[PATCH v2 13/15] media: microchip-isc: use weighted averages for Grey World AWB

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

Replace pixel counts with intensity-weighted averages. Add 2% outlier
rejection at histogram tails.

Signed-off-by: Balakrishnan Sambath <balakrishnan.s@microchip.com>
---
 .../platform/microchip/microchip-isc-base.c   | 167 +++++++++++++-----
 .../media/platform/microchip/microchip-isc.h  |   2 +
 2 files changed, 125 insertions(+), 44 deletions(-)

diff --git a/drivers/media/platform/microchip/microchip-isc-base.c b/drivers/media/platform/microchip/microchip-isc-base.c
index e6386f8852e5..8fcf64708dc8 100644
--- a/drivers/media/platform/microchip/microchip-isc-base.c
+++ b/drivers/media/platform/microchip/microchip-isc-base.c
@@ -40,6 +40,12 @@
 	(((mbus_code) == MEDIA_BUS_FMT_Y10_1X10) | \
 	(((mbus_code) == MEDIA_BUS_FMT_Y8_1X8)))
 
+/* 4.0 in Q9 fixed-point: cap grey-world correction at 4x. */
+#define ISC_AWB_GW_GAIN_MAX	(4u << 9)
+
+/* Outlier rejection: skip darkest/brightest 2% of histogram. */
+#define ISC_AWB_OUTLIER_DIV	50
+
 static inline void isc_update_v4l2_ctrls(struct isc_device *isc)
 {
 	struct isc_ctrls *ctrls = &isc->ctrls;
@@ -1404,6 +1410,11 @@ static void isc_hist_count(struct isc_device *isc, u32 *min, u32 *max)
 	u32 *hist_count = &ctrls->hist_count[ctrls->hist_id];
 	u32 *hist_entry = &ctrls->hist_entry[0];
 	u32 i;
+	u32 total_pixels;
+	u32 dark_threshold, bright_threshold;
+	u32 cumulative;
+	u64 weighted_sum;
+	u32 pixel_count;
 
 	*min = 0;
 	*max = HIST_ENTRIES;
@@ -1411,44 +1422,103 @@ static void isc_hist_count(struct isc_device *isc, u32 *min, u32 *max)
 	regmap_bulk_read(regmap, ISC_HIS_ENTRY + isc->offsets.his_entry,
 			 hist_entry, HIST_ENTRIES);
 
-	*hist_count = 0;
-	/*
-	 * we deliberately ignore the end of the histogram,
-	 * the most white pixels
-	 */
+	/* Calculate total pixels */
+	total_pixels = 0;
+	for (i = 0; i < HIST_ENTRIES; i++)
+		total_pixels += hist_entry[i];
+
+	/* Handle empty histogram case */
+	if (total_pixels == 0) {
+		*hist_count = 0;
+		ctrls->channel_avg[ctrls->hist_id] = 256; /* Default middle value */
+		ctrls->total_pixels[ctrls->hist_id] = 0;
+		*min = 1;
+		*max = HIST_ENTRIES - 1;
+		dev_dbg(isc->dev,
+			"isc wb: no pixels in histogram for channel %u\n",
+			ctrls->hist_id);
+		return;
+	}
+
+	/* Outlier rejection: skip darkest/brightest 2% of histogram */
+	dark_threshold = total_pixels / ISC_AWB_OUTLIER_DIV;
+	bright_threshold = total_pixels / ISC_AWB_OUTLIER_DIV;
+	cumulative = 0;
+
+	/* Find effective minimum (skip dark noise) */
+	*min = 1;
 	for (i = 1; i < HIST_ENTRIES; i++) {
-		if (*hist_entry && !*min)
+		cumulative += hist_entry[i];
+		if (cumulative > dark_threshold) {
 			*min = i;
-		if (*hist_entry)
+			break;
+		}
+	}
+
+	/* Find effective maximum (skip bright saturation) */
+	cumulative = 0;
+	*max = HIST_ENTRIES - 1;
+	for (i = HIST_ENTRIES - 1; i > *min; i--) {
+		cumulative += hist_entry[i];
+		if (cumulative > bright_threshold) {
 			*max = i;
-		*hist_count += i * (*hist_entry++);
+			break;
+		}
 	}
 
