drivers: display: ssd1322: fix low and uneven pixel brightness

In case more than one segment per pixel is required, only part of the
segments were written resulting in low and uneven pixel brightness (at
least on NHD-2.7-12864WDW3). Fix it by writing all required segments.

Signed-off-by: Johan Lafon <johan.lafon@syslinbit.com>

drivers/display/ssd1322.c

@@ -40,6 +40,9 @@ LOG_MODULE_REGISTER(ssd1322, CONFIG_DISPLAY_LOG_LEVEL);
 #define SSD1322_SET_MUX_RATIO        0xCA
 #define SSD1322_COMMAND_LOCK         0xFD
 
+#define BITS_PER_SEGMENT  4
+#define SEGMENTS_PER_BYTE (8 / BITS_PER_SEGMENT)
+
 struct ssd1322_config {
 	const struct device *mipi_dev;
 	struct mipi_dbi_config dbi_config;
@@ -57,7 +60,7 @@ struct ssd1322_config {
 	bool remap_com_dual;
 	uint8_t segments_per_pixel;
 	uint8_t *conversion_buf;
-	uint16_t conversion_buf_size;
+	size_t conversion_buf_size;
 };
 
 static inline int ssd1322_write_command(const struct device *dev, uint8_t cmd, const uint8_t *buf,
@@ -96,19 +99,28 @@ static int ssd1322_conv_mono01_grayscale(const uint8_t **buf_in, uint32_t *pixel
 	/* Output buffer size gets rounded down to avoid splitting chunks in the middle of input
 	 * bytes
 	 */
-	uint16_t pixels_in_chunk = MIN(*pixel_count, ROUND_DOWN(buf_out_size / 2, 8));
+	uint16_t pixels_in_chunk =
+		MIN(*pixel_count,
+		    ROUND_DOWN((buf_out_size * SEGMENTS_PER_BYTE) / segments_per_pixel, 8));
 
 	for (uint16_t in_idx = 0; in_idx < pixels_in_chunk; in_idx++) {
-		uint16_t seg_idx = in_idx * segments_per_pixel;
 		uint8_t color = ((*buf_in)[in_idx / 8] & BIT(in_idx % 8)) ? 0xF : 0;
 
-		buf_out[seg_idx / 2] =
-			(seg_idx % 2 == 0) ? color : ((color << 4) | buf_out[seg_idx / 2]);
+		for (size_t i = 0; i < segments_per_pixel; i++) {
+			size_t seg_idx = in_idx * segments_per_pixel + i;
+			size_t shift = BITS_PER_SEGMENT * (seg_idx % SEGMENTS_PER_BYTE);
+
+			if (shift == 0) {
+				buf_out[seg_idx / SEGMENTS_PER_BYTE] = color;
+			} else {
+				buf_out[seg_idx / SEGMENTS_PER_BYTE] |= color << shift;
+			}
+		}
 	}
 
 	buf_in += pixels_in_chunk / 8;
 	*pixel_count -= pixels_in_chunk;
-	return pixels_in_chunk * segments_per_pixel / 2;
+	return pixels_in_chunk * segments_per_pixel / SEGMENTS_PER_BYTE;
 }
 
 static int ssd1322_write_pixels(const struct device *dev, const uint8_t *buf, uint32_t pixel_count)