[PATCH 04/18] media: microchip-isc: Improve histogram calculation with outlier rejection

[Balamanikandan Gunasundar]

From: Balakrishnan Sambath <balakrishnan.s at microchip.com>

Replace simple min/max detection with smart outlier rejection that skips
bottom/top 2% of histogram to avoid noise and saturation. Add channel
average calculation using weighted pixel intensity instead of simple
pixel counting for more accurate color analysis.

Signed-off-by: Balakrishnan Sambath <balakrishnan.s at microchip.com>
---
 .../platform/microchip/microchip-isc-base.c   | 83 ++++++++++++++++---
 .../media/platform/microchip/microchip-isc.h  |  2 +
 2 files changed, 75 insertions(+), 10 deletions(-)

diff --git a/drivers/media/platform/microchip/microchip-isc-base.c b/drivers/media/platform/microchip/microchip-isc-base.c
index c138e92a1aca..956bdea830e3 100644
--- a/drivers/media/platform/microchip/microchip-isc-base.c
+++ b/drivers/media/platform/microchip/microchip-isc-base.c
@@ -262,6 +262,10 @@ static void isc_set_histogram(struct isc_device *isc, bool enable)
 	struct isc_ctrls *ctrls = &isc->ctrls;
 
 	if (enable) {
+		/* Initialize histogram data storage for clean start */
+		memset(ctrls->total_pixels, 0, sizeof(ctrls->total_pixels));
+		memset(ctrls->hist_minmax, 0, sizeof(ctrls->hist_minmax));
+
 		regmap_write(regmap, ISC_HIS_CFG + isc->offsets.his,
 			     ISC_HIS_CFG_MODE_GR |
 			     (isc->config.sd_format->cfa_baycfg
@@ -1231,24 +1235,83 @@ 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 */
+	u32 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", ctrls->hist_id);
+		return;
+	}
+
+	/* Smart outlier rejection - skip bottom/top 2% */
+	u32 dark_threshold = total_pixels / 50;   /* Bottom 2% */
+	u32 bright_threshold = total_pixels / 50; /* Top 2% */
+	u32 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;
+	u64 weighted_sum = 0;
+
+	for (i = *min; i <= *max; i++) {
+		u32 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",
+		ctrls->hist_id, ctrls->channel_avg[ctrls->hist_id],
+		*hist_count, *min, *max, total_pixels);
 }
 
 static void isc_wb_update(struct isc_ctrls *ctrls)
diff --git a/drivers/media/platform/microchip/microchip-isc.h b/drivers/media/platform/microchip/microchip-isc.h
index ad4e98a1dd8f..bd75ff4f109b 100644
--- a/drivers/media/platform/microchip/microchip-isc.h
+++ b/drivers/media/platform/microchip/microchip-isc.h
@@ -156,6 +156,8 @@ 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 */
 };
 
 #define ISC_PIPE_LINE_NODE_NUM	15
-- 
2.34.1