libipa: camera_sensor_helper: Implement exponential gain model

The CameraSensorHelper specifies two gain models, linear and
exponential. They are modelled after the MIPI CCS specification. Only
the linear model has been implemented, the exponential model was left
for later.

We now need to support sensors that configure their gain in a hardware
register with a value expressed in dB. This has similarities with the
MIPI CCS exponential gain model, but is only has an exponential factor,
while CCS also allows sensors to support a configurable linear factor.

The full CCS exponential model needs two values (for the linear and
exponential factors) to express a gain, while IPAs use a single linear
gain value internally. However, the exponential gain model example in
the CCS specification has a fixed linear factor, which may indicate that
it could be common for sensors that implement the exponential gain model
to only use the exponential factor. For this reason, implement the
exponential gain model with a fixed linear factor, but with a
sensor-specific coefficient for the exponential factor that allows
expressing the gain in dB (or other logarithmical units) instead of
limiting it to powers of 2 as in the MIPI CCS specification.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/ipa/libipa/camera_sensor_helper.h b/src/ipa/libipa/camera_sensor_helper.h
index 6b96520b..7351fc7c 100644
--- a/src/ipa/libipa/camera_sensor_helper.h
+++ b/src/ipa/libipa/camera_sensor_helper.h
@@ -41,8 +41,14 @@ protected:
 		int16_t c1;
 	};
 
+	struct AnalogueGainExpConstants {
+		double a;
+		double m;
+	};
+
 	union AnalogueGainConstants {
 		AnalogueGainLinearConstants linear;
+		AnalogueGainExpConstants exp;
 	};
 
 	AnalogueGainType gainType_;