linux-kernel - Re: [PATCH v4 1/2] backlight: pwm_bl: Move the checks for initial power state to a separate function

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Message-ID: <CAMuHMdWAHpJKf1wrc-ADY1KzEtRJK1-qH-wgRqLeQD6TWP-rPQ@mail.gmail.com>
Date:   Wed, 22 Mar 2017 13:30:32 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Peter Ujfalusi <peter.ujfalusi@...com>,
        Lee Jones <lee.jones@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] backlight: pwm_bl: Move the checks for initial
 power state to a separate function

Hi Philip,

On Wed, Mar 22, 2017 at 11:46 AM, Philipp Zabel <p.zabel@...gutronix.de> wrote:
> On Tue, 2017-03-21 at 19:48 +0100, Geert Uytterhoeven wrote:
>> On Tue, Nov 22, 2016 at 2:41 PM, Peter Ujfalusi <peter.ujfalusi@...com> wrote:
>> > Move the checks to select the initial state for the backlight to a new
>> > function and document the checks we are doing.
>>
>> This is far from a simple "move"...
>>
>> > With the separate function it is going to be easier to fix or improve the
>> > initial power state configuration later and it is easier to read the code.
>> >
>> > Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
>> > Reviewed-by: Philipp Zabel <p.zabel@...gutronix.de>
>> > Reviewed-by: Thierry Reding <treding@...dia.com>
>>
>> This patch (commit 7613c922315e308a in v4.11-rc1) broke the display on
>> r8a7740/armadillo.
>>
>> > @@ -267,20 +292,16 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>> >                 pb->enable_gpio = gpio_to_desc(data->enable_gpio);
>> >         }
>> >
>> > -       if (pb->enable_gpio) {
>> > -               /*
>> > -                * If the driver is probed from the device tree and there is a
>> > -                * phandle link pointing to the backlight node, it is safe to
>> > -                * assume that another driver will enable the backlight at the
>> > -                * appropriate time. Therefore, if it is disabled, keep it so.
>> > -                */
>> > -               if (node && node->phandle &&
>> > -                   gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_OUT &&
>> > -                   gpiod_get_value(pb->enable_gpio) == 0)
>> > -                       initial_blank = FB_BLANK_POWERDOWN;
>> > -               else
>> > -                       gpiod_direction_output(pb->enable_gpio, 1);
>>
>> In my case, "node" points to the "/backlight" node, but phandle is NULL.
>> Hence before, gpiod_direction_output() was called to enable the GPIO...
>>
>> > -       }
>> > +       /*
>> > +        * If the GPIO is configured as input, change the direction to output
>> > +        * and set the GPIO as active.
>> > +        * Do not force the GPIO to active when it was already output as it
>> > +        * could cause backlight flickering or we would enable the backlight too
>> > +        * early. Leave the decision of the initial backlight state for later.
>> > +        */
>> > +       if (pb->enable_gpio &&
>> > +           gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_IN)
>> > +               gpiod_direction_output(pb->enable_gpio, 1);
>>
>> ... while now it's no longer called, as gpiod_get_direction() returns
>> -EINVAL.
>>
>> Indeed, r8a7740_pfc does not implement the .get_direction() callback,
>> so gpiod_get_direction() always returns -EINVAL, which is never equal
>> to GPIOF_DIR_IN.
>
> Oh, I didn't think about this at all, anymore. Though I believe to
> remember that this was the reason that I checked for
> (gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_OUT) before, so ...
>
>> Restoring the old behavior by changing the above test to
>>
>>         if (pb->enable_gpio &&
>>             (!node || !node->phandle ||
>>              gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_IN))
>>
>> fixes the display for me, but leads to a more complex expression.
>>
>> However, changing the test to
>>
>>         if (pb->enable_gpio &&
>>             gpiod_get_direction(pb->enable_gpio) != GPIOF_DIR_OUT)
>>
>> also fixes the display, as an error is always different from GPIOF_DIR_OUT.
>>
>> Anyone with comments or suggestions to fix this for real?
>
> ... I'm in favor of the latter, as this is closer to the initial
> intention. I'd also mention this in the comment:
>
>        /*
>         * If the GPIO is not known to be already configured as output, that is,
>         * if gpiod_get_direction returns either GPIOF_DIR_IN or -EINVAL, change
>         * the direction to output and set the GPIO as active.
>         * Do not force the GPIO to active when it was already output as it
>         * could cause backlight flickering or we would enable the backlight too
>         * early. Leave the decision of the initial backlight state for later.
>         */

Thanks, I'll cook up a patch.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds