaround libcxx duration limitation

A bug in libcxx [0] used by clang version 11.0.2 is prevalent when
building libcamera for Android SDK30. This has been fixed and
integrated upstream with [1].

As a workaround, directly cast libcamera::utils::Duration objects to
std::chrono::duration when dividing.

Alternatives evaluated:
Considered: Enable public inheritance of std::chrono::duration and
override operator/ in the class.
Outcome: Does not fix the original compiler error.

Considered: Enable public inheritance of std::chrono::duration and
override operator/ in the libcamera namespace.
Outcome: new compiler error:
ld.lld: error: duplicate symbol: libcamera::operator/
(libcamera::utils::Duration const&, libcamera::utils::Duration const&)

Considered: Use private inheritance of std::chrono::duration and
re-implement a pass-through version of each std::chrono::duration
operator within libcamera::utils::Duration and use template
metaprogramming to fix the division operator.
Outcome: Testing shows that this would introduce substantial
limitations, i.e. requring the Duration object to be on the LHS of any
arithmetic operation with other numeric types. This also substantially
increases implementation complexity.

Considered: Extract double values from libcamera::utils::Duration
objects and use those to divide.
Outcome: This creates substantial readability and unit-safety issues.

[0] https://github.com/llvm/llvm-project/issues/40475
[1] https://github.com/llvm/llvm-project/commit/efa6d803c624f9251d0ab7881122501bb9d27368
Bug: https://bugs.libcamera.org/show_bug.cgi?id=156

Signed-off-by: Nicholas Roth <nicholas@rothemail.net>

2 files changed, 10 insertions, 5 deletions

diff --git a/src/ipa/raspberrypi/controller/rpi/agc.cpp b/src/ipa/raspberrypi/controller/rpi/agc.cpp
index a30e50c1..4feb8dd8 100644
--- a/src/ipa/raspberrypi/controller/rpi/agc.cpp
+++ b/src/ipa/raspberrypi/controller/rpi/agc.cpp
@@ -413,7 +413,8 @@ void Agc::prepare(Metadata *imageMetadata)
 			Duration actualExposure = deviceStatus.shutterSpeed *
 						  deviceStatus.analogueGain;
 			if (actualExposure) {
-				status_.digitalGain = status_.totalExposureValue / actualExposure;
+				status_.digitalGain = std::chrono::duration(status_.totalExposureValue) /
+							std::chrono::duration(actualExposure);
 				LOG(RPiAgc, Debug) << "Want total exposure " << status_.totalExposureValue;
 				/*
 				 * Never ask for a gain < 1.0, and also impose
@@ -818,7 +819,8 @@ void Agc::divideUpExposure()
 			}
 			if (status_.fixedAnalogueGain == 0.0) {
 				if (exposureMode_->gain[stage] * shutterTime >= exposureValue) {
-					analogueGain = exposureValue / shutterTime;
+					analogueGain = std::chrono::duration(exposureValue) /
+							std::chrono::duration(shutterTime);
 					break;
 				}
 				analogueGain = exposureMode_->gain[stage];
@@ -833,10 +835,12 @@ void Agc::divideUpExposure()
 	 */
 	if (!status_.fixedShutter && !status_.fixedAnalogueGain &&
 	    status_.flickerPeriod) {
-		int flickerPeriods = shutterTime / status_.flickerPeriod;
+		int flickerPeriods = std::chrono::duration(shutterTime) /
+				std::chrono::duration(status_.flickerPeriod);
 		if (flickerPeriods) {
 			Duration newShutterTime = flickerPeriods * status_.flickerPeriod;
-			analogueGain *= shutterTime / newShutterTime;
+			analogueGain *= std::chrono::duration(shutterTime) /
+					std::chrono::duration(newShutterTime);
 			/*
 			 * We should still not allow the ag to go over the
 			 * largest value in the exposure mode. Note that this
diff --git a/src/ipa/raspberrypi/controller/rpi/lux.cpp b/src/ipa/raspberrypi/controller/rpi/lux.cpp
index 9759186a..410f6f44 100644
--- a/src/ipa/raspberrypi/controller/rpi/lux.cpp
+++ b/src/ipa/raspberrypi/controller/rpi/lux.cpp
@@ -94,7 +94,8 @@ void Lux::process(StatisticsPtr &stats, Metadata *imageMetadata)
 		double currentY = sum / (double)num + .5;
 		double gainRatio = referenceGain_ / currentGain;
 		double shutterSpeedRatio =
-			referenceShutterSpeed_ / deviceStatus.shutterSpeed;
+			std::chrono::duration(referenceShutterSpeed_) /
+			std::chrono::duration(deviceStatus.shutterSpeed);
 		double apertureRatio = referenceAperture_ / currentAperture;
 		double yRatio = currentY * (65536 / numBins) / referenceY_;
 		double estimatedLux = shutterSpeedRatio * gainRatio *