linux-kernel - Re: [PATCH] backlight: pwm_bl: Avoid backlight flicker applying initial PWM state

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Message-ID: <20231018210741.6t3yfj6qgmpwhhlo@pengutronix.de>
Date:   Wed, 18 Oct 2023 23:07:41 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee@...nel.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Helge Deller <deller@....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: Avoid backlight flicker applying
 initial PWM state

Hello Philipp,

On Thu, Jun 08, 2023 at 04:11:14PM +0200, Philipp Zabel wrote:
> The initial PWM state returned by pwm_init_state() has a duty cycle
> of 0 ns.

This is only true for drivers without a .get_state() callback, isn't it?

> To avoid backlight flicker when taking over an enabled
> display from the bootloader, skip the initial pwm_apply_state()
> and leave the PWM be until backlight_update_state() will apply the
> state with the desired brightness.
> 
> Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> ---
> With a PWM driver that allows to inherit PWM state from the bootloader,
> postponing the initial pwm_apply_state() with 0 ns duty cycle allows to
> set the desired duty cycle before the PWM is set, avoiding a short flicker
> if the backlight was previously enabled and will be enabled again.
> ---
>  drivers/video/backlight/pwm_bl.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index fce412234d10..47a917038f58 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -531,12 +531,10 @@ 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_state(pb->pwm, &state);
> -	if (ret) {
> -		dev_err(&pdev->dev, "failed to apply initial PWM state: %d\n",
> -			ret);
> -		goto err_alloc;
> -	}
> +	/*
> +	 * No need to apply initial state, except in the error path.

Why do you want to modify the PWM in the error path? I would have
expected not touching it at all in .probe() is fine?!

> +	 * State will be applied by backlight_update_status() on success.
> +	 */
>  
>  	memset(&props, 0, sizeof(struct backlight_properties));
>  

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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