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

From: "Nuno Sá" <noname.nuno@gmail.com>
To: Jonathan Cameron <jic23@kernel.org>
Cc: David Lechner <dlechner@baylibre.com>,
	nuno.sa@analog.com, linux-kernel@vger.kernel.org,
	devicetree@vger.kernel.org, linux-iio@vger.kernel.org,
	Olivier MOYSAN <olivier.moysan@foss.st.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	Frank Rowand <frowand.list@gmail.com>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Michael Hennerich <Michael.Hennerich@analog.com>
Subject: Re: [PATCH 04/12] iio: adc: ad9467: fix reset gpio handling
Date: Mon, 04 Dec 2023 17:41:48 +0100	[thread overview]
Message-ID: <9b11a42d83dfe77215bafae1d116375ee2398ae6.camel@gmail.com> (raw)
In-Reply-To: <20231204151514.4e2c8ada@jic23-huawei>

On Mon, 2023-12-04 at 15:15 +0000, Jonathan Cameron wrote:
> On Sat, 02 Dec 2023 09:36:47 +0100
> Nuno Sá <noname.nuno@gmail.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@gmail.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@kernel.org> wrote:  
> > > > > > 
> > > > > > From: Nuno Sa <nuno.sa@analog.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.
> 

Ahh I see. Well would not really be a fix :)

- Nuno Sá