linux-kernel - Re: [PATCH 3/3] iio: magnetometer: ak8975: Add gpio reset support

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Date:   Mon, 18 May 2020 19:43:17 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Jonathan Albrieux <jonathan.albrieux@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hartmut Knaack <knaack.h@....de>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        "open list:IIO SUBSYSTEM AND DRIVERS" <linux-iio@...r.kernel.org>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Steve Winslow <swinslow@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Cameron <jic23@...nel.org>
Subject: Re: [PATCH 3/3] iio: magnetometer: ak8975: Add gpio reset support

On Mon, May 18, 2020 at 06:01:20PM +0200, Jonathan Albrieux wrote:
> On Mon, May 18, 2020 at 05:55:51PM +0300, Andy Shevchenko wrote:
> > On Mon, May 18, 2020 at 4:38 PM Jonathan Albrieux
> > <jonathan.albrieux@...il.com> wrote:
> > 
> > > +       gpiod_set_value_cansleep(data->reset_gpiod, 1);
> > 
> > (1)
> > 
> > ...
> > 
> > > +       /*
> > > +        * If reset pin is provided then will be set to high on power on
> > > +        * and to low on power off according to AK09911 datasheet
> > > +        */
> > 
> > Wording is confusing, perhaps you have to use 'asserted / deasserted'.
> 
> Thank you for the suggestion, I'll be working on rewording as soon as
> possible.
> 
> > Btw, in (1) it's also "high" (asserted). I barely understand how it's
> > supposed to work in all cases?
> > 
> > > +       reset_gpiod = devm_gpiod_get_optional(&client->dev,
> > > +                                             "reset", GPIOD_OUT_HIGH);
> > > +       if (IS_ERR(reset_gpiod))
> > > +               return PTR_ERR(reset_gpiod);
> > 
> 
> I'm sorry but I'm not sure about what you mean by saying all cases.
> Currently  I'm testing this driver on a msm8916 device having AK09911
> magnetometer. At the current stage the driver is failing on probe 
> because reset pin is not connected to VID (as datasheet requires in case
> of pin not being used). In case of reset pin not asserted, register's
> reset is triggered resulting in empty registers, leading to probe fail.
> For this reason pin is asserted during power on in order to have 
> informations in registers and deasserted before power off triggering
> a reset.
> 
> A workaround that gets AK09911 working on device is by setting the
> reset pin always high on device tree. This way registers gets reset by
> a Power On Reset circuit autonomously and reset pin never triggers the
> reset.

You need to distinguish electrical level from logical (GPIO flag defines
logical). So, I'm talking about active-high vs. active-low case.

Now I re-read above, and see that here you assert the reset signal. But where
is desertion?

-- 
With Best Regards,
Andy Shevchenko