linux-kernel - Re: [PATCH 04/12] iio: adc: ad9467: fix reset gpio handling

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Date:   Mon, 4 Dec 2023 15:15:14 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Nuno Sá <noname.nuno@...il.com>
Cc:     David Lechner <dlechner@...libre.com>, nuno.sa@...log.com,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-iio@...r.kernel.org,
        Olivier MOYSAN <olivier.moysan@...s.st.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>
Subject: Re: [PATCH 04/12] iio: adc: ad9467: fix reset gpio handling

On Sat, 02 Dec 2023 09:36:47 +0100
Nuno Sá <noname.nuno@...il.com> wrote:

> On Fri, 2023-12-01 at 11:01 -0600, David Lechner wrote:
> > On Fri, Dec 1, 2023 at 2:47 AM Nuno Sá <noname.nuno@...il.com> wrote:  
> > > 
> > > On Thu, 2023-11-30 at 15:41 -0600, David Lechner wrote:  
> > > > On Tue, Nov 21, 2023 at 4:17 AM Nuno Sa via B4 Relay
> > > > <devnull+nuno.sa.analog.com@...nel.org> wrote:  
> > > > > 
> > > > > From: Nuno Sa <nuno.sa@...log.com>
> > > > > 
> > > > > The reset gpio was being requested with GPIOD_OUT_LOW which means, not
> > > > > asserted. Then it was being asserted but never de-asserted which means
> > > > > the devices was left in reset. Fix it by de-asserting the gpio.  
> > > > 
> > > > It could be helpful to update the devicetree bindings to state the
> > > > expected active-high or active-low setting for this gpio so it is
> > > > clear which state means asserted.
> > > >   
> > > 
> > > You could state that the chip is active low but I don't see that change that
> > > important for now. Not sure if this is clear and maybe that's why your comment.
> > > GPIOD_OUT_HIGH has nothing to do with active high or low. It just means, "get me
> > > the
> > > pin in the asserted state".
> > >   
> > 
> > I would assume that this bug happened in the first place because
> > someone forgot GPIOD_OUT_LOW in the devicetree when they were
> > developing the driver. So this is why I suggested that updating the
> > devicetree binding docs so that future users are less likely to make
> > the same mistake. Currently, the bindings don't even have reset-gpios
> > in the examples.  
> 
> Hmm, I think you're missing the point... The bug has nothing to do with devicetree.
> This is what was happening:
> 
> 1) We were calling devm_gpiod_get_optional() with GPIOD_OUT_LOW. What this means is
> that you get an output gpio deasserted. Hence the device is out of reset. And here is
> the important part... what you have in dts does not matter. If you have active low,
> it means the pin level will be 1. If you have high, the pin level is 0. And this is
> all handled by gpiolib for you. 
> 
> 2) Then, we called gpiod_direction_output(..., 1), which means set the direction out
> (which is actually not needed since it was already done when getting the pin) and
> assert the pin. Hence, reset the device. And we were never de-asserting the pin so
> the device would be left in reset.

Functionally I believe David is correct.   Flipping the DT would 'fix' this.
It's all down to a nreset vs reset pin description.

In this case I guess it's defined a a 'not reset' on the datasheet which is what
is causing the confusion.  It's not uncommon for people to refer to a reset when
they mean a "not reset" with assumptions on polarity to match.

Jonathan



> 
> - Nuno Sá