ipa: ipu3: Change limits and split loops in calculateBdsGrid()

The loops over the width and height of the image when calculating the
BDS grid parameters are nested, but they're actually independent. Split
them to reduce the complexity.

While at it, split out the constants to documented const expressions
for the grid sizes.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 46 insertions, 24 deletions

diff --git a/src/ipa/ipu3/ipu3.cpp b/src/ipa/ipu3/ipu3.cpp
index b3ac96ed..757a5d50 100644
--- a/src/ipa/ipu3/ipu3.cpp
+++ b/src/ipa/ipu3/ipu3.cpp
@@ -19,6 +19,7 @@
 #include <linux/v4l2-controls.h>
 
 #include <libcamera/base/log.h>
+#include <libcamera/base/utils.h>
 
 #include <libcamera/control_ids.h>
 #include <libcamera/framebuffer.h>
@@ -136,8 +137,18 @@
  * <linux/intel-ipu3.h> struct ipu3_uapi_gamma_corr_lut for further details.
  */
 
-static constexpr uint32_t kMaxCellWidthPerSet = 80;
-static constexpr uint32_t kMaxCellHeightPerSet = 60;
+/* Minimum grid width, expressed as a number of cells */
+static constexpr uint32_t kMinGridWidth = 16;
+/* Maximum grid width, expressed as a number of cells */
+static constexpr uint32_t kMaxGridWidth = 80;
+/* Minimum grid height, expressed as a number of cells */
+static constexpr uint32_t kMinGridHeight = 16;
+/* Maximum grid height, expressed as a number of cells */
+static constexpr uint32_t kMaxGridHeight = 60;
+/* log2 of the minimum grid cell width and height, in pixels */
+static constexpr uint32_t kMinCellSizeLog2 = 3;
+/* log2 of the maximum grid cell width and height, in pixels */
+static constexpr uint32_t kMaxCellSizeLog2 = 6;
 
 namespace libcamera {
 
@@ -281,45 +292,56 @@ int IPAIPU3::start()
 }
 
 /**
+ * \brief Calculate a grid for the AWB statistics
+ *
  * This function calculates a grid for the AWB algorithm in the IPU3 firmware.
  * Its input is the BDS output size calculated in the ImgU.
  * It is limited for now to the simplest method: find the lesser error
  * with the width/height and respective log2 width/height of the cells.
  *
- * \todo The frame is divided into cells which can be 8x8 => 128x128.
+ * \todo The frame is divided into cells which can be 8x8 => 64x64.
  * As a smaller cell improves the algorithm precision, adapting the
  * x_start and y_start parameters of the grid would provoke a loss of
  * some pixels but would also result in more accurate algorithms.
  */
 void IPAIPU3::calculateBdsGrid(const Size &bdsOutputSize)
 {
-	uint32_t minError = std::numeric_limits<uint32_t>::max();
 	Size best;
 	Size bestLog2;
 
 	/* Set the BDS output size in the IPAConfiguration structure */
 	context_.configuration.grid.bdsOutputSize = bdsOutputSize;
 
-	for (uint32_t widthShift = 3; widthShift <= 6; ++widthShift) {
-		uint32_t width = std::min(kMaxCellWidthPerSet,
-					  bdsOutputSize.width >> widthShift);
-		width = width << widthShift;
-		for (uint32_t heightShift = 3; heightShift <= 6; ++heightShift) {
-			int32_t height = std::min(kMaxCellHeightPerSet,
-						  bdsOutputSize.height >> heightShift);
-			height = height << heightShift;
-			uint32_t error  = std::abs(static_cast<int>(width - bdsOutputSize.width))
-							+ std::abs(static_cast<int>(height - bdsOutputSize.height));
-
-			if (error > minError)
-				continue;
-
-			minError = error;
-			best.width = width;
-			best.height = height;
-			bestLog2.width = widthShift;
-			bestLog2.height = heightShift;
-		}
+	uint32_t minError = std::numeric_limits<uint32_t>::max();
+	for (uint32_t shift = kMinCellSizeLog2; shift <= kMaxCellSizeLog2; ++shift) {
+		uint32_t width = std::clamp(bdsOutputSize.width >> shift,
+					    kMinGridWidth,
+					    kMaxGridWidth);
+
+		width = width << shift;
+		uint32_t error = std::abs(static_cast<int>(width - bdsOutputSize.width));
+		if (error >= minError)
+			continue;
+
+		minError = error;
+		best.width = width;
+		bestLog2.width = shift;
+	}
+
+	minError = std::numeric_limits<uint32_t>::max();
+	for (uint32_t shift = kMinCellSizeLog2; shift <= kMaxCellSizeLog2; ++shift) {
+		uint32_t height = std::clamp(bdsOutputSize.height >> shift,
+					     kMinGridHeight,
+					     kMaxGridHeight);
+
+		height = height << shift;
+		uint32_t error = std::abs(static_cast<int>(height - bdsOutputSize.height));
+		if (error >= minError)
+			continue;
+
+		minError = error;
+		best.height = height;
+		bestLog2.height = shift;
 	}
 
 	struct ipu3_uapi_grid_config &bdsGrid = context_.configuration.grid.bdsGrid;