linux-kernel - Re: [PATCH V3 3/3] gpio: modepin: Add driver support for modepin GPIO controller

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Date:   Wed, 18 Aug 2021 10:52:02 +0200
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Piyush Mehta <piyush.mehta@...inx.com>, arnd@...db.de,
        zou_wei@...wei.com, gregkh@...uxfoundation.org,
        linus.walleij@...aro.org, michal.simek@...inx.com,
        wendy.liang@...inx.com, iwamatsu@...auri.org,
        bgolaszewski@...libre.com, robh+dt@...nel.org,
        rajan.vaja@...inx.com
Cc:     linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
        git@...inx.com, sgoud@...inx.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Re: [PATCH V3 3/3] gpio: modepin: Add driver support for modepin GPIO
 controller

On 18.08.21 10:10, Piyush Mehta wrote:
> This patch adds driver support for the zynqmp modepin GPIO controller.
> GPIO modepin driver set and get the value and status of the PS_MODE pin,
> based on device-tree pin configuration. These four mode pins are
> configurable as input/output. The mode pin has a control register, which
> have lower four-bits [0:3] are configurable as input/output, next four-bits
> can be used for reading the data  as input[4:7], and next setting the
> output pin state output[8:11].
> 
> Signed-off-by: Piyush Mehta <piyush.mehta@...inx.com>
> Acked-by: Michal Simek <michal.simek@...inx.com>
> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> ---

> +/**
> + * modepin_gpio_dir_in - Set the direction of the specified GPIO pin as input
> + * @chip:	gpio_chip instance to be worked on
> + * @pin:	gpio pin number within the device
> + *
> + * Return: 0 always
> + */
> +static int modepin_gpio_dir_in(struct gpio_chip *chip, unsigned int pin)
> +{
> +	return 0;
> +}

You say the gpio controller can configure pins as inputs or outputs.
Yet, .direction_input is doing nothing. So it's not clear to me,
how this sequence could work:

 - set gpio output high (writes bootmode)
 - set gpio to input (no-op, pin will remain high, not high impedance)

I didn't check the previous discussions, but if this indeed works as intended,
the how should be written here into the driver. That is a more useful comment
than kernel doc for a stub function.

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