+	/* Ensure reasonable range */
+	if (*max <= *min) {
+		*min = HIST_ENTRIES / 4;
+		*max = (HIST_ENTRIES * 3) / 4;
+	}
+
+	/* Calculate both pixel count and weighted average for useful range */
+	*hist_count = 0;
+	weighted_sum = 0;
+
+	for (i = *min; i <= *max; i++) {
+		pixel_count = hist_entry[i];
+		*hist_count += pixel_count;
+		weighted_sum += (u64)i * pixel_count;
+	}
+
+	/* Store total useful pixels for this channel */
+	ctrls->total_pixels[ctrls->hist_id] = *hist_count;
+
+	/* Calculate channel average */
+	if (*hist_count > 0)
+		ctrls->channel_avg[ctrls->hist_id] =
+			div64_u64(weighted_sum, *hist_count);
+	else
+		/* Default middle value */
+		ctrls->channel_avg[ctrls->hist_id] = 256;
+
 	if (!*min)
 		*min = 1;
 
-	dev_dbg(isc->dev, "isc wb: hist_id %u, hist_count %u",
-		ctrls->hist_id, *hist_count);
+	dev_dbg(isc->dev,
+		"isc wb: hist_id %u, avg %u, count %u, range [%u,%u], total %u\n",
+		ctrls->hist_id, ctrls->channel_avg[ctrls->hist_id],
+		*hist_count, *min, *max, total_pixels);
 }
 
 static void isc_wb_update(struct isc_ctrls *ctrls)
 {
 	struct isc_device *isc = container_of(ctrls, struct isc_device, ctrls);
-	u32 *hist_count = &ctrls->hist_count[0];
 	u32 c, offset[4];
 	u64 avg = 0;
-	/* We compute two gains, stretch gain and grey world gain */
-	u32 s_gain[4], gw_gain[4];
+	u32 gain, gw_gain, s_gain;
+	u32 min_pixels;
+	u32 frame_pixels;
 
 	/*
 	 * According to Grey World, we need to set gains for R/B to normalize
 	 * them towards the green channel.
-	 * Thus we want to keep Green as fixed and adjust only Red/Blue
-	 * Compute the average of the both green channels first
+	 * Thus we want to keep Green as fixed and adjust only Red/Blue.
+	 * Compute the average of the both green channels first.
 	 */
-	avg = (u64)hist_count[ISC_HIS_CFG_MODE_GR] +
-		(u64)hist_count[ISC_HIS_CFG_MODE_GB];
-	avg >>= 1;
+	avg = (ctrls->channel_avg[ISC_HIS_CFG_MODE_GR] +
+		ctrls->channel_avg[ISC_HIS_CFG_MODE_GB]) >> 1;
 
 	dev_dbg(isc->dev, "isc wb: green components average %llu\n", avg);
 
@@ -1456,7 +1526,23 @@ static void isc_wb_update(struct isc_ctrls *ctrls)
 	if (!avg)
 		return;
 
+	/*
+	 * Require a minimum pixel count for both black-level offset and
+	 * grey-world gain: 1/64 of the frame area, which equals ~6.25% of
+	 * one Bayer channel's expected pixel count.  This scales with sensor
+	 * resolution and prevents noise-dominated histograms (from very small
+	 * crops or a nearly-empty frame) from producing wild corrections.
+	 * A floor of 64 ensures the guard is non-zero for tiny crops.
+	 */
+	frame_pixels = isc->fmt.fmt.pix.width * isc->fmt.fmt.pix.height;
+	min_pixels = frame_pixels ? max(frame_pixels >> 6, 64u) : 64u;
+
 	for (c = ISC_HIS_CFG_MODE_GR; c <= ISC_HIS_CFG_MODE_B; c++) {
+		u32 hist_min = ctrls->hist_minmax[c][HIST_MIN_INDEX];
+		u32 hist_max = ctrls->hist_minmax[c][HIST_MAX_INDEX];
+		u32 channel_avg = ctrls->channel_avg[c];
+		u32 total_pixels = ctrls->total_pixels[c];
+
 		/*
 		 * the color offset is the minimum value of the histogram.
 		 * we stretch this color to the full range by substracting
@@ -1482,40 +1568,33 @@ static void isc_wb_update(struct isc_ctrls *ctrls)
 		ctrls->offset[c] = -ctrls->offset[c];
 
