Re: [RFC PATCH 1/2] Input: rotary-encoder- Add support for absolute encoder

[Vignesh R <vigneshr@ti.com>]

Hi,

On 05/23/2016 06:48 PM, Uwe Kleine-König wrote:
> Hello,
> 
> On Mon, May 23, 2016 at 04:48:40PM +0530, R, Vignesh wrote:
>> On 5/22/2016 3:56 PM, Uwe Kleine-König wrote:
>>> Hello,
>>>
>>> On Thu, May 19, 2016 at 02:34:00PM +0530, Vignesh R wrote:
>>>> There are rotary-encoders where GPIO lines reflect the actual position
>>>> of the rotary encoder dial. For example, if dial points to 9, then four
>>>> GPIO lines connected to the rotary encoder will read HLLH(1001b = 9).
>>>> Add support for such rotary-encoder.
>>>> The driver relies on rotary-encoder,absolute-encoder DT property to
>>>> detect such encoders.
>>>> Since, GPIO IRQs are not necessary to work with
>>>> such encoders, optional polling mode support is added using
>>>
>>> I don't understand this. It's necessary in the same way as with the
>>> already supported devices. I.e. you want to trigger an irq when the
>>> encoder is moved and then check for it's position in the handler.
>>>
>>
>> Unlike already supported device, there is no need to count steps or
>> determine the direction of rotation. Hence, IRQ is not a requirement,
>> periodically polling the gpio lines is more than sufficient. With
>> absolute encoder, you are able to determine the current position at any
>> time just by looking at the gpio inputs.
> 
> The timing might not be that critical, but there is no reason to operate
> this device without irqs, is there?

No, I am not against the use of IRQs. In fact, my patches add support
for both IRQ based(default) and polling based operation(optional). If
rotary-encoder does not have interrupt capable gpio lines connected to
it (like am335x-ice) then one can make use of the polling mode support.

> 
>> Suppose we poll device at t=0ms and see gpio values are LLLH(1), if we
>> again poll device at t=500ms(which is what input_poll_dev helps to do)
>> and see that gpio values is LLHH(3), then we know that rotary encoder
>> has changed to 3. This can be done w/o IRQ for absolute encoders.
>>
>>>> diff --git a/Documentation/devicetree/bindings/input/rotary-encoder.txt b/Documentation/devicetree/bindings/input/rotary-encoder.txt
>>>> index 6c9f0c8a846c..9c928dbd1500 100644
>>>> --- a/Documentation/devicetree/bindings/input/rotary-encoder.txt
>>>> +++ b/Documentation/devicetree/bindings/input/rotary-encoder.txt
>>>> @@ -12,6 +12,10 @@ Optional properties:
>>>>  - rotary-encoder,relative-axis: register a relative axis rather than an
>>>>    absolute one. Relative axis will only generate +1/-1 events on the input
>>>>    device, hence no steps need to be passed.
>>>> +- rotary-encoder,absolute-encoder: support encoders where GPIO lines
>>>> +  reflect the actual position of the rotary encoder dial. For example,
>>>> +  if dial points to 9, then four GPIO lines read HLLH(1001b = 9).
>>>> +  In this case, rotary-encoder,steps-per-period needed not be defined.
>>>
>>> IMHO this is wrong, I'd formalize this device as:
>>>
>>> 	{
>>> 		compatible = "rotary-encoder";
>>> 		gpios = <&gpio 19 1>, <&gpio 20 0>, <...>, <...>;
>>> 		rotary-encoder,encoding = "binary";
>>> 		rotary-encoder,steps = <16>;
>>> 		rotary-encoder,steps-per-period = <16>;
>>
>> The above bindings essential means quarter_period device. I would not
>> like to bother with all the logic in rotary_encoder_quarter_period_irq()
>> when we can know encoder->pos by directly reading state of gpio lines.
> 
> OK, we have code that is more complex than it needs to be for your
> device. But your device is a special case of the supported devices, so
> I'd say don't bother that there is more logic in the driver than you
> need and be lucky.
> 

More complexity is just a overhead. Since, encoder can be turned at a
rate faster than handling of IRQs (rotary_encoder_quarter_period_irq()
is threaded IRQ hence, priority is not close to real time), some states
can be missed. rotary_encoder_quarter_period_irq() is not robust in this
case, reading gpios directly is more suitable option. I see similar
views expressed in previously[1]

[1]http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/391196.html

-- 
Regards
Vignesh
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