Date:   Tue, 23 Apr 2019 10:57:40 +0200
From:   Alexandre <amergnat@...libre.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     robh+dt@...nel.org, mark.rutland@....com, knaack.h@....de,
        lars@...afoo.de, pmeerw@...erw.net, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, baylibre-upstreaming@...ups.io,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        linux-input@...r.kernel.org
Subject: Re: [PATCH 3/3] iio: Add PAT9125 optical tracker sensor

Hi Jonathan,

On 4/22/19 10:42, Jonathan Cameron wrote:
> On Tue, 16 Apr 2019 14:49:19 +0200
> Alexandre <amergnat@...libre.com> wrote:
>
>> Hello Jonathan,
>>
>> On 4/7/19 12:20, Jonathan Cameron wrote:
>>> Hi Alexandre,
>>>
>>> So I have no problem with this as an IIO driver, but for devices that
>>> are somewhat 'on the edge' I always like to get a clear answer to the
>>> question: Why not input?
>>>
>>> I would also argue that, to actually be 'useful' we would typically need
>>> some representation of the 'mechanicals' that are providing the motion
>>> being measured.  Looking at the datasheet this includes, rotating shafts
>>> (side or end on), disk edges and flat surface tracking (mouse like).
>>>
>>> That's easy enough to do with the iio in kernel consumer interface. These
>>> are similar to when we handle analog electronic front ends.
>>>
>>> I you can, please describe what it is being used for in your application
>>> as that may give us somewhere to start!
>>>
>>> + CC Dmitry and linux-input.
>> I developed this driver to detect the board movement which can't be
>> detected by accelerometer (very slow motion). I admit this use case can
>> be handled by an input, and I'm agree with you, PAT9125 driver could be
>> an input. But, like you said, this chip is able to track different kind
>> of motion, and additionally have an interrupt GPIO, so using it like
>> input limit the driver potential. This chip is designed to work in
>> industrial measurement or embedded systems, and the IIO API match with
>> these environments, so it's the best way to exploit the entire potential
>> of this chip.
>>
>> As I understand (from
>> https://www.kernel.org/doc/html/v4.12/input/event-codes.html#mice ),
>> mouse driver must report values when the device move. This feature
>> souldn't be mandatory for an optical tracker driver, specially for cases
>> where user prefers to use buffer or poll only when he need data.
>>
>>> If 1 or 2, I would suggest that you provide absolute position to
>>> Linux.  So add the value to a software counter and provide that.
>>> 32 bits should be plenty of resolution for that.
>> I can't provide absolute position, only relative. Do you mean using
>> input driver to do that ? If not, how is built the position data?
> Sorry, I should have been clearer on this.
> I mean absolute relative to the start point.  So on startup you assume
> absolute position is 0 and go from there.  What I can't work out is
> if the device does internal tracking, or whether each time you read
> it effectively resets it's internal counters to 0 so the next measurement
> is relative to the previous one.
Each time you read that reset internal counters to 0.
>>> Silly question for you.  What happens if you set the delta values to 0?
>>> Do we get an interrupt which is effectively data ready?
>>> If we do, you might want to think about a scheme where that is an option.
>>> As things currently stand we have a confusing interface where changing this
>>> threshold effects the buffered data output.   That should only be the
>>> case if this interface is for a trigger, not an event.
>> I'm not sure to understand your question. Is it possible to read delta_x
>> and delta_y = 0 in special/corner case because internal value continue
>> to be updated after toggled motion_detect pin (used for IRQ) until
>> values registers are read and then motion_detect pin is released:
>>
>>    * Chip move (i.e. +2 on X axis and 0 on Y axis)
>>    * Motion_detect IRQ trigger and internal reg value is updated (i.e.
>>      delta_x = 2 and delta_y = 0.
>>    * GPIO IRQ handled but read_value isn't executed yet (timing reason)
>>    * Chip move back to it origin point (i.e. -2 on X axis and 0 on Y axis)
>>    * Motion_detect IRQ still low because it hasn't been reset by read
>>      value and internal reg value is updated (i.e. delta_x = 0 and
>>      delta_y = 0)
>>    * Read_value is executed, we get delta values = 0.
> Again, I was unclear.  Is it possible to set the device to interrupt
> every time it evaluates whether motion has occured? Not only when it
> concludes that there has been some motion.  That would allow the interrupt
> to be used as a signal that the device has taken a measurement (data
> ready signal in other sensors).
>
I don't know, the datasheet don't describe the role of each bit in 
registers and I don't found documentation which provide that. I had to 
do research on example code to retrieve some bits, but got nothing on 
motion detection pin configuration.

>>> If it is actually not possible to report the two channels separately
>>> then don't report them at all except via the buffered interface and
>>> set the available scan masks so that both are on.
>> I found a way to keep the consistency between delta x and delta y
>> (without losing data). The first part is to reset a value only when user
>> read it (also when it's buffered). The second part is to add the new
>> value to the old value. With these two mechanism, X and Y will always be
>> consistent:
>>
>>    * as possible during a move.
>>    * perfectly when move is finished.
> Ah. This adding old value to a new value point is what I was getting
> at (I think) with 'absolute' position above.
>
> In industrial control for example you have absolute position by using
> limit switches to set your baseline.  Measurement devices are then
> capable of either reporting relative position, which is the movement
> since the last reading was taken, or 'absolute' position which is
> referenced to some known point.  It was this form of absolute position
> that I was suggesting you use.  If you use such a system without a
> limit switch it is normally called unreference motion.  You can do
> it but then the 0 is where ever your device was at power on.
> For some systems it doesn't actually matter (conveyor belts for
> instance where the positions you care about are between things
> on the belt, not the position of the belt itself).

Ok, I decided to return delta between last read/buffering to stay closer 
to the hardware mechanism and still coherent with "IIO_CHAN_INFO_RAW". 
If user want absolute position, he can make an addition of all received 
value in user space, and that allow him to reset/replace the initial 
position when he want it.

> Thanks,
>
> Jonathan
>
>>
>> Regards,
>>
>> Alexandre
>>

Thanks for your comments,

Alexandre

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