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ad714x wheel support and other shortcomings

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Hi guys,
(sorry for the long message, but there are A LOT of issues here in this 
driver...)

Ok, I took a step back after my failed mod 
(1335460639-1362-2-git-send-email-jeff.dagenais@xxxxxxxxx), and discovered many 
shortcomings in the driver code around the wheel feature, hw_init and more 
generically the abs_pos calculation algorithm. It looks like we're the only 
"kernel-participating" party that has tried to integrate the wheel in a real 
system...?

This is sort of a story based account of my recent dealings with the ad714x 
driver, I know it's chatty, but please bear with me...

The motion of the wheel near the roll around point (ex. between stages 7 and 0 
for an 8 stage wheel) has a dead zone. This is because the slices of max_coord 
being added up are too large, and near the last segment, the value is greater 
than max_coord, but is capped at max_coord, hence the dead zone. Now this 
effect, caused by the enlarged slices, is tolerable for a slider since there is 
no rolling around, but for the wheel, this is unusable.

Simply shrinking the slice size didn't fix the problem, the values capped at 
max_coord before the mid-point between the last and first stages, making a dead 
zone, then a skip when the finger nears the center of start_stage. So I came up 
with a new algorithm which relocates the positioning one turn of the wheel 
ahead, then modulo's the value back into the max_coord range to eliminate this 
problem.

I had to stepped away from the a_param and b_param based mean calculation 
because (and this is true for the slider as well) it has bumps in it. The bumps 
appear when the determined "highest_stage" changes. The recalculated values 
near this frontier skips ahead or backward by a noticeable amount, hence the 
"bump". It is especially annoying when you keep your finger around a tipping 
point between two stages. The value then skips by a large quantity rapidly back 
and forth. IMPORTANT NOTE: since the slider uses a similar algorithm, I tried 
telling the driver my wheel as a slider to invoke that code, and did the bump 
test there in the middle of my wheel, SAME PROBLEM!

My new algo still grabs the largest response and the two adjacent stages, the 
response "floor" (or 0) is brought up to the smallest of the two adjacent 
stages. This basically eliminates one of the adjacent stages and while 
adjusting the ratio between the largest response and the next largest one. With 
these two stages left, a proportion is given to the largest vs. the other. This 
becomes a vector which offsets the coord (+/-) from the largest response 
stage's center coordinate. Ultra simple and works really well. IT COULD/SHOULD 
BE PORTED TO THE SLIDER and/or to the generic ad714x_cal_abs_pos function.

Once I got that working, I got jerky behaviour from the reported position 
around the edges (i.e. 0 and max_coord). The cause was the flt_pos calculation 
which is basically broken for circular coordinates.

The problem is that when using a max_coord of 1024 for example, then coord 0 
equals coord 1024. So the abs_pos and old flt_pos have to be brought in the 
same "quadrant" (for lack of a better word) for the calculation to be valid. 
But this is still not enough for things to be smooth in the whole range of 
values.

The other issue one encounters is that, even if the values are in the same 
"quadrant" and you modulo the end value, when you add several turns to the 
coordinates for the flt_pos calculation, it doesn't yield the same result as if 
you don't. My solution was to offset the abs_pos and old flt_pos around 
max_coord, make the calculation and "de-offset" the result after. This means 
the calculation is always done using the same scale (i.e. max_coord).

The resulting position is regular and smooth. But then again, my abs_pos was 
fine without the flt_pos calculation. It made me wonder if the filtering, which 
is really just a time-base smoothing function, had been added because of the 
bump problem I talked about earlier. Any thoughts?

BTW, just so it doesn't go un-noticed in my upcoming patch, while refactoring 
this, I noticed a clear bug in the current ad714x_wheel_cal_abs_pos :
	first_before = (sw->highest_stage + stage_num - 1) % stage_num;
	highest = sw->highest_stage;
	first_after = (sw->highest_stage + stage_num + 1) % stage_num;
... this will fail IF start_stage IS NOT 0 for this wheel. I have changed it to 
something like this :
	int highest_idx_rel = sw->highest_stage - hw->start_stage;
	...
	first_before = ((highest_idx_rel + stage_num - 1) % stage_num)
	                            + hw->start_stage ;
	...
Agreed?

So now, using this strategy, the wheel motion is both precise and has no breaks 
or bumps in it. I even tested the code using stages 1..8 instead of 0..7 and it 
still works correctly. This suggests that my index calculations are ok.

