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

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Message-ID: <20231020112727.GF23755@aspen.lan>
Date:   Fri, 20 Oct 2023 12:27:27 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee@...nel.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

On Wed, Oct 18, 2023 at 11:07:41PM +0200, Uwe Kleine-König wrote:
> 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?

pwm_init_state() explicitly zeros the duty-cycle in order to avoid
problems when the default args have a different period to the currently
applied config:
https://elixir.bootlin.com/linux/latest/source/include/linux/pwm.h#L174


Daniel.


> > 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/ |