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

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Message-ID: <20200518174335.GA6392@jonathan-N53SV>
Date:   Mon, 18 May 2020 19:43:35 +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>,
        "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 07:43:17PM +0300, Andy Shevchenko wrote:
> 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?

Oh I see, I'll try explaining by points the proposed approach:
- reset pin is active low
- during power on gpio is set to 0 so the reset pin is high, thus no reset
- during power off gpio is set to 1 so the reset pin becomes low, thus resetting

this is a possible solution but maybe there are other ways to achieve that, 
do you have suggestions on how to get a better approach for solving this issue?

> -- 
> With Best Regards,
> Andy Shevchenko
> 
> 

Best regards,
Jonathan Albrieux