Re: [PATCH v2] HID: playstation: Center initial joystick axes to prevent spurious events

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Nov 11, 2025 at 03:45:19PM -0800, Siarhei Vishniakou wrote:
> When a new PlayStation gamepad (DualShock 4 or DualSense) is initialized,
> the input subsystem sets the default value for its absolute axes (e.g.,
> ABS_X, ABS_Y) to 0.
> 
> However, the hardware's actual neutral/resting state for these joysticks
> is 128 (0x80). This creates a mismatch.
> 
> When the first HID report arrives from the device, the driver sees the
> resting value of 128. The kernel compares this to its initial state of 0
> and incorrectly interprets this as a delta (0 -> 128). Consequently, it
> generates EV_ABS events for this initial, non-existent movement.
> 
> This behavior can fail userspace 'sanity check' tests (e.g., in
> Android CTS) that correctly assert no motion events should be generated
> from a device that is already at rest.
> 
> This patch fixes the issue by explicitly setting the initial value of the
> main joystick axes (e.g., ABS_X, ABS_Y, ABS_RX, ABS_RY) to 128 (0x80)
> in the common ps_gamepad_create() function.
> 
> This aligns the kernel's initial state with the hardware's expected
> neutral state, ensuring that the first report (at 128) produces no
> delta and thus, no spurious event.
> 
> Signed-off-by: Siarhei Vishniakou <svv@google.com>
> Reviewed-by: Benjamin Tissoires <bentiss@kernel.org>
> ---
>  drivers/hid/hid-playstation.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/hid/hid-playstation.c b/drivers/hid/hid-playstation.c
> index 1468fb11e39d..a145b5ea4405 100644
> --- a/drivers/hid/hid-playstation.c
> +++ b/drivers/hid/hid-playstation.c
> @@ -718,11 +718,16 @@ static struct input_dev *ps_gamepad_create(struct hid_device *hdev,
>  	if (IS_ERR(gamepad))
>  		return ERR_CAST(gamepad);
>  
> +	/* Set initial resting state for joysticks to 128 (center) */
>  	input_set_abs_params(gamepad, ABS_X, 0, 255, 0, 0);
> +	gamepad->absinfo[ABS_X].value = 128;

input_set_abs_params() can fail to allocate absinfo. If that happens,
this will crash. AI suggests setting the value with input_abs_set_val()
to ensure that the value is checked before dereferencing.

Guenter

>  	input_set_abs_params(gamepad, ABS_Y, 0, 255, 0, 0);
> +	gamepad->absinfo[ABS_Y].value = 128;
>  	input_set_abs_params(gamepad, ABS_Z, 0, 255, 0, 0);
>  	input_set_abs_params(gamepad, ABS_RX, 0, 255, 0, 0);
> +	gamepad->absinfo[ABS_RX].value = 128;
>  	input_set_abs_params(gamepad, ABS_RY, 0, 255, 0, 0);
> +	gamepad->absinfo[ABS_RY].value = 128;
>  	input_set_abs_params(gamepad, ABS_RZ, 0, 255, 0, 0);
>  
>  	input_set_abs_params(gamepad, ABS_HAT0X, -1, 1, 0, 0);
> -- 
> 2.51.2.1041.gc1ab5b90ca-goo