Re: [PATCH] Fix autocentering command in hid-lgff driver

On Tuesday 14 of June 2011 17:20:52 you wrote:

quoted

Hello,

this patch fixes two issues with autocentering in lgff driver. Current
implementation incorrectly assumes that the saturation force is always
0x80 which is
inconsistent with behavior of the official driver. It also makes it
impossible to disable autocentering on some wheels - at least Logitech
Formula Force RX is
the case. Values of stiffness coefficient were also calculated
incorrectly. Formula used in this patch appears to generate the same
commands as the official
Logitech drivers. The patch also fixes two minor coding style issues.

Hi Michal,
Can you elaborate more on the difference between stiffness and saturation?
And how these might work in the Linux FF system which only accounts a
single parameter.

On my reverse engineering of the Wii Wheel (1) I noticed that the
AutoCentre consisted of 3 variables, but did realise the precise details.
Perhaps I can factor in a similar adjustment....

BTW - I also discovered some other modes of feedback beyond AutoCentre and
CF which might be applicable to other Logitech wheels. (2)

Simon

1.
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=dr
ivers/hid/hid-lg4ff.c;h=fa550c8e1d1bc639bd3e9c34a299903391bca44a;hb=3a2289a4
a317e0290a8bc7af28c62c9830cb12e5#l72

2. http://wiibrew.org/wiki/Logitech_USB_steering_wheel

Sure, but please don't take it like I have some inside knowledge of Logitech FF implementation. I've been given few bits of useful information, the rest I 
figured out from sniffed USB communication.

Centering force seems to be modeled as a spring effect. Force created by a spring can be calculated as "F = -k*x" where F = force, k = stiffness coefficient 
and x = deflection from centre. The higher the k is, the faster the centering force increases when the wheel is turned. Saturation force seems to be the limit 
centering force, so when kx > saturation_force => F = saturation_force.
Logitech driver has a slider which sets the centering force from 0 % to 150 % (it's actually from 0 % to 200 %, at least the values in the command suggest 
so) and both the stiffness coeff and saturation force increase gradually with the centering force according to the formula I provide in the patch, so just the 
magnitude value is enough. It looks like the stiffness coeff actually wraps around, because 0 = no stiffness, 7 = max stiffness, 8 = low stiffness, 15 = max 
stiffness. It'd be interesting to see if you can observe the same behavior on the Wii wheel.

As for the other FF effects, I guess these are things like "rumble" etc. I'm planning to look into it more when I'll have some extra free time.

Regards, Michal.