[PATCH v2 14/15] media: microchip-isc: smooth AWB gains with EMA filter

[Balakrishnan Sambath <balakrishnan.s@microchip.com>]

Apply exponential moving average (alpha=0.25) to reduce per-frame
flicker from sensor noise.

Signed-off-by: Balakrishnan Sambath <balakrishnan.s@microchip.com>
---
 .../platform/microchip/microchip-isc-base.c   | 19 ++++++++++++++++---
 .../media/platform/microchip/microchip-isc.h  |  1 +
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/media/platform/microchip/microchip-isc-base.c b/drivers/media/platform/microchip/microchip-isc-base.c
index 8fcf64708dc8..072386aa2d73 100644
--- a/drivers/media/platform/microchip/microchip-isc-base.c
+++ b/drivers/media/platform/microchip/microchip-isc-base.c
@@ -134,6 +134,7 @@ static inline void isc_reset_awb_ctrls(struct isc_device *isc)
 	for (c = ISC_HIS_CFG_MODE_GR; c <= ISC_HIS_CFG_MODE_B; c++) {
 		/* gains have a fixed point at 9 decimals */
 		isc->ctrls.gain[c] = 1 << 9;
+		isc->ctrls.gain_smooth[c] = 1 << 9;
 		/* offsets are in 2's complements */
 		isc->ctrls.offset[c] = 0;
 	}
@@ -1590,11 +1591,23 @@ static void isc_wb_update(struct isc_ctrls *ctrls)
 		/* Combine stretch and grey-world gains; result stays in Q9. */
 		gain = (s_gain * gw_gain) >> 9;
 
-		ctrls->gain[c] = clamp_val(gain, 0, GENMASK(12, 0));
+		/*
+		 * Smooth gain updates with an exponential weighted average
+		 * to suppress per-frame flicker:
+		 *   smooth[n] = (3 * smooth[n-1] + gain) / 4
+		 * Clamp to the hardware register width to prevent unbounded
+		 * accumulation under degenerate (near-empty histogram) inputs.
+		 */
+		ctrls->gain_smooth[c] = (3 * ctrls->gain_smooth[c] + gain) / 4;
+		ctrls->gain_smooth[c] = min_t(u32, ctrls->gain_smooth[c],
+					      GENMASK(12, 0));
+
+		ctrls->gain[c] = ctrls->gain_smooth[c];
 
 		dev_dbg(isc->dev,
-			"isc wb: c=%u black=%u avg=%u s_gain=%u gw_gain=%u gain=%u",
-			c, hist_min, channel_avg, s_gain, gw_gain, gain);
+			"isc wb: c=%u black=%u avg=%u s_gain=%u gw_gain=%u gain=%u smooth=%u\n",
+			c, hist_min, channel_avg, s_gain, gw_gain, gain,
+			ctrls->gain_smooth[c]);
 	}
 }
 
diff --git a/drivers/media/platform/microchip/microchip-isc.h b/drivers/media/platform/microchip/microchip-isc.h
index 44d54404250d..e558f1a65b33 100644
--- a/drivers/media/platform/microchip/microchip-isc.h
+++ b/drivers/media/platform/microchip/microchip-isc.h
@@ -155,6 +155,7 @@ struct isc_ctrls {
 
 	/* one for each component : GR, R, GB, B */
 	u32 gain[HIST_BAYER];
+	u32 gain_smooth[HIST_BAYER];
 	s32 offset[HIST_BAYER];
 
 	u32 hist_entry[HIST_ENTRIES];
-- 
2.34.1