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:

quoted

On 2023-12-15 03:40, Chris Morgan wrote:

quoted

From: Chris Morgan <redacted>

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 <redacted>
---
 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