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

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Message-ID: <9d330824-58b4-4c19-b778-9b000eb4d20a@oss.qualcomm.com>
Date: Fri, 21 Feb 2025 20:34:08 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Abel Vesa <abel.vesa@...aro.org>, Lee Jones <lee@...nel.org>,
        Pavel Machek <pavel@...nel.org>,
        Anjelique Melendez <quic_amelende@...cinc.com>
Cc: Kamal Wadhwa <quic_kamalw@...cinc.com>,
        Jishnu Prakash <jishnu.prakash@....qualcomm.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Johan Hovold <johan@...nel.org>, Sebastian Reichel <sre@...nel.org>,
        Pavel Machek <pavel@....cz>, linux-leds@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] leds: rgb: leds-qcom-lpg: Fix pwm resolution for Hi-Res
 PWMs

On 20.02.2025 11:31 AM, Abel Vesa wrote:
> 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@...r.kernel.org    # 6.4
> Fixes: b00d2ed37617 ("leds: rgb: leds-qcom-lpg: Add support for high resolution PWM")
> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> ---

Maybe Anjelique would know better, but the computer tells me PMK8550 has
a 1*4*(not 15)-bit PWM controller.. I don't know if it's related, but
something to keep in mind

Konrad