[PATCH 1/2] drm/ssd130x: Scale ssd133x per-channel contrast by brightness on init

[Amit Barzilai]

ssd133x_init() wrote the SSD133X_CONTRAST_A/B/C commands with magic hex
values (0x91/0x50/0x7d).  These are a per-channel white-balance
calibration: the A/B/C channels drive sub-pixels whose OLED materials
differ in luminous efficiency, so the values set the white point at full
brightness.

Extract them into ssd133x_set_contrast(), which scales each channel by a
requested brightness via ssd130x_scale_contrast(), instead of writing the
calibration unconditionally.  This makes the sequence readable, avoids
repetition, and is a prerequisite for wiring up an ssd133x backlight
controller that dims while preserving the white point.

Note this changes the ssd133x power-on brightness.  Previously the init
wrote the calibration unscaled and ignored ssd130x->contrast, so the
panel always booted at full brightness.  It now scales by the shared
default contrast of 127, i.e. half of MAX_CONTRAST (255).  This is
intentional and matches ssd130x, whose contrast register also defaults
to 127 (mid-scale), so all families now power on at ~50% and report
props.brightness = 127 / max_brightness = 255 to userspace.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Amit Barzilai <amit.barzilai22@xxxxxxxxx>
---
 drivers/gpu/drm/solomon/ssd130x.c | 43 ++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/solomon/ssd130x.c b/drivers/gpu/drm/solomon/ssd130x.c
index 0940eae7a2e4..f49e5245b216 100644
--- a/drivers/gpu/drm/solomon/ssd130x.c
+++ b/drivers/gpu/drm/solomon/ssd130x.c
@@ -14,6 +14,7 @@
 #include <linux/bits.h>
 #include <linux/delay.h>
 #include <linux/gpio/consumer.h>
+#include <linux/math.h>
 #include <linux/property.h>
 #include <linux/pwm.h>
 #include <linux/regulator/consumer.h>
@@ -140,6 +141,11 @@
 #define SSD133X_SET_PRECHARGE_VOLTAGE		0xbb
 #define SSD133X_SET_VCOMH_VOLTAGE		0xbe
 
+/* ssd133x A/B/C channel contrast at full brightness (white balance) */
+#define SSD133X_DEFAULT_CONTRAST_A		0x91
+#define SSD133X_DEFAULT_CONTRAST_B		0x50
+#define SSD133X_DEFAULT_CONTRAST_C		0x7d
+
 #define MAX_CONTRAST 255
 
 const struct ssd130x_deviceinfo ssd130x_variants[] = {
@@ -582,12 +588,37 @@ static int ssd132x_init(struct ssd130x_device *ssd130x)
 	return ssd130x_run_cmd_seq(ssd130x, cmds);
 }
 
-static int ssd133x_init(struct ssd130x_device *ssd130x)
+/* Scale a channel's white-balance calibration contrast by the requested brightness */
+static u8 ssd130x_scale_contrast(u8 calibration, u32 brightness)
+{
+	return DIV_ROUND_CLOSEST(calibration * brightness, MAX_CONTRAST);
+}
+
+/*
+ * The A/B/C contrast channels drive sub-pixels whose OLED materials differ
+ * in luminous efficiency, so the per-channel values are a white-balance
+ * calibration.  Scale them by the requested brightness instead of
+ * overwriting them, to keep the white point while dimming.
+ */
+static int ssd133x_set_contrast(struct ssd130x_device *ssd130x, u32 brightness)
 {
 	const u8 cmds[] = {
-		2, SSD133X_CONTRAST_A, 0x91,
-		2, SSD133X_CONTRAST_B, 0x50,
-		2, SSD133X_CONTRAST_C, 0x7d,
+		2, SSD133X_CONTRAST_A,
+		ssd130x_scale_contrast(SSD133X_DEFAULT_CONTRAST_A, brightness),
+		2, SSD133X_CONTRAST_B,
+		ssd130x_scale_contrast(SSD133X_DEFAULT_CONTRAST_B, brightness),
+		2, SSD133X_CONTRAST_C,
+		ssd130x_scale_contrast(SSD133X_DEFAULT_CONTRAST_C, brightness),
+		0,
+	};
+
+	return ssd130x_run_cmd_seq(ssd130x, cmds);
+}
+
+static int ssd133x_init(struct ssd130x_device *ssd130x)
+{
+	int ret;
+	const u8 cmds[] = {
 		2, SSD133X_SET_MASTER_CURRENT, 0x06,
 		3, SSD133X_SET_COL_RANGE, 0x00, ssd130x->width - 1,
 		3, SSD133X_SET_ROW_RANGE, 0x00, ssd130x->height - 1,
@@ -614,6 +645,10 @@ static int ssd133x_init(struct ssd130x_device *ssd130x)
 		0,
 	};
 
+	ret = ssd133x_set_contrast(ssd130x, ssd130x->contrast);
+	if (ret < 0)
+		return ret;
+
 	return ssd130x_run_cmd_seq(ssd130x, cmds);
 }
 
-- 
2.55.0