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drm/ssd130x: Scale ssd133x per-channel contrast by brightness on init

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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@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patch.msgid.link/20260729053054.29374-2-amit.barzilai22@gmail.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
This commit is contained in:
Amit Barzilai
2026-07-29 08:30:53 +03:00
committed by Javier Martinez Canillas
parent 2b209e52a5
commit a16d5be8ea
1 changed files with 39 additions and 4 deletions
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43
drivers/gpu/drm/solomon/ssd130x.c
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@@ -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);
}