ipa: workaround libcxx duration limitationjmondi/android/vndk

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, 5 insertions, 4 deletions

diff --git a/src/ipa/rpi/cam_helper/cam_helper.cpp b/src/ipa/rpi/cam_helper/cam_helper.cpp
index ddd5e9a4..7e29373b 100644
--- a/src/ipa/rpi/cam_helper/cam_helper.cpp
+++ b/src/ipa/rpi/cam_helper/cam_helper.cpp
@@ -71,7 +71,7 @@ void CamHelper::process([[maybe_unused]] StatisticsPtr &stats,
 
 uint32_t CamHelper::exposureLines(const Duration exposure, const Duration lineLength) const
 {
-	return exposure / lineLength;
+	return std::chrono::duration(exposure) / std::chrono::duration(lineLength);
 }
 
 Duration CamHelper::exposure(uint32_t exposureLines, const Duration lineLength) const
@@ -93,8 +93,8 @@ std::pair<uint32_t, uint32_t> CamHelper::getBlanking(Duration &exposure,
 	 * frameLengthMax gets calculated on the smallest line length as we do
 	 * not want to extend that unless absolutely necessary.
 	 */
-	frameLengthMin = minFrameDuration / mode_.minLineLength;
-	frameLengthMax = maxFrameDuration / mode_.minLineLength;
+	frameLengthMin = std::chrono::duration(minFrameDuration) / std::chrono::duration(mode_.minLineLength);
+	frameLengthMax = std::chrono::duration(maxFrameDuration) / std::chrono::duration(mode_.minLineLength);
 
 	/*
 	 * Watch out for (exposureLines + frameIntegrationDiff_) overflowing a
diff --git a/src/ipa/rpi/cam_helper/cam_helper_imx296.cpp b/src/ipa/rpi/cam_helper/cam_helper_imx296.cpp
index ecb845e7..c5180de5 100644
--- a/src/ipa/rpi/cam_helper/cam_helper_imx296.cpp
+++ b/src/ipa/rpi/cam_helper/cam_helper_imx296.cpp
@@ -57,7 +57,8 @@ double CamHelperImx296::gain(uint32_t gainCode) const
 uint32_t CamHelperImx296::exposureLines(const Duration exposure,
 					[[maybe_unused]] const Duration lineLength) const
 {
-	return std::max<uint32_t>(minExposureLines, (exposure - 14.26us) / timePerLine);
+	return std::max<uint32_t>(minExposureLines, std::chrono::duration(exposure - 14.26us) /
+							std::chrono::duration(timePerLine));
 }
 
 Duration CamHelperImx296::exposure(uint32_t exposureLines,