(patch form was too noisy, I will send a patch after I get feedback if you guys 
don't mind)

static void ad714x_wheel_cal_abs_pos(struct ad714x_chip *ad714x, int idx)
{
	struct ad714x_wheel_plat *hw = &ad714x->hw->wheel[idx];
	struct ad714x_wheel_drv *sw = &ad714x->sw->wheel[idx];
	int stage_num = hw->end_stage - hw->start_stage + 1;
	/* index of the highest stage relative to start_stage */
	int highest_idx_rel = sw->highest_stage - hw->start_stage;
	/* the number of positions between each stages */
	int slice_size = DIV_ROUND_CLOSEST(hw->max_coord, stage_num);
	int a, b, c; /* the 3 vals to consider */
	int dir; /* direction of the adjustment from the highest stage pos */

	/* Init abs_pos at the highest stage's physical location, but one turn
	 * of the wheel ahead (modulo'd later down), then add half the slice
	 * size because we want coordinate 0 to be half way between end_stage
	 * and start_stage.
	 */
	sw->abs_pos = (slice_size * highest_idx_rel)
		       + hw->max_coord + (slice_size/2);

	/* grab the three values we are interested in. These are the highest
	 * index, and the one before and after, in a circular roll-over type
	 * increment and decrement, also considering start_stage != 0.
	 */
	a = ad714x->sensor_val[((highest_idx_rel + stage_num - 1) % stage_num)
			       + hw->start_stage];
	b = ad714x->sensor_val[sw->highest_stage];
	c = ad714x->sensor_val[((highest_idx_rel + stage_num + 1) % stage_num)
			       + hw->start_stage];

	/* eliminate the smallest val from the equation, by substracting the
	 * smallest to all values, in other words, bring the signal reference
	 * up to the smallest value of the 3. After this "if-else", 'bM is
	 * still the highest val, 'a' contains the second biggest val, and
	 * 'dir' contains a record of the direction we need to adjust abs_pos.
	 *        : .                          . :
	 *      : : :                          : : :
	 *  if: a b c  adjust right (1), else: a b c adjust left (-1)
	 *
	 */
	if(a < c) {
		c -= a;
		b -= a;
		a = c;
		dir = 1;
	} else {
		a -= c;
		b -= c;
		dir = -1;
	}
	/* add/substract a proportional to a/a+b quantity to abs_pos */
	sw->abs_pos = (sw->abs_pos +
		       DIV_ROUND_CLOSEST(a * dir * slice_size, a+b)) %
		       hw->max_coord;
}

static void ad714x_wheel_cal_flt_pos(struct ad714x_chip *ad714x, int idx)
{
	struct ad714x_wheel_plat *hw = &ad714x->hw->wheel[idx];
	struct ad714x_wheel_drv *sw = &ad714x->sw->wheel[idx];
	int half_coord_range = hw->max_coord/2;
	int abs_pos = sw->abs_pos;
	int diff = sw->abs_pos - sw->flt_pos;

	/* try to put both pos within max_coord/2 of each other by adding
	 * one turn of the wheel, this turn is removed by modulo after calc.
	 */
	if (diff > half_coord_range)
		sw->flt_pos += hw->max_coord;
	else if (diff < -half_coord_range)
		abs_pos += hw->max_coord;

	/* if difference is still too great, just use abs_pos */
	if (abs(abs_pos - sw->flt_pos) > half_coord_range)
		sw->flt_pos = sw->abs_pos;
	else {
		/* for the filter to work without "breakage" around the wheel,
		 * we need to offset the values to bring the two values around
		 * max_coord. Pretend the old flt_pos is max_coord.
		 */
		diff = hw->max_coord - sw->flt_pos;
		abs_pos += diff;

		sw->flt_pos = (DIV_ROUND_CLOSEST(((hw->max_coord * 30) +
						 (abs_pos * 71)), 100) - diff)
			       % hw->max_coord;
	}
}



Alright, while I have your attention... some more questions:

In hw_init, why do we read back all the sys registers but do nothing with the 
data?

Also, a few lines further in hw_init:
ad714x->write(ad714x, AD714X_STG_CAL_EN_REG, 0xFFF);
...which completely disregards the settings provided by platform init 
(ad714x->hw->sys_cfg_reg[1]) which are programmed a few lines before for 
nothing basically. I can understand that the driver could "hard-code" the 
calib_en feature for it's behaviour, but writing 0xfff overwrites AVG_F/LP_SKIP 
registers to 0. Since the settings are provided by platform, I would just 
delete the line that does this , and trust the platform to init those properly, 
it is already responsible for writing most of the registers anyway.

Another weird thing is the presence of the 3 STAGE_(LOW/HIGH/COMP)_INT_ENABLE 
registers in the platform init structure, even though the driver specifically 
overwrites those in the ad714x_use_(com/thr)_int functions. I would shrink the 
platform data's sys_cfg_reg array to 5 since these last three registers are 
under the control of the driver, and the other configuration item in these 3 
regs is the GPIO feature, which is not useable by the current driver code 
anyway.


Thanks for reading through!--
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