[PATCH v2 03/10] leds: st1202: fix pattern duration register calculation

[Manuel Fombuena]

The duration register (PATy_DUR) uses a direct encoding: register value N
corresponds to N × 22.2 ms, with 0 reserved as "pattern skip" (§7.10).
The driver incorrectly subtracted 1 from the register value, based on the
assumption that register 0 was the minimum duration rather than a skip
indicator. This caused two problems:

  - All programmed durations were off by one step (~22 ms too short).
  - Writing the minimum duration (22 ms) produced register value 0,
    silently skipping the pattern step instead of setting a 22 ms duration.

The maximum duration constant was also wrong at 5660 ms. With the correct
formula the 8-bit register saturates at 255, giving a maximum of 5610 ms
(22 ms × 255). Values above 5653 ms were already producing a uint8_t
overflow and writing 0 to the hardware.

Fix the formula and derive the maximum from the register width so the
relationship is explicit.

Update the documentation to reflect the correct maximum.

Fixes: 259230378c65 ("leds: Add LED1202 I2C driver")
Signed-off-by: Manuel Fombuena <fombuena@xxxxxxxxxxx>
Assisted-by: Claude:claude-sonnet-4-6
---
 Documentation/leds/leds-st1202.rst | 2 +-
 drivers/leds/leds-st1202.c         | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/leds/leds-st1202.rst b/Documentation/leds/leds-st1202.rst
index 1a09fbfcedcf..a2353549469e 100644
--- a/Documentation/leds/leds-st1202.rst
+++ b/Documentation/leds/leds-st1202.rst
@@ -17,7 +17,7 @@ To be compatible with the hardware pattern format, maximum 8 tuples of
 brightness (PWM) and duration must be written to hw_pattern.
 
 - Min pattern duration: 22 ms
-- Max pattern duration: 5660 ms
+- Max pattern duration: 5610 ms
 
 The format of the hardware pattern values should be:
 "brightness duration brightness duration ..."
diff --git a/drivers/leds/leds-st1202.c b/drivers/leds/leds-st1202.c
index 5f4238181057..02db1006fb53 100644
--- a/drivers/leds/leds-st1202.c
+++ b/drivers/leds/leds-st1202.c
@@ -31,7 +31,7 @@
 #define ST1202_ILED_REG0                   0x09
 #define ST1202_MAX_LEDS                    12
 #define ST1202_MAX_PATTERNS                8
-#define ST1202_MILLIS_PATTERN_DUR_MAX      5660
+#define ST1202_MILLIS_PATTERN_DUR_MAX      (ST1202_MILLIS_PATTERN_DUR_MIN * U8_MAX)
 #define ST1202_MILLIS_PATTERN_DUR_MIN      22
 #define ST1202_PATTERN_DUR                 0x16
 #define ST1202_PATTERN_PWM                 0x1E
@@ -85,7 +85,7 @@ static int st1202_write_reg(struct st1202_chip *chip, int reg, uint8_t val)
 
 static uint8_t st1202_prescalar_to_miliseconds(unsigned int value)
 {
-	return value / ST1202_MILLIS_PATTERN_DUR_MIN - 1;
+	return value / ST1202_MILLIS_PATTERN_DUR_MIN;
 }
 
 static int st1202_pwm_pattern_write(struct st1202_chip *chip, int led_num,
-- 
2.54.0