linux-kernel - Re: [PATCH] backlight: pwm_bl: Improve bootloader/kernel device handover

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Date:   Thu, 19 Aug 2021 10:59:29 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Jingoo Han <jingoohan1@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-pwm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Marek Vasut <marex@...x.de>,
        stable@...r.kernel.org
Subject: Re: [PATCH] backlight: pwm_bl: Improve bootloader/kernel device
 handover

On Thu, 22 Jul 2021, Daniel Thompson wrote:

> Currently there are (at least) two problems in the way pwm_bl starts
> managing the enable_gpio pin. Both occur when the backlight is initially
> off and the driver finds the pin not already in output mode and, as a
> result, unconditionally switches it to output-mode and asserts the signal.
> 
> Problem 1: This could cause the backlight to flicker since, at this stage
> in driver initialisation, we have no idea what the PWM and regulator are
> doing (an unconfigured PWM could easily "rest" at 100% duty cycle).
> 
> Problem 2: This will cause us not to correctly honour the
> post_pwm_on_delay (which also risks flickers).
> 
> Fix this by moving the code to configure the GPIO output mode until after
> we have examines the handover state. That allows us to initialize
> enable_gpio to off if the backlight is currently off and on if the
> backlight is on.
> 
> Reported-by: Marek Vasut <marex@...x.de>
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> Cc: stable@...r.kernel.org
> Acked-by: Marek Vasut <marex@...x.de>
> Tested-by: Marek Vasut <marex@...x.de>
> ---
>  drivers/video/backlight/pwm_bl.c | 54 +++++++++++++++++---------------
>  1 file changed, 28 insertions(+), 26 deletions(-)

Applied, thanks.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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