linux-kernel - Re: [PATCH 5/5] iio: addac: ad74413r: add support for reset-gpio

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Date:   Tue, 15 Nov 2022 16:10:52 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Nuno Sá <noname.nuno@...il.com>
CC:     Jonathan Cameron <jic23@...nel.org>,
        "Tanislav, Cosmin" <Cosmin.Tanislav@...log.com>,
        Rob Herring <robh+dt@...nel.org>,
        "Rasmus Villemoes" <linux@...musvillemoes.dk>,
        Lars-Peter Clausen <lars@...afoo.de>,
        "Hennerich, Michael" <Michael.Hennerich@...log.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] iio: addac: ad74413r: add support for reset-gpio

On Tue, 15 Nov 2022 15:49:46 +0100
Nuno Sá <noname.nuno@...il.com> wrote:

> On Mon, 2022-11-14 at 19:44 +0000, Jonathan Cameron wrote:
> > On Mon, 14 Nov 2022 13:52:26 +0000
> > "Tanislav, Cosmin" <Cosmin.Tanislav@...log.com> wrote:
> >   
> > > > -----Original Message-----
> > > > From: Jonathan Cameron <jic23@...nel.org>
> > > > Sent: Saturday, November 12, 2022 7:07 PM
> > > > To: Rasmus Villemoes <linux@...musvillemoes.dk>
> > > > Cc: Tanislav, Cosmin <Cosmin.Tanislav@...log.com>; Lars-Peter
> > > > Clausen
> > > > <lars@...afoo.de>; Hennerich, Michael
> > > > <Michael.Hennerich@...log.com>;
> > > > devicetree@...r.kernel.org; Rob Herring <robh+dt@...nel.org>;
> > > > linux-
> > > > iio@...r.kernel.org; linux-kernel@...r.kernel.org
> > > > Subject: Re: [PATCH 5/5] iio: addac: ad74413r: add support for
> > > > reset-gpio
> > > > 
> > > > [External]
> > > > 
> > > > On Fri, 11 Nov 2022 15:39:21 +0100
> > > > Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
> > > >     
> > > > > We have a board where the reset pin of the ad74412 is connected
> > > > > to a
> > > > > gpio, but also pulled low by default. Hence to get the chip out
> > > > > of
> > > > > reset, the driver needs to know about that gpio and set it high
> > > > > before
> > > > > attempting to communicate with it.    
> > > > 
> > > > I'm a little confused on polarity here.  The pin is a !reset so
> > > > we need to drive it low briefly to trigger a reset.
> > > > I'm guessing for your board the pin is set to active low? (an
> > > > example
> > > > in the dt would have made that clearer) Hence the pulse
> > > > in here to 1 is actually briefly driving it low before restoring
> > > > to high?
> > > > 
> > > > For a pin documented as !reset that seems backwards though you
> > > > have
> > > > called it reset so that is fine, but this description doesn't
> > > > make that
> > > > celar.    
> > > 
> > > My opinion is that the driver shouldn't exactly know the polarity
> > > of the reset,
> > > and just assume that setting the reset GPIO to 1 means putting it
> > > in reset,
> > > and setting it to 0 means bringing out of reset.  
> > 
> > Agreed. I'd just like a comment + example in the dt-binding to make
> > the point
> > that the pin is !reset.
> > 
> > Preferably with an example in the dt binding of the common case of it
> > being wired
> > up to an active low pin.
> > 
> > The main oddity here is the need to pulse it rather than request it
> > directly as
> > in the reset state and then just set that to off.
> > 
> >   
> 
> Agreed... In theory we should be able to request the gpio with
> GPIOD_OUT_HIGH and then just bring the device out of reset

If I recall correctly the datasheet specifically calls out that a pulse
should be used.  No idea if that's actually true, or if it was meant
to be there just to say it needs to be set for X nsecs.

Jonathan

> 
> - Nuno Sá
>