linux-kernel - Re: [linux-sunxi] Re: [PATCH] arm: sunxi: input: RFC: Add sysfs voltage for sun4i-lradc driver

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Date:	Tue, 27 Jan 2015 11:34:01 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Priit Laes <plaes@...es.org>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	linux-sunxi@...glegroups.com, Hans de Goede <hdegoede@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-input@...r.kernel.org
Subject: Re: [linux-sunxi] Re: [PATCH] arm: sunxi: input: RFC: Add sysfs
 voltage for sun4i-lradc driver

On Tue, Jan 27, 2015 at 11:49:49AM +0200, Priit Laes wrote:
> 
> On Tue, 2015-01-27 at 10:18 +0100, Maxime Ripard wrote:
> > Hi,
> > 
> > On Mon, Jan 26, 2015 at 06:58:32PM +0200, Priit Laes wrote:
> > > ---
> > 
> > Like Hans was pointing out, commit log and signed-off-by please
> > 
> > >  .../ABI/testing/sysfs-driver-input-sun4i-lradc     |  4 ++
> > >  drivers/input/keyboard/sun4i-lradc-keys.c          | 49 
> > > +++++++++++++++++-----
> > >  2 files changed, 43 insertions(+), 10 deletions(-)
> > >  create mode 100644 Documentation/ABI/testing/sysfs-driver-input-
> > > sun4i-lradc
> > > 
> > > diff --git a/Documentation/ABI/testing/sysfs-driver-input-sun4i-
> > > lradc b/Documentation/ABI/testing/sysfs-driver-input-sun4i-lradc
> > > new file mode 100644
> > > index 0000000..e4e6448
> > > --- /dev/null
> > > +++ b/Documentation/ABI/testing/sysfs-driver-input-sun4i-lradc
> > > @@ -0,0 +1,4 @@
> > > +What:          /sys/class/input/input(x)/device/voltage
> > > +Date:          February 2015
> > > +Contact:       Priit Laes <plaes@...es.org>
> > > +Description:   ADC output voltage in microvolts or 0 if device is 
> > > not opened.
> > 
> > Why is it returning 0 when "device is not opened" ? What does that 
> > even mean? You can't read that file without opening it.
> 
> It means that something has to open the /dev/input/inputX device which 
> sets up the ADC before the voltage can be read from the sysfs file.

I'd consider this a bug. Why someone has to use an unrelated interface for
another interface to work correctly?

> 
> [...]
> 
> 
> > 
> > As I told you already, if you're going to expose this an ADC in the 
> > end, the proper solution is to use the IIO framework, not adding a 
> > custom sysfs file.
> 
> My intention was to expose just a simple debug output, so one can 
> press the buttons and read the voltages for devicetree keymap.
> 
> If anyone can suggest a simpler approach than current sysfs based one, 
> I would do it. But full blown iio driver is currently out of scope. 
> Also, Carlo's (ccaione) initially submitted (?) driver for lradc 
> utilized iio subsystem.

For stuff like this debugfs (AKA dumping ground) is better suited as it does
not establish ABI.  i

Thanks.

-- 
Dmitry
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