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References: <20230720061105.154821-1-victor.liu@nxp.com> <20230720112742.GA2525277@aspen.lan>
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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 20 Jul 2023 19:28:26 +0300
Message-ID: <CAHp75Vdw_-nKUdNm0TNRnMFLhUEoFAtTpt4ApVNmeHFOi_16Xg@mail.gmail.com>
Subject: Re: [PATCH] backlight: gpio_backlight: Drop output gpio direction
 check for initial power state
To:     Daniel Thompson <daniel.thompson@linaro.org>
Cc:     Ying Liu <victor.liu@nxp.com>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "lee@kernel.org" <lee@kernel.org>,
        "jingoohan1@gmail.com" <jingoohan1@gmail.com>,
        "deller@gmx.de" <deller@gmx.de>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Andy Shevchenko <andy@kernel.org>
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On Thu, Jul 20, 2023 at 2:27=E2=80=AFPM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
>
> On Thu, Jul 20, 2023 at 06:06:27AM +0000, Ying Liu wrote:
> > Bootloader may leave gpio direction as input and gpio value as logical =
low.
> > It hints that initial backlight power state should be FB_BLANK_POWERDOW=
N
> > since the gpio value is literally logical low.
>
> To be honest this probably "hints" that the bootloader simply didn't
> consider the backlight at all :-) . I'd rather the patch description
> focus on what circumstances lead to the current code making a bad
> decision. More like:
>
>   If the GPIO pin is in the input state but the backlight is currently
>   off due to default pull downs then ...
>
> > So, let's drop output gpio
> > direction check and only check gpio value to set the initial power stat=
e.
>
> This check was specifically added by Bartosz so I'd be interested in his
> opinion of this change (especially since he is now a GPIO maintainer)!
>
> What motivates (or motivated) the need to check the direction rather
> than just read that current logic level on the pin?

...

> > -     else if (gpiod_get_direction(gbl->gpiod) =3D=3D 0 &&
> > -              gpiod_get_value_cansleep(gbl->gpiod) =3D=3D 0)
> > +     else if (gpiod_get_value_cansleep(gbl->gpiod) =3D=3D 0)
> >               bl->props.power =3D FB_BLANK_POWERDOWN;

The code before this patch needs a bit of elaboration. There is no
prohibition on reading value for the pin that is in any direction.
I.o.w. if the direction here is a problem it should have been
configured beforehand.

--=20
With Best Regards,
Andy Shevchenko