 		/*
-		 * the stretch gain is the total number of histogram bins
-		 * divided by the actual range of color component (Max - Min)
-		 * If we compute gain like this, the actual color component
-		 * will be stretched to the full histogram.
-		 * We need to shift 9 bits for precision, we have 9 bits for
-		 * decimals
+		 * Stretch gain: scale the histogram range [hist_min, hist_max]
+		 * to the full 512-bin span.  Result is in Q9 fixed-point
+		 * (1.0 = 512).
 		 */
-		s_gain[c] = (HIST_ENTRIES << 9) /
-			(ctrls->hist_minmax[c][HIST_MAX_INDEX] -
-			ctrls->hist_minmax[c][HIST_MIN_INDEX] + 1);
+		s_gain = (HIST_ENTRIES << 9) / (hist_max - hist_min + 1);
 
 		/*
-		 * Now we have to compute the gain w.r.t. the average.
-		 * Add/lose gain to the component towards the average.
-		 * If it happens that the component is zero, use the
-		 * fixed point value : 1.0 gain.
+		 * Grey-world gain: scale each channel towards the green
+		 * average.  Require a minimum pixel count so noise-dominated
+		 * channels do not produce wild corrections.
 		 */
-		if (hist_count[c])
-			gw_gain[c] = div_u64(avg << 9, hist_count[c]);
+		if (channel_avg > 0 && total_pixels >= min_pixels)
+			gw_gain = div64_u64((avg << 9), channel_avg);
 		else
-			gw_gain[c] = 1 << 9;
+			gw_gain = 1 << 9;
 
-		dev_dbg(isc->dev,
-			"isc wb: component %d, s_gain %u, gw_gain %u\n",
-			c, s_gain[c], gw_gain[c]);
-		/* multiply both gains and adjust for decimals */
-		ctrls->gain[c] = s_gain[c] * gw_gain[c];
-		ctrls->gain[c] >>= 9;
+		/* Cap grey-world correction at 4x to avoid over-amplification. */
+		gw_gain = min_t(u32, gw_gain, ISC_AWB_GW_GAIN_MAX);
 
-		/* make sure we are not out of range */
-		ctrls->gain[c] = clamp_val(ctrls->gain[c], 0, GENMASK(12, 0));
+		/* Combine stretch and grey-world gains; result stays in Q9. */
+		gain = (s_gain * gw_gain) >> 9;
 
-		dev_dbg(isc->dev, "isc wb: component %d, final gain %u\n",
-			c, ctrls->gain[c]);
+		ctrls->gain[c] = clamp_val(gain, 0, GENMASK(12, 0));
+
+		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);
 	}
 }
 
diff --git a/drivers/media/platform/microchip/microchip-isc.h b/drivers/media/platform/microchip/microchip-isc.h
index a4f1e6c22e44..44d54404250d 100644
--- a/drivers/media/platform/microchip/microchip-isc.h
+++ b/drivers/media/platform/microchip/microchip-isc.h
@@ -164,6 +164,8 @@ struct isc_ctrls {
 #define HIST_MIN_INDEX		0
 #define HIST_MAX_INDEX		1
 	u32 hist_minmax[HIST_BAYER][2];
+	u32 channel_avg[HIST_BAYER];      /* Average pixel intensity per channel */
+	u32 total_pixels[HIST_BAYER];     /* Total pixels per channel */
 
 	/*
 	 * Custom per-channel gamma LUT (10-bit output values, 64 entries).
-- 
2.34.1