Re: [RFC] dt-bindings: input: Clarify that abs_min must be less than abs_max

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Sorry, meant to add Peter Hutterer to the conversation, but forgot
before hitting send...

On Tue, Dec 19, 2023 at 12:32:44PM -0800, Dmitry Torokhov wrote:
> On Fri, Dec 15, 2023 at 12:19:51PM +0100, Artur Rojek wrote:
> > On 2023-12-15 03:40, Chris Morgan wrote:
> > > From: Chris Morgan <macromorgan@hotmail.com>
> > > 
> > > uinput refuses to work with abs devices where the min value is greater
> > > than the max value. uinput_validate_absinfo() returns -EINVAL if this
> > > is the case and prevents using uinput on such a device. Since uinput
> > > has worked this way since at least kernel 2.6 (or prior) I presume that
> > > this is the correct way of doing things, and that this documentation
> > > needs to be clarified that min must always be less than max.
> > > 
> > > uinput is used in my use case to bind together adc-joystick devices
> > > with gpio-keys devices to create a single unified gamepad for
> > > userspace.
> > > 
> > > Note that there are several boards that will need to be corrected,
> > > all but a few of them I maintain. Submitting as an RFC for now to get
> > > comments from the input team and the original author in case there is
> > > something I am missing.
> > > 
> > > Fixes: 7956b0d4694f ("dt-bindings: input: Add docs for ADC driven
> > > joystick")
> > > 
> > > Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
> > > ---
> > >  Documentation/devicetree/bindings/input/adc-joystick.yaml | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/input/adc-joystick.yaml
> > > b/Documentation/devicetree/bindings/input/adc-joystick.yaml
> > > index 6c244d66f8ce..8f5cdd5ef190 100644
> > > --- a/Documentation/devicetree/bindings/input/adc-joystick.yaml
> > > +++ b/Documentation/devicetree/bindings/input/adc-joystick.yaml
> > > @@ -73,8 +73,9 @@ patternProperties:
> > >          description: >
> > >            Minimum and maximum values produced by the axis.
> > >            For an ABS_X axis this will be the left-most and right-most
> > > -          inclination of the joystick. If min > max, it is left to
> > > userspace to
> > > -          treat the axis as inverted.
> > > +          inclination of the joystick. The axis must always be
> > > expressed as
> > > +          min < max, if the axis is inverted it is left to userspace to
> > > handle
> > > +          the inversion.
> > 
> > Hi Chris,
> > 
> > Device Tree is supposed to depict the actual state of the hardware.
> > I worded the adc-joytick's adc-range property specifically, so that it
> > covers a case of GCW Zero hardware [1], which has a joystick,  where the
> > ABS_X axis reports increasing values for the left-wards inclination of
> > the joystick, and decreasing values for the right-wards inclination. You
> > are saying that there are even more boards that need to be corrected -
> > those are all situations, where DT depicts the actual behavior of the
> > hardware.
> > What you are trying to do is change hardware description, because of how
> > a driver in an OS works. You should instead fix behavior of said driver,
> > even if nobody stumbled upon that issue since 2.6 :) We fixed libSDL [2]
> > for the same reason.
> 
> We have several places in the kernel (such as mousedev and joydev) where
> we expect that max is greater or equal to min if they are specified. I
> am sure that at least some userspace components also have this
> assumption. In general, we expect min to be a minimum value reported and
> max being maximum value reported, and orientation expressed via
> different properties (see [1]).
> 
> Since we codified min > max as inversion for adc-joystick devices in the
> bindings, I think we need to handle this *in that driver* and leave the
> rest alone.
> 
> [1] Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml
> 

-- 
Dmitry