linux-kernel - Re: [PATCH v5 1/2] wilc1000: Add reset/enable GPIO support to SPI driver

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Date:   Wed, 15 Dec 2021 07:59:05 -0700
From:   David Mosberger-Tang <davidm@...uge.net>
To:     Claudiu.Beznea@...rochip.com, Ajay.Kathat@...rochip.com
Cc:     adham.abozaeid@...rochip.com, davem@...emloft.net,
        devicetree@...r.kernel.org, kuba@...nel.org, kvalo@...eaurora.org,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, robh+dt@...nel.org
Subject: Re: [PATCH v5 1/2] wilc1000: Add reset/enable GPIO support to SPI
 driver

On Wed, 2021-12-15 at 06:41 +0000, Claudiu.Beznea@...rochip.com wrote:
> On 15.12.2021 05:05, David Mosberger-Tang wrote:
> > 
> +static int wilc_parse_gpios(struct wilc *wilc)
> > +{
> > +       struct spi_device *spi = to_spi_device(wilc->dev);
> > +       struct wilc_spi *spi_priv = wilc->bus_data;
> > +       struct wilc_gpios *gpios = &spi_priv->gpios;
> > +
> > +       /* get ENABLE pin and deassert it (if it is defined): */
> > +       gpios->enable = devm_gpiod_get_optional(&spi->dev,
> > +                                               "enable", GPIOD_OUT_LOW);
> > +       /* get RESET pin and assert it (if it is defined): */
> > +       if (gpios->enable) {
> > +               /* if enable pin exists, reset must exist as well */
> > +               gpios->reset = devm_gpiod_get(&spi->dev,
> > +                                             "reset", GPIOD_OUT_HIGH);
> 
> As far as I can tell form gpiolib code the difference b/w GPIOD_OUT_HIGH
> and GPIOD_OUT_LOW in gpiolib is related to the initial value for the GPIO.

Yes.

> Did you used GPIOD_OUT_HIGH for reset to have the chip out of reset at this
> point?

No, ~RESET is an active-low signal.  GPIOD_OUT_LOW should really be
called GPIOD_OUT_DEASSERTED or something like that.  The code ensures
that the chip is in RESET and ~ENABLEd after parsing the GPIOs.

> > +               if (IS_ERR(gpios->reset)) {
> > +                       dev_err(&spi->dev, "missing reset gpio.\n");
> > +                       return PTR_ERR(gpios->reset);
> > +               }
> > +       } else {
> > +               gpios->reset = devm_gpiod_get_optional(&spi->dev,
> > +                                                      "reset", GPIOD_OUT_HIGH);
> > +       }
> > +       return 0;
> > +}
> > +
> > +static void wilc_wlan_power(struct wilc *wilc, bool on)
> > +{
> > +       struct wilc_spi *spi_priv = wilc->bus_data;
> > +       struct wilc_gpios *gpios = &spi_priv->gpios;
> > +
> > +       if (on) {
> > +               gpiod_set_value(gpios->enable, 1);      /* assert ENABLE */
> > +               mdelay(5);
> > +               gpiod_set_value(gpios->reset, 0);       /* deassert RESET */
> 
> From what I can tell from gpiolib code, requesting the pin from device tree
> with:
> 
> +        reset-gpios = <&pioA 6 GPIO_ACTIVE_LOW>;
> 
> makes the value written with gpiod_set_value() to be negated, thus the 0
> written here is translated to a 1 on the pin. Is there a reason you did it
> like this?

Yes, of course.  RESET is an active-low signal, as defined in the
datasheet.

> Would it have been simpler to have both pins requested with
> GPIO_ACTIVE_HIGH and here to do gpiod_set_value(gpio, 1) for both of the
> pin. In this way, at the first read of the code one one would have been
> telling that it does what datasheet specifies: for power on toggle enable
> and reset gpios from 0 to 1 with a delay in between.

I think you're confusing 0 and 1 with low-voltage and high-voltage.  0
means de-assert the signal, 1 means assert the signal.  Whether that
translates to a low voltage or a high voltage depends on whether the
signal a active-low or active-high.

> 
> 
> > +       } else {
> > +               gpiod_set_value(gpios->reset, 1);       /* assert RESET */
> > +               gpiod_set_value(gpios->enable, 0);      /* deassert ENABLE */
> 
> I don't usually see comments near the code line in kernel. Maybe move them
> before the actual code line or remove them at all as the code is impler enough?

You're kidding, right?

  --david