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

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Date:   Mon, 18 May 2020 18:01:20 +0200
From:   Jonathan Albrieux <jonathan.albrieux@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "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 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.

> -- 
> With Best Regards,
> Andy Shevchenko

Hoping to having answered to your question,
Best regards,
Jonathan Albrieux