linux-kernel - Re: [PATCH] backlight: gpio_backlight: Drop output gpio direction check for initial power state

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Date:   Thu, 20 Jul 2023 19:28:26 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Ying Liu <victor.liu@....com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lee@...nel.org" <lee@...nel.org>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "deller@....de" <deller@....de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Andy Shevchenko <andy@...nel.org>
Subject: Re: [PATCH] backlight: gpio_backlight: Drop output gpio direction
 check for initial power state

On Thu, Jul 20, 2023 at 2:27 PM Daniel Thompson
<daniel.thompson@...aro.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_POWERDOWN
> > 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 state.
>
> 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) == 0 &&
> > -              gpiod_get_value_cansleep(gbl->gpiod) == 0)
> > +     else if (gpiod_get_value_cansleep(gbl->gpiod) == 0)
> >               bl->props.power = 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.

-- 
With Best Regards,
Andy Shevchenko