linux-kernel - Re: [PATCH] backlight: pwm_bl: apply the initial backlight state with sane defaults

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Message-ID: <n6rltuxqwybh2mwzz3hxi3tzix2c7q3mbovscobzzmkj6puo6w@gc3qnchjlagq>
Date: Fri, 1 Aug 2025 08:32:20 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: Michael Grzeschik <m.grzeschik@...gutronix.de>
Cc: Lee Jones <lee@...nel.org>, Daniel Thompson <danielt@...nel.org>, 
	Jingoo Han <jingoohan1@...il.com>, Helge Deller <deller@....de>, 
	Pengutronix <kernel@...gutronix.de>, linux-pwm@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] backlight: pwm_bl: apply the initial backlight state
 with sane defaults

Hallo Michael,

On Thu, Jul 31, 2025 at 10:47:18AM +0200, Michael Grzeschik wrote:
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 237d3d3f3bb1a..5924e0b9f01e7 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -518,13 +518,6 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	if (!state.period && (data->pwm_period_ns > 0))
>  		state.period = data->pwm_period_ns;
>  
> -	ret = pwm_apply_might_sleep(pb->pwm, &state);
> -	if (ret) {
> -		dev_err_probe(&pdev->dev, ret,
> -			      "failed to apply initial PWM state");
> -		goto err_alloc;
> -	}
> -
>  	memset(&props, 0, sizeof(struct backlight_properties));
>  
>  	if (data->levels) {
> @@ -582,6 +575,15 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>  	pb->lth_brightness = data->lth_brightness * (div_u64(state.period,
>  				pb->scale));
>  
> +	state.duty_cycle = compute_duty_cycle(pb, data->dft_brightness, &state);
> +
> +	ret = pwm_apply_might_sleep(pb->pwm, &state);
> +	if (ret) {
> +		dev_err_probe(&pdev->dev, ret,
> +			      "failed to apply initial PWM state");
> +		goto err_alloc;
> +	}
> +

I wonder why the PWM is updated at all in .probe(). Wouldn't it be the
natural thing to keep the PWM configured as it was (in its reset default
state or how the bootloader set it up)?

Orthogonal to your change, while looking at the driver I wondered about:

        bl = backlight_device_register(dev_name(&pdev->dev), &pdev->dev, pb,
                                       &pwm_backlight_ops, &props);
        if (IS_ERR(bl)) {
                ret = dev_err_probe(&pdev->dev, PTR_ERR(bl),
                                    "failed to register backlight\n");
                goto err_alloc;
        }

        if (data->dft_brightness > data->max_brightness) {
                dev_warn(&pdev->dev,
                         "invalid default brightness level: %u, using %u\n",
                         data->dft_brightness, data->max_brightness);
                data->dft_brightness = data->max_brightness;
        }

        bl->props.brightness = data->dft_brightness;
        bl->props.power = pwm_backlight_initial_power_state(pb);
        backlight_update_status(bl);

Shoudn't setting data->dft_brightness, bl->props.brightness and
bl->props.power better happen before backlight_device_register()? Also
calling backlight_update_status() after backlight_device_register()
seems wrong to me, I'd claim the backend driver shouldn't call that.

Best regards
Uwe

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