Re: [PATCH-V2 1/1] HID: Force feedback support for the Logitech G920

On Mon, Jan 11, 2016 at 12:25 AM, Edwin Velds [off-list ref] wrote:

This patch implements force feedback support for the Logitech
G920 Driving Force Racing Wheel. It is a generic implementation
of feature 0x8123 of the Logitech HID++ protocol and should be
usable for any future devices that implement this feature.

This patch should be applied after the basic G920 support patch
by Simon Wood:
http://www.spinics.net/lists/linux-input/msg42174.html

The driving supports everything that is supported by the G920
firmware:
    FF_CONSTANT
    FF_PERIODIC
    FF_SINE
    FF_SQUARE
    FF_SAW_UP
    FF_SAW_DOWN
    FF_TRIANGLE
    FF_SPRING
    FF_DAMPER
    FF_AUTOCENTER
    FF_GAIN
and for version 2 firmware also:
    FF_FRICTION
    FF_INERTIA
    FF_RAMP

Both envelopes and replay values are supported as well, but some
problems may occur when using firmware release 1. There is also a
small residual clockwise damper in the wheel when using the first
firmware release. All problems are fixed in the soon te be released
firmware version 2.

The default spring is disabled by permanently placing a spring
force in the wheel. This spring is also used as the autocenter
spring.

Signed-off-by: Edwin Velds <redacted>

Tested-by: Elias Vanderstuyft <redacted>

Tested with both firmware version 1 and firmware version 2.

@Dmitry:
As side note, Edwin intentionally did *not* put a clamp(0, 0x7FFF, x)
in the driver,
with x being envelope.attack_level or envelope.fade_level,
since this parameter checking is (/ should be) the task of "ff_core.c".
The device will behave odd when playing an effect that violates this rule,
although most user-space programs don't let them exceed 0x7FFF.
So this is a TODO for "ff-core.c", but not a blocker for this patch,
since the odd behavior will only be triggered during playback of such
violating effect,
and vanishes when the effect stops.

Other note:
For anyone who is interested on which spec this patch is based,
take a look at the excellently written "Logitech Force Feedback
Protocol" document:
https://opensource.logitech.com/opensource/index.php/Technical_Information

Thanks,
Elias