Related Wine issue: https://bugs.winehq.org/show_bug.cgi?id=52714
I'm starting work on updating/extending Linux force feedback API and one of my goals is to expose the number and an array that contains the device's ffb-enabled axes. As I'm mainly working with USB PID driver, I already have a POC of obtaining this data.
What prompted this is that currently Wine always creates virtual joysticks with two PID ffb axes. This causes issues with [Richard Burns Rally](https://github.com/ValveSoftware/Proton/issues/6702#issuecomment-2670946977), as this game incorrectly initializes a `CONSTANT_FORCE` effect with all the FFB axes that contain `DIDOI_FFACTUATOR` flag and then trying to update `cAxes` and `rgdwAxes` values which leads to `DIERR_INVALIDPARAM`.
Now, this flag is set by wine while enumerating virtual device's axes and while `(axis index < FFB axes)`, flag is applied. This means, that every device which has FFB functionality and at least two axes, will report FFB on `X`, `Y`.
While updated API would mean a lot of steering wheels would correctly report only 1 axis, a lot of USB PID wheels still include `X` and `Y` in their `AXES_ENABLE` and `DIRECTION` usages. This, again, would lead to broken FFB (in affected games).
On Windows, there's a possibility of overwriting some joystick capabilities with registry entries, which ends up removing `DIDOI_FFACTUATOR` flag from a selected axis. This doesn't work in Wine.
This MR allows Wine to add an arbitrary number (up to `PID_AXES_MAX`) of axes to the PID descriptor, based on the value of Haptic Axes from SDL or from Linux FF api in the future (I'm working on it). Additionally, to work around RBR and other games with similar, wrong FFB handling, I introduced a [hint](https://github.com/libsdl-org/SDL/issues/12341) to SDL that allows a dynamic overwrite of the number of haptic axes. For the Linux API, I'll figure out a nice way to override when the number of axes will be exposed.
With these changes, I was able to successfully get force feedback on my Moza Racing R9 with all axes usable. With additional traces in the dinput code, I confirmed that with one axis defined in the PID descriptor, the game created a constant force effect with just one axis and `cAxes = 1`.
-- v25: dinput/tests: Add tests for 6-axis ff joystick