linux-kernel - Re: [PATCH v2 1/2] leds: rgb: leds-qcom-lpg: Fix pwm resolution max for Hi-Res PWMs

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Message-ID: <knuma4xplv6ipwnqgfvzu3bw5tgbq3qzfasaybjwd7kzpf5ipu@74gqmk5xcr6a>
Date: Fri, 28 Feb 2025 10:49:32 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Abel Vesa <abel.vesa@...aro.org>
Cc: Lee Jones <lee@...nel.org>, Pavel Machek <pavel@...nel.org>, 
	Anjelique Melendez <quic_amelende@...cinc.com>, Kamal Wadhwa <quic_kamalw@...cinc.com>, 
	Jishnu Prakash <jishnu.prakash@....qualcomm.com>, 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 v2 1/2] leds: rgb: leds-qcom-lpg: Fix pwm resolution max
 for Hi-Res PWMs

On Wed, Feb 26, 2025 at 03:58:54PM +0200, Abel Vesa wrote:
> Ideally, the requested duty cycle should never translate to a PWM
> value higher than the selected resolution (PWM size), but currently the
> best matched period is never reported back to the PWM consumer, so the
> consumer will still be using the requested period which is higher than
> the best matched one. This will result in PWM consumer requesting
> duty cycle values higher than the allowed PWM value.
> 
> Currently, the consumer driver known to fail this way is the PWM backlight
> (pwm_bl) and should be reworked in such a way that the best matched period
> is used instead.
> 

As you're rebasing the patch (per Anjelique's addition of 6-bit pwm) I
think you should omit this paragraph. Whether we like it or not, this
seems to be the way the PWM api is designed, and there's no need to
promise a redesign of the API in this bug fix.

> As for the current implementation of the duty cycle calculation, it is
> capping the max value, fix that by using the resolution to figure out the
> maximum allowed PWM value.
> 
> 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>

Reviewed-by: Bjorn Andersson <andersson@...nel.org>

Regards,
Bjorn

> ---
>  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;
> 
> -- 
> 2.34.1
>