[PATCH] leds: rgb: leds-qcom-lpg: Fix pwm resolution for Hi-Res PWMs

Abel Vesa
2025-02-20T10:31:13Z

Currently, for the high resolution PWMs, the resolution, clock,
pre-divider and exponent are being selected based on period. Basically,
the implementation loops over each one of these and tries to find the
closest (higher) period based on the following formula:

period * refclk
prediv_exp = log2 -------------------------------------
NSEC_PER_SEC * pre_div * resolution

Since the resolution is power of 2, the actual period resulting is
usually higher than what the resolution allows. That's why the duty
cycle requested needs to be capped to the maximum value allowed by the
resolution (known as PWM size).

Here is an example of how this can happen:

For a requested period of 5000000, the best clock is 19.2MHz, the best
prediv is 5, the best exponent is 6 and the best resolution is 256.

Then, the pwm value is determined based on requested period and duty
cycle, best prediv, best exponent and best clock, using the following
formula:

duty * refclk
pwm_value = ----------------------------------------------
NSEC_PER_SEC * prediv * (1 << prediv_exp)

So in this specific scenario:

(5000000 * 19200000) / (1000000000 * 5 * (1 << 64)) = 300

With a resolution of 8 bits, this pwm value obviously goes over.

Therefore, the max pwm value allowed needs to be 255.

If not, the PMIC internal logic will only value that is under the set PWM
size, resulting in a wrapped around PWM value.

This has been observed on Lenovo Thinkpad T14s Gen6 (LCD panel version)
which uses one of the PMK8550 to control the LCD backlight.

Fix the value of the PWM by capping to a max based on the chosen
resolution (PWM size).

Cc: stable@vger.kernel.org # 6.4
Fixes: b00d2ed37617 ("leds: rgb: leds-qcom-lpg: Add support for high resolution PWM")
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
---
Note: This fix is blocking backlight support on Lenovo Thinkpad T14s
Gen6 (LCD version), for which I have patches ready to send once this
patch is agreed on (review) and merged.
---
drivers/leds/rgb/leds-qcom-lpg.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/leds/rgb/leds-qcom-lpg.c b/drivers/leds/rgb/leds-qcom-lpg.c
index f3c9ef2bfa572f9ee86c8b8aa37deb8231965490..146cd9b447787bf170310321e939022dfb176e9f 100644
--- a/drivers/leds/rgb/leds-qcom-lpg.c
+++ b/drivers/leds/rgb/leds-qcom-lpg.c
@@ -529,7 +529,7 @@ static void lpg_calc_duty(struct lpg_channel *chan, uint64_t duty)
unsigned int clk_rate;

if (chan->subtype == LPG_SUBTYPE_HI_RES_PWM) {
- max = LPG_RESOLUTION_15BIT - 1;
+ max = BIT(lpg_pwm_resolution_hi_res[chan->pwm_resolution_sel]) - 1;
clk_rate = lpg_clk_rates_hi_res[chan->clk_sel];
} else {
max = LPG_RESOLUTION_9BIT - 1;

---
base-commit: 50a0c754714aa3ea0b0e62f3765eb666a1579f24
change-id: 20250220-leds-qcom-lpg-fix-max-pwm-on-hi-res-067e8782a79b

Best regards,
--
Abel Vesa <abel.vesa@linaro.org>