linux-kernel - Re: [PATCH] HID: playstation: Remap joystick axes to be centered at 0

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Message-ID: <CAKF84v3zxT5siCOnXwn6Xc80T4vTDpP08K1Pd-J8XyBtv-V3Mw@mail.gmail.com>
Date: Wed, 5 Nov 2025 11:41:03 -0800
From: Siarhei Vishniakou <svv@...gle.com>
To: Jiri Kosina <jikos@...nel.org>
Cc: Benjamin Tissoires <bentiss@...nel.org>, 
	Roderick Colenbrander <roderick.colenbrander@...y.com>, linux-input@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] HID: playstation: Remap joystick axes to be centered at 0

пт, 31 окт. 2025 г. в 02:00, Jiri Kosina <jikos@...nel.org>:
>
> On Fri, 10 Oct 2025, Siarhei Vishniakou wrote:
>
> > The joystick axes (ABS_X, ABS_Y, ABS_RX, ABS_RY) for PlayStation
> > gamepads report a neutral state of 128 over HID, with a full range of
> > [0, 255]. The driver previously mapped this directly, resulting in an
> > evdev range of [0, 255] with a resting point of 128.
> >
> > This approach is unconventional for Linux gamepad drivers and has several
> > drawbacks: it requires userspace applications to be aware of the
> > non-zero resting state, and it is incompatible with the input
> > subsystem's 'flat' (deadzone) logic, which assumes a resting point of 0.
> >
> > This patch remaps the four joystick axes to the conventional signed
> > 8-bit range of [-128, 127], with 0 as the neutral state. This is
> > accomplished by changing their evdev range in ps_gamepad_create() and
> > translating the incoming hardware value in the report parsing functions
> > by subtracting 128.
> >
> > The analog trigger axes (ABS_Z, ABS_RZ) are handled separately. Their
> > resting state is 0 (un-pressed), and their hardware range of [0, 255]
> > is already the conventional representation. They are left unmodified by
> > this patch.
> >
> > This makes the joystick behavior consistent with other gamepad drivers
> > while preserving the standard behavior for the triggers.
> >
> > Signed-off-by: Siarhei Vishniakou <svv@...gle.com>
>
> Hmmm ... as I am rather unaware of the existing playstation userspace
> ecosystem -- is there any way how we could now be breaking existing
> playstation-specific userspace assumptions that have already developed
> based on asumptions that we're centered at 128?
>
> Thanks,
>
> --
> Jiri Kosina
> SUSE Labs
>

Thanks Jiri for looking! I chatted with Roderick, and he brought up
similar concerns. Generally, most of the joysticks have the "normal"
ranges with the 0 value at the center. However, I understand that some
user-space apps might have hardcoded some of these Sony-specific
assumptions.

Instead, I will change this patch to provide correct initial
(centered) values of 0x80 when the driver first starts. This is needed
in order to fix our Sony gamepad tests on Android (and will help
everyone else, because now there won't be a stray "bring to center"
event emitted from the gamepad in the beginning). I will follow up
later this week.