libcamera: raspberrypi: Add control of sensor vblanking

Add support for setting V4L2_CID_VBLANK appropriately when setting
V4L2_CID_EXPOSURE. This will allow adaptive framerates during
viewfinder use cases (e.g. when the exposure time goes above 33ms, we
can reduce the framerate to lower than 30fps).

The minimum and maximum frame durations are provided via libcamera
controls, and will prioritise exposure time limits over any AGC request.

V4L2_CID_VBLANK is controlled through the staggered writer, just like
the exposure and gain controls.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Tested-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 33 insertions, 2 deletions

diff --git a/src/ipa/raspberrypi/cam_helper.cpp b/src/ipa/raspberrypi/cam_helper.cpp
index 6efa0d7f..b7b8faf0 100644
--- a/src/ipa/raspberrypi/cam_helper.cpp
+++ b/src/ipa/raspberrypi/cam_helper.cpp
@@ -34,8 +34,10 @@ CamHelper *CamHelper::Create(std::string const &cam_name)
 	return nullptr;
 }
 
-CamHelper::CamHelper(MdParser *parser)
-	: parser_(parser), initialized_(false)
+CamHelper::CamHelper(MdParser *parser, unsigned int maxFrameLength,
+		     unsigned int frameIntegrationDiff)
+	: parser_(parser), initialized_(false), maxFrameLength_(maxFrameLength),
+	  frameIntegrationDiff_(frameIntegrationDiff)
 {
 }
 
@@ -56,6 +58,35 @@ double CamHelper::Exposure(uint32_t exposure_lines) const
 	return exposure_lines * mode_.line_length / 1000.0;
 }
 
+uint32_t CamHelper::GetVBlanking(double &exposure, double minFrameDuration,
+				 double maxFrameDuration) const
+{
+	uint32_t frameLengthMin, frameLengthMax, vblank;
+	uint32_t exposureLines = ExposureLines(exposure);
+
+	assert(initialized_);
+
+	/*
+	 * Clamp frame length by the frame duration range and the maximum allowable
+	 * value in the sensor, given by maxFrameLength_.
+	 */
+	frameLengthMin = std::clamp<uint32_t>(1e3 * minFrameDuration / mode_.line_length,
+					      mode_.height, maxFrameLength_);
+	frameLengthMax = std::clamp<uint32_t>(1e3 * maxFrameDuration / mode_.line_length,
+					      mode_.height, maxFrameLength_);
+	/*
+	 * Limit the exposure to the maximum frame duration requested, and
+	 * re-calculate if it has been clipped.
+	 */
+	exposureLines = std::min(frameLengthMax - frameIntegrationDiff_, exposureLines);
+	exposure = Exposure(exposureLines);
+
+	/* Limit the vblank to the range allowed by the frame length limits. */
+	vblank = std::clamp(exposureLines + frameIntegrationDiff_,
+			    frameLengthMin, frameLengthMax) - mode_.height;
+	return vblank;
+}
+
 void CamHelper::SetCameraMode(const CameraMode &mode)
 {
 	mode_ = mode;