leds: st1202: Scale pattern brightness to the 12-bit PWM range

The LED1202 PWM registers are 12 bits wide, but st1202_led_pattern_set()
writes the 8-bit brightness value carried by struct led_pattern into
them unscaled. A pattern step asking for full brightness therefore
drives the channel at 255 out of 4095, about 6% duty cycle, so a pattern
is dimmer than the same brightness set directly.

Scale the value to the full PWM range before programming it.

Scale against max_brightness rather than the register maximum. The LED
core takes max_brightness from the DT max-brightness property, so a
board can declare a ceiling below full scale, and the pattern trigger
already rejects steps above it. Scaling against a hardcoded 255 would
leave such a board unable to reach its own declared maximum from the
pattern path while the brightness path reaches it.

Scaling divides by max_brightness. The LED core replaces a zero
max-brightness from the device tree with LED_FULL, so the divisor should
never be zero, but the helper returns zero rather than rely on that.
max_brightness is capped at U8_MAX, the driver's own maximum, so a
device tree declaring more than the part supports is treated as full
scale. struct led_pattern brightness is signed, so clamp both ends and
floor a negative value to LED_OFF rather than let it wrap.

Document that a step is relative to max_brightness, since the range
given in the documentation no longer holds when the device tree sets
max-brightness.

Fixes: 259230378c ("leds: Add LED1202 I2C driver")
Signed-off-by: Manuel Fombuena <fombuena@outlook.com>
Assisted-by: LLM
Link: https://patch.msgid.link/GV1PR08MB8497B938D3592582E17CB757C5BA2@GV1PR08MB8497.eurprd08.prod.outlook.com
Signed-off-by: Lee Jones <lee@kernel.org>

Documentation/leds/leds-st1202.rst

@@ -20,6 +20,11 @@ brightness (PWM) and duration must be written to hw_pattern.
 - Min pattern duration: 22 ms
 - Max pattern duration: 5610 ms
 
+If the device tree sets max-brightness, that value replaces 255 as the
+upper limit, and a step equal to it drives the channel at full scale.
+The hardware cannot go above 255, so with a larger max-brightness every
+step from 255 upwards is full scale.
+
 The format of the hardware pattern values should be:
 "brightness duration brightness duration ..."
 

drivers/leds/leds-st1202.c

@@ -91,6 +91,16 @@ static u8 st1202_milliseconds_to_prescaler(unsigned int value)
 	return value / ST1202_MILLIS_PATTERN_DUR_MIN;
 }
 
+static u16 st1202_brightness_to_pwm(int brightness, unsigned int max_brightness)
+{
+	if (!max_brightness)
+		return 0;
+
+	brightness = clamp_t(int, brightness, LED_OFF, max_brightness);
+
+	return brightness * ST1202_PATTERN_PWM_FULL / max_brightness;
+}
+
 static int st1202_pwm_pattern_write(struct st1202_chip *chip, int led_num,
 				int pattern, unsigned int value)
 {
@@ -235,8 +245,11 @@ static int st1202_led_pattern_set(struct led_classdev *ldev,
 {
 	struct st1202_led *led = cdev_to_st1202_led(ldev);
 	struct st1202_chip *chip = led->chip;
+	unsigned int max_brightness;
 	int ret;
 
+	max_brightness = min_t(unsigned int, ldev->max_brightness, U8_MAX);
+
 	if (len > ST1202_MAX_PATTERNS)
 		return -EINVAL;
 
@@ -254,7 +267,8 @@ static int st1202_led_pattern_set(struct led_classdev *ldev,
 
 	for (int pattern = 0; pattern < len; pattern++) {
 		ret = st1202_pwm_pattern_write(chip, led->led_num, pattern,
-						patterns[pattern].brightness);
+					st1202_brightness_to_pwm(patterns[pattern].brightness,
+								max_brightness));
 		if (ret != 0)
 			return